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5′ cap The 7-methylguanosine cap structure
found on mRNA in eukaryotes.
A band A wide, dark band produced by the
orderly parallel arrangement of the thick
filaments in the middle of each sarcomere.
A junction Specialised for chemical and
electrical communication between cells.
abiotic Pertaining to physical and inorganic
components.
ABO blood group antigen A major group
of carbohydrate antigens on the surface
of human blood cells; it is important in
determining the compatibility of blood
transfusions.
aboral Refers to the region opposite the mouth.
abortion Procedures or circumstances that
cause the death of an embryo or foetus after
implantation.
abscisic acid (ABA) A growth-inhibitory
hormone controlling responses of plants to
stress (such as drought), frost tolerance and
seed dormancy; synthesised in the carotenoid
pigment pathway.
abscission zone A zone of tissue at the base of a
petiole that regulates abscission or shedding
of the leaf.
absorption The process in which digested
nutrients are transported from the digestive
cavity into an animal’s circulatory system.
absorption spectrum The pattern of absorption
of photons at different wavelengths of light.
absorptive capacity The surface area of that
part of the gut over which small molecules
produced by digestive enzymes can be
absorbed into the blood or lymph.
absorptive nutrition The process whereby
an organism secretes enzymes into food
substrates, breaking down complex organic
molecules into small organic molecules that
are absorbed as food.
absorptive state One of two alternating phases
in the utilisation of nutrients; occurs when
ingested nutrients enter the blood from the
gastrointestinal tract. The other phase is the
postabsorptive state.
abundance The number of individuals (size) of
a population.
Acacia The wattles, plants from the family
Mimosaceae. They are legumes, characterised
by compound leaves that are often replaced by
phyllodes, and flowers with showy anthers.
acanthodian An extinct spiny fish from the
Silurian period, 440–417 million years ago.
accessory genetic elements A term used to
describe plastids and phages because they are
not vital to the host bacterium and can be
lost from the bacterial cell.
acclimation The response of an organism to
change in a single environmental factor,
usually within a laboratory.
acclimatisation The process of fine-tuning an
animal’s adaptive mechanisms to a changing
environment.
acetyl CoA A two-carbon compound that is the
substrate for the citric acid cycle; produced
in mitochondria during the final stage of
glycolysis when O2 is available, and as a
product of b-oxidation.
acid A molecule that releases hydrogen ions in
solution.
acid growth hypothesis The hypothesis that
acidification of the plant cell wall leads to
hydrolysis of restraining chemical bonds
within the cell wall, allowing cell elongation
driven by the turgor pressure of the wall.
acid hydrolase A hydrolytic enzyme found in
lysosomes that functions at acidic pH and
uses a molecule of water to break a covalent
bond.
acid rain Precipitation with a pH of less than
5.6; acid rain results from the burning of
fossil fuels, which releases sulfur dioxide and
nitrogen oxide into the atmosphere. These
in turn react with oxygen in the air to form
sulfuric acid and nitric acid, which fall to
the surface in rain or snow.
acidophile A type of bacteria (Archaea) that
survives in highly acidic environments
(acid-loving).
acoelomate Containing no coelom (body cavity
within mesoderm).
acquired antibiotic resistance The common
phenomenon of a previously susceptible strain
becoming resistant to a specific antibiotic.
acquired immune deficiency syndrome
(AIDS) A disease caused by the human
immunodeficiency virus (HIV) that leads to
a defect in the immune system of infected
individuals.
acquired immunity The specific immune
response that is triggered after infection or
other encounter with an immunogen.
acritarch Phytoplankton fossils; tiny spherical
cells (algal cysts), some with spines.
acrocentric A chromosome in which the
centromere is near one end.
acromegaly A condition in which a person’s
GH levels remain elevated after growth has
ceased, and the continued excess GH causes
many bones, like those of the hands and feet,
to thicken and enlarge.
acrosomal reaction An event in fertilisation in
which the binding of a sperm cell to proteins
located in the egg cell plasma membrane
triggers a series of events producing the fast
block to polyspermy and the entry of the
sperm cell’s nucleus into the egg cell.
acrosome Secretory vesicle in the head of the
sperm containing hydrolytic enzymes, which
are released during fertilisation.
actin A globular protein; the main structural
component of microfilaments.
actinomorphic Describes a flower with
parts arranged in a regular way (radially
symmetric).
actinopod A single-celled radially symmetrical
protist with long slender projections
(axopods) on the surface.
Actinopterygii Ray-finned fishes including about
25 000 living species; birchirs, sturgeons,
paddle fishes and teleosts; skeleton of true bone
with ray-like supports in the fins.
actinula A small polyp stage in the life cycle of
a jellyfish.
action potential An electrical event that is
conducted the full length of an axon without
loss of amplitude because it regenerates
itself at successive points. It is triggered by
depolarisation that reaches the threshold
potential for the membrane and involves
a rapid non-linear opening of voltagedependent sodium channels, followed by
an opening of voltage-dependent potassium
channels.
action spectrum The absorption spectrum of
light that activates photosynthesis.
activation energy The energy required to
initiate a reaction; it is more than the
minimal level to break existing bonds at the
moment molecules collide.
activator A transcription factor that binds to
DNA and increases the rate of transcription.
active immunity The acquired response to
exposure to any type of antigen.
active response An amplification of a local
depolarisation of a neuron that dies away
with distance from the point of initiation; it
is triggered by depolarisation that reaches the
threshold potential for the membrane.
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active site A specialised region of an enzyme
into which substrate molecules fit; a pocket
or groove formed by the folding of the
polypeptide chains of the enzyme (quaternary
structure).
active transport A carrier-mediated process
requiring energy derived from hydrolysis of
adenosine triphosphate (ATP); it can move
substances against a concentration gradient.
acute phase protein A primitive recognition
system, present normally in serum
but increasing upon infection up to
several hundred-fold; they decrease the
invasive ability of pathogens and increase
phagocytosis.
adaptation The processes and structures by
which organisms adjust to short-term or
long-term changes in their environment.
adaptive radiation The rapid evolution and
divergence of members of one lineage into
different niches.
adenine (A) A purine base found in DNA and
RNA.
adenosine triphosphate (ATP) An adeninecontaining compound that releases free
energy when its terminal phosphate bond
is hydrolysed; this energy is used to drive
energy-requiring reactions in cells.
adenylyl cyclase An enzyme in the plasma
membrane that synthesises cAMP from
ATP.
adherens junction A subclass of anchoring
junction that provides structural support
and cell adhesion by cross-linking between
cytoskeletons of neighbouring cells; adherens
junctions may be focal (occur as a patch) or
belt-like.
adhesion molecule A membrane protein found
in animal cells that promotes cell adhesion.
adhesion The tendency of water to cling to
other polar compounds due to hydrogen
bonding.
adipose cells Fat cells, found in loose connective
tissue, usually in large groups that form
adipose tissue. Each adipose cell can store a
droplet of fat (triacylglyceride).
a-diversity The concept of the diversity of a
biological community within a local area that
takes account of the relative abundance of
species.
adrenal cortex An endocrine gland that secretes
steroid hormones—mineralocorticoids (such
as aldosterone) and glucocorticoids (such as
corticosterone).
adrenal medulla A neurosecretory gland
that secretes catecholamines—adrenaline,
noradrenaline and dopamine.
advanced See apomorphic.
adventitious Referring to a structure arising
from an unusual place, such as stems from
roots or roots from stems.
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adventitious root A root that arises from deep
within the stem of certain plants, such as at
the nodes of grasses and palms.
aerenchyma A type of parenchyma found in
aquatic plants; spongy in appearance with
large gas-filled intercellular spaces.
aerobic Refers to a process that occurs in the
presence of oxygen; a form of metabolism
that requires oxygen.
aerofoil A structure that generates lift forces as it
passes through a fluid (generally air or water
for animals) by virtue of its asymmetrical
cross-section, such as seen in the wings of
birds or the pectoral fins of sharks.
aerotolerant anaerobe A microorganism that
does not use oxygen but is not poisoned by
it either.
aestivation Torpor in response to hot,
dry summer conditions (as opposed to
hibernation during cold winter conditions).
afferent arterioles Blood vessels that provide a
pathway for blood into a tissue or organ. For
example, afferent arterioles in the kidney that
supply each glomerulus with blood.
aflatoxins Fungal toxins that cause liver cancer
and are a major health concern worldwide.
agar A gelatinous product of red seaweeds used
in microbiology and as food.
aggregate fruits Fruits that develop from a
cluster of separate carpels on one flower.
AIDS (acquired immunodeficiency syndrome)
A disease caused by the retrovirus human
immunodeficiency lentivirus (HIV).
air capillary A small-diameter air-filled tube
that branches from the parabronchi in the
bird lung and forms the site for gas exchange;
as such, the space between the air
capillaries is filled with a network of
blood capillaries.
air sac A component of the avian respiratory
system: air sacs—not lungs—expand when a
bird inhales and shrink when it exhales, and
they do not participate in gas exchange.
akinete A spore of a cyanobacterium that is an
enlarged cell filled with food reserves and
that can remain dormant.
aldosterone A hormone secreted by the adrenal
cortex that controls the concentrations of
sodium and potassium in the blood through its
actions mainly on the renal tubules. It promotes
sodium reabsorption and potassium secretion.
aleurone layer The outer layer(s) of the
endosperm of cereal grains that produces
enzymes required for endosperm breakdown.
algae A general name for unicellular, colonial
or multicellular eukaryotic photosynthetic
organisms of simple structure of marine and
freshwater habitats.
algal bloom A population explosion of ‘algal’
cells in water bodies, for example green algae
or cyanobacteria (blue-green algae).
alimentary canal The single elongated tube of
a digestive system with an opening at either
end, through which food and eventually
wastes pass from one end to the other.
alkaline A solution that has a pH above 7.
alkaloids A group of structurally related
secondary metabolites that all contain
nitrogen and usually have a cyclic, ringlike
structure.
allantois In amniotes; an outgrowth of the
embryonic hindgut used for excretion during
development.
allatostatin A small neuropeptide hormone
secreted by neurosecretory cells that inhibits
juvenile hormone production and reduces
food intake in insects and crustaceans.
allatotropin A small neuropeptide hormone
secreted by neurosecretory cells in insects that
stimulates juvenile hormone production.
allee effect The phenomenon where some
individuals will fail to mate successfully
purely by chance, for example because of the
failure to find a mate.
allele One of two or more forms of a gene
located in the same position on homologous
chromosomes.
allele frequency The proportion of a particular
allele in a population.
allelochemical A powerful plant chemical, often
a root exudate, that kills other plant species.
allelopathy The suppressed growth of one
species due to the release of toxic chemicals
by another species.
allergen An agent that provokes an overreaction
of the immune system.
allodiploid An alloploid that has only one set of
chromosomes from two different species.
allograft A graft of tissue from another
individual of the same species.
allometric The growth pattern whereby
different parts of the body grow at different
rates with respect to each other.
allopatric speciation A geographic separation
of populations leading to divergent evolution
and formation of new species.
allopolyploid An organism that contains two
or more complete sets of chromosomes from
two or more different species.
allostasis The re-establishment of homeostasis;
the dynamic maintenance of constancy of the
internal environment.
allosteric interaction This occurs when the
binding of a compound to a protein induces
a shape change in the protein at a site distant
to the binding site.
allotetraploid A type of allopolyploid that
contains two complete sets of chromosomes
from two species for a total of four sets.
alternation of generations In eukaryotes: the
alternation of haploid and diploid stages in
the life cycle.
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alternation of generations In plants, a
reproductive cycle in which a haploid (n)
phase (the gametophyte), gives rise to
gametes which, after fusion to form a zygote,
germinate to produce a diploid (2n) phase
(the sporophyte). Spores produced by meiotic
division from the sporophyte give rise to new
gametophytes, completing the cycle.
alternative splicing The splicing of pre-mRNA
in more than one way to create two or more
different polypeptides.
altruism behaviour Where individuals reduce
their own reproductive success to enhance
the reproductive success of others (e.g.
co-operating to help others raise their young).
alveolar ducts The final branch of the airways
in lungs that terminates in the alveoli.
alveolus (pl. alveoli) A sac-like structure of the
lungs where gas exchange occurs.
Alzheimer’s disease (AD) The leading
worldwide cause of dementia; characterised
by a loss of memory and intellectual and
emotional function.
AM fungi Arbuscular mycorrhizal fungi;
glomalean fungi that form symbiotic
associations with the roots of vascular plants.
amber The hardened resin of conifers in which
fossils may be preserved.
amensalism One-sided competition, where the
interaction is detrimental to one species but
not to the other.
amino acid An organic molecule with an amino
group (–NH2, except for proline, which has
an imino group –NH–), an acidic carboxyl
group (–COOH), a hydrogen atom and a
unique side chain (R-group), all bonded to
a central carbon atom; the structural unit of
proteins.
aminoacyl site (A site) One of the three sites
for tRNA binding to the ribsome; the others
are the peptidyl site (P site) and the exit site
(E site). The A site is the site where incoming
tRNA molecules bind to the mRNA (except
for the initiator tRNA).
aminoacyl-tRNA synthetase An enzyme that
catalyses the attachment of amino acids to
tRNA molecules.
ammonia (NH3) One of the most highly toxic
of the nitrogenous wastes because it disrupts
pH, ion electrochemical gradients and many
chemical reactions that involve oxidations
and reductions; typically produced in many
aquatic species.
ammonoid A group of extinct cephalopod
animals dominant in the Devonian period.
ammonotely The pattern of nitrogenous waste
excretion where excess nitrogen (mainly from
digested protein) is excreted as ammonia.
amnion The extra-embryonic membrane
enclosing an amniote embryo in a
fluid-filled sac.
amniote A member of a clade of tetrapods with
amniotic eggs that includes turtles, lizards,
snakes, crocodiles, birds and mammals.
amniotic egg The structure that contains the
developing embryo and the four separate
extraembryonic membranes that it produces:
the amnion, the yolk sac, the allantois and
the chorion.
amoeba (pl. amoebae) A single-celled organism
that produces extensions of the cell surface
(pseudopodia or ‘false feet’) involved in
locomotion or feeding.
amoebocyte Amoeboid cells in the mesophyl of
sponges; they are capable of engulfing and
ingesting material.
Amphibia A class of vertebrate, including
salamanders, frogs and toads.
amphipathic molecule A molecule in which
there is a difference in water solubility
between one end and the other, such as
a phospholipid, which has a phosphate
head (hydrophilic) and a fatty acid tail
(hydrophobic).
amplexus A mating position used by frogs and
toads, involving the male climbing onto the
female’s back and clasping her around the
abdomen.
ampulla A contractable sac at the base of a
tube foot in the water vascular system of
echinoderms.
amygdala An area of the brain known to be
critical for understanding and remembering
emotional situations.
amylase A digestive enzyme involved in the
digestion of starch.
amylopectin One of the polysaccharide
components of starch; made of
α-D-glucose units.
amyloplast A kind of plastid containing
large starch granules and very few, if any,
membranes within the stroma.
amylose One of the polysaccharide components
of starch; made of sparsely branched
α-D-glucose units.
anabolic The favouring of the synthesis of
tissue.
anabolism Metabolic reactions involving the
building or synthesis of molecules.
anaerobic Not requiring oxygen.
anaerobic respiration Cellular respiration
occurring in the absence of oxygen; glycolysis
or fermentation.
anagenesis The pattern of speciation in which
a single species is transformed into a
different species over the course of many
generations.
anal pore The posterior opening to the digestive
system.
analogous Structures that have a similar
function as a result of convergent evolution;
compare homologous.
anamorph A fungus that has lost the ability to
reproduce sexually.
anaphase A phase of mitosis in which the two
kinetochores of the centromeres separate and
sister chromatids move apart.
anapsid A vertebrate skull that lacks one or two
pairs of openings in the temples.
anastomose Hyphal fusion in fungi allowing
migration of nuclei from one hyphal cell to
another.
anatomy The study of the morphology of living
organisms, such as plants and animals.
anchoring junction A junction linking
neighbouring cells and providing
mechanical support; includes desmosomes,
hemidesmosomes and adherant junctions.
androecium The floral whorl that comprises the
stamens.
androgen A steroid hormone, including
testosterone and dihydrotestosterone; secreted
by the Leydig cells of the testes.
anemia A condition characterised by lower than
normal levels of hemoglobin, which reduces
the amount of oxygen that can be stored in
the blood.
aneuploidy An alteration in the number of
particular chromosomes so that the total
number of chromosomes is not an exact
multiple of a set.
angiosperm A flowering plant.
angiotensin II A peptide hormone derived from
the action of a kidney hormone, renin, on a
blood protein, angiotensinogen, produced in
the liver; its actions include vasoconstriction
of peripheral blood vessels, renal retention
of ions and water and the control of
mineralocorticoid secretion from the
adrenal cortex.
angle of attack The orientation (angle) of an
aerofoil or hydrofoil in relation to the direction
of travel; a disc travelling through air edge-on
has an angle of attack of 0° and, travelling flat
surface first, an angle of 90°.
animal contest The interactions between
individuals that are attempting to gain
exclusive access to a resource.
animal pole The pole of the egg containing
relatively low concentrations of yolk.
Animalia The kingdom including all animals.
anion A negatively charged ion (e.g. Cl–).
Annelida A phylum of segemented worms.
annual A plant that dies after producing seed
during its first year of life.
annual ring A ring of xylem in trees and shrubs
that represents secondary growth from one
season to the next; annual rings can be used
to determine the age of trees and climatic
changes.
annulus (pl. annuli) A ring of enlarged cells
with lignified walls found in sporangia of
leptosporangiate ferns.
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anoxygenic photosynthesis A photosynthetic
pathway in some bacteria that does not
generate oxygen; early type of photosynthesis
in evolution.
antagonist A muscle or group of muscles that
produces oppositely directed movements at
a joint.
antenna (pl. antennae) The paired appendages
located on the head of an arthropod and
bearing sensory receptors.
antenna complex A complex of hundreds of
pigment molecules in a photosystem that
collects photons and feeds the light energy to
a reaction centre.
antennal gland The paired metanephridial
excretory organs of crustaceans, which have
excretory openings at the base of the antennae.
antennule The small antenna on the head of an
arthropod; see antenna.
anterior Refers to the end of an animal where
the head is found.
anterior pituitary gland (adenohypophysis)
A non-neural endocrine gland in vertebrates
derived from the gut; it secretes proteinbased hormones under the control of
neurosecretions from the hypothalamus.
anteroposterior axis One of the three axes along
which the adult body pattern is organised;
the others are the dorsoventral axis and the
right–left axis.
anther In a flower, part of the stamen that
houses developing male reproductive cells.
antheridium (pl. antheridia) In a plant: the male
gametangium (sex organ) producing sperm (or
male haploid nuclei); antheridia are unicellular
in algae and fungi and multicellular in plants
(bryophytes and lower vascular plants).
Anthocerophyta The phylum of hornworts, a
group of small spore-producing, non-vascular
land plants.
Anthozoa A class of cnidarians that includes
corals and sea anemones.
anthropoid A member of a class of primates
that includes the monkeys and the hominoid:
species are larger brained and diurnal.
antibiotic A naturally occurring inhibitor
of bacterial protein synthesis and thus
bacterial growth.
antibody A protein molecule produced by
B cells and plasma cells in response to
an antigen, and that reacts specifically with
that antigen.
anticodon A three-nucleotide sequence in tRNA
that is complementary to a codon in mRNA.
antidiuretic hormone A hormone that increases
the permeability of the renal collecting duct
to water and thus osmoconcentrates urine.
antigen Any molecule that can be recognised
by and binds to one of the specific molecules
(antibodies or T-cell receptors) of the
immune system.
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antimicrobial peptide Found in both plants and
animals; the most ancient form of defence
against pathogens.
antiparallel An arrangement in DNA where one
strand runs in the 5′ to 3′ direction while
the other strand is oriented in the 3′ to 5′
direction.
antipodal cell A nutritive cell of the embryo sac
of flowering plants; it lies at the end opposite
the micropyle.
antiport A type of transporter that binds two or
more ions or molecules and transports them
in opposite directions.
antiseptic The preventing of infection by
inhibiting the action of microorganisms.
Anura An order of modern amphibians,
including frogs and toads.
anus The terminal opening of the gut; the solid
residues of digestion are eliminated through
the anus.
aorta The main artery that carries oxygenated
blood from the heart to the body in higher
vertebrates.
apical complex A structure characteristic of
apicomplexan, protistan parasites, that
enables the parasite to enter a host cell.
apical constriction A cellular process during
gastrulation that is crucial to development; a
reduction in the diameter of the actin rings
connected to the adherens junctions causes
the cells to elongate toward their basal end.
apical dominance This occurs in plants where the
apical bud inhibits the growth of axillary buds
further down the stem, resulting in a single
dominant shoot; it is thought to be maintained
by the auxin hormone, indole-3-acetic acid.
apical meristem A specialised growth region
at the tip of shoots and roots; cells divide
continually to produce the primary tissues
and organs of the plant.
apical organ The site of the balance organ
(statocyst) in a comb jelly (ctenophore).
apical region The region of a plant that projects
upwards, usually from the soil, and produces
the leaves and flowers.
apicomplexan Endoparasites of animals, such
as Plasmodium, the causal agent of malaria; a
type of unicellular alveolate protist.
apomixis A type of seed formation in plants
where the embryo is derived only from cells
in the female ovule rather than from the
fusion of male and female gametes.
apomorphic An advanced character, one that
has evolved more recently than general,
ancestral features.
apoplast The portion of a plant tissue that lies
outside the cell membranes, that is, mainly
the cell walls.
apoplastic pathway The pathway of least
resistance of water from soil into the plant,
through cell walls and intercellular spaces.
apoptosis Programmed cell death. The process
of cell suicide that involves a characteristic
series of events leading to death of the cell.
aposematic coloration An ecological strategy of
some organisms that ‘advertise’ their poisonous
nature by the use of bright colours.
appendix A finger-like projection in the
gastrointestinal tract of animals that has no
known essential function, but that may at
one time have been an important part of the
body’s defence mechanisms.
appressorium The swelling at the tip of a fungal
hypha that adheres to the surface of a host.
Apterygota A class of minute insects that
primitively lack wings, for example springtails
and silverfish.
aquaporin A membrane channel that conducts
water, present in the plasma membrane of
cells in which rapid water transfer is required.
aqueous humor A thin liquid in the anterior
cavity behind the cornea of the vertebrate
eye.
aqueous solution A solution made with water.
Arachnida A class of arthropods that includes
spiders, scorpions and ticks.
arbuscular mycorrhizae Symbiotic associations
between glomalean fungi and the roots of
vascular plants.
arbuscule The finely branched haustoria
of glomalean fungi within arbuscular
mycorrhizal roots.
Archaea One of the super kingdoms (or
domains) of prokaryotes (bacteria).
archegonium (pl. archegonia) A multicellular,
female gametangium (sex organ) producing
egg cells; in all plants except flowering plants.
archenteron An embryonic cavity in animals
that becomes the gut.
Archosauria A group of vertebrates that
includes crocodiles, birds and extinct
dinosaurs.
aridity Very dry environmental conditions.
arteriole Small muscular arteries leading to the
capillaries.
artery A large blood vessel carrying blood from
the heart.
Arthropoda The animal phylum that includes
spiders, crustaceans and insects.
artificial selection See selective breeding.
ascocarp A fruiting body formed from
vegetative hyphae of an ascomycete fungus;
encloses asci with ascospores.
ascogonium A female gametangium enclosing
female gametes or female haploid nuclei (e.g.
in ascomycete fungi).
Ascomycota A phylum of the kingdom Fungi;
ascomycetes, including truffles and yeasts,
that produce ascospores inside a sack-like
cell, the ascus.
ascospore A haploid, sexual spore produced in
an ascus of an ascomycete fungus.
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ascus (pl. asci) A sac-like cell that produces
ascospores (ascomycete fungi).
aseptate The condition of not being partitioned
into smaller cells; usually refers to fungal cells.
asexual reproduction Reproduction in which
offspring are clones of the parent organism.
aspect ratio This describes wing shape; either
the ratio of the span to the mean chord
(width) of a wing, or the ratio of the square
of the span to the surface area.
aspirating pump In terms of gas exchange, a
pump that draws air into the lungs under
negative pressure, hence the term aspiration.
assemblage A group of co-existing species.
assortative mating Non-random mating
in which phenotypically alike (positive
assortative mating) or non-alike (negative
assortative mating) individuals mate
preferentially.
aster An organelle associated with nuclear
division; it comprises bundles of
microtubules produced from a centrosome.
Asteraceae The Angiosperm daisy family.
Asteroidea A class of echinoderms; sea stars.
atlas The anterior first neck vertebra supporting
the skull of a tetrapod.
atom The smallest part of an element that
can exist and retain the properties of that
element; comprising a central nucleus made
of protons (positively charged) and neutrons
(neutral charge) surrounded by one or more
orbiting electrons (negatively charged).
atomic number The number of protons in the
nucleus of an atom; characteristic for each
element.
ATP synthase A protein complex located in
the inner membrane of mitochiondria; a
molecular ‘motor’ that both pumps protons
and catalyses the synthesis of ATP.
atrial natriuretic factor (ANF) A hormone
secreted by specific cells of the cardiac atria
in response to stretch; its actions include
increase of urine flow and loss of body fluid
volume by the inhibition of sodium and water
reabsorption by the renal tubules.
atrioventricular bundle (Bundle of His)
A group of rapidly conducting cells that
leads from the atrioventricular node to the
Purkinje fibres; together, they produce a
co-ordinated ventricular contraction.
atrioventricular node A patch of modified
muscle cells lying between the right atrium
and ventricle in higher vertebrates; it slows
the conduction of excitation between atria
and ventricles.
atrium (pl. atria) The heart chamber that
receives blood from veins or the sinus
venosus and delivers it to the ventricle; see
also spongocoel of sponges.
atrophy A reduction in the size of a structure,
such as a muscle.
atropine A potent toxin derived from the deadly
nightshade plant.
Australian region The biogeographic region
including the Australian mainland and
islands on the continental shelf, such as
Tasmania and New Guinea.
australopithecine Early hominid fossils from
Africa (dated back 4.4 million years), bipedal,
with a forward jutting face, a ridge above the
eyes and small brain.
autapomorphy The unique characters of each
species (as opposed to synapomorphy, the
shared advanced characters of the organisms
on a particular branch of a phylogenetic tree).
autocrine hormones Hormones that interact
with receptors on the surface of the cell that
releases them.
autograft A graft of tissue transferred from
another site on the same individual.
autoimmune disease A disease resulting from
the development of an immune response
to an individual’s own antigens; usually a
chronic diseases, such as diabetes or arthritis.
autoimmunity An immune response to one’s
own antigens; does not necessarily lead to
disease.
autonomic nervous system The nervous system
in animals that innervates the visceral organs
of the body so that their functions are not
consciously controlled.
autophagy Meaning ‘the eating of one’s self ’.
A process whereby cellular material, such as
a worn-out organelle, becomes enclosed in a
double membrane and degraded.
autoregulation The control of blood flow
to a tissue caused by the direct effects of
metabolites on smooth muscle of arterioles
and precapillary sphincters.
autosome Chromosomes that exist in pairs in
diploid organisms.
autotomy In echinoderms, the ability to
intentionally detach a body part, such as a
limb, that will later regenerate.
autotroph An organism able to synthesise
its own food by photosynthesis or
chemosynthesis.
auxin A type of plant hormone controlling stem
elongation; synthesised in the growing shoot
and root tips of plants; synthetic auxins are
used as selective herbicides.
Auxin-binding protein A protein to which the
plant hormone auxin specifically binds; it is
involved in auxin signalling.
Aves A class of vertebrates; birds.
avirulence gene (Avr gene) A gene that encodes
a virulence-enhancing elicitor, which causes
plant disease.
avirulent pathogen Any type of normally
pathogenic organism or virus that utilises
host resources but does not cause extensive
damage or death.
Avogadro’s number As first described by Italian
physicist Amedeo Avogadro, 1 mole of
any element contains the same number of
particles—6.022 ⫻ 1023.
axil In plants, the angle between a leaf ’s petiole
and the stem to which it is attached.
axillary bud A bud that occurs in the axil
(the upper angle where a twig or leaf joins
the stem).
axis The second vertebra in the neck of a
tetrapod.
axon A long neuronal process that carries the
output of the neuron to the next cell.
axon hillock The first part of an axon, the
membrane properties of which allow action
potentials to be generated.
axoneme A precise array of microtubules
covered by the plasma membrane; the
structural basis of cilia or flagella.
axopod See actinopod.
axostyle A stiff, rod-like bunch of microtubules
characteristic of parabasilid unicellular
flagellates (e.g. protist Trichomonas vaginalis
found in the genital tract of humans).
B lymphocyte (B cell) A type of lymphocyte
responsible for specific immunity.
bacilli rods one of the five major shapes of
prokaryotic cells.
backbone The linear arrangement of phosphates
and sugar molecules in a DNA or RNA
strand.
backcross In plant breeding, a cross between F1
(heterozygous) individuals and either of their
pure-breeding parents.
bacteria (sing. bacterium) Smallest cellular life
form (see prokaryote); when not capitalised,
common name that refers to prokaryotic cells
or species within the super kingdoms Bacteria
and Archaea
Bacteria A super kingdom of prokaryotes; also
termed the Eubacteria or true bacteria.
bacterial artificial chromosome (BAC) A vector
used to generate recombinant DNA molecules
that carry very large genomic fragments.
bacterial species In the Bergey classification
system for bacteria, a group of bacteria with
many common phenotypic characteristics,
occupying similar habitats, whose DNA
shows no major compositonal differences,
and whose 16S ribosomal RNA sequences are
97% or more similar.
bacteriophage A virus that infects and multiplies
in bacteria; commonly called phage.
balanced polymorphism Genetic polymorphism
that is stable and can be maintained in balance
in terms of the Hardy–Weinberg principle;
occurs if heterozygotes for particular alleles are
fitter than either homozygote.
balancing selection A type of natural selection
that maintains genetic diversity in a
population.
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bark All tissues to the outside of the vascular
cambium in woody stems and woody roots.
baroreceptor The nerve ending in the walls of
blood vessels that senses blood pressure.
basal lamina The extracellular matrix that
underlies epithelial cell layers.
basal metabolic rate (BMR) The metabolic
rate of an animal under resting conditions,
in good health, and not under stress of
any kind.
basal nuclei Clusters of neuronal cell bodies
that surround the thalamus on both sides and
lie beneath the cerebral cortex; involved in
planning and learning movements and also
function via a complex circuitry to initiate or
inhibit movements.
basal region The region of a plant that produces
the roots.
basal transcription A low level of transcription
resulting from the core promoter.
base A substance that can accept hydrogen ions
(H+), causing a decrease in their number in
solution; possessing a pH above 7; see also
nitrogenous base.
base pairing The pairing of nitrogenous bases
(C pairs with G and A with T) on opposite
sides of the two chains in the double helix
DNA molecule.
base substitution A mutation that involves the
substitution of a single base in the DNA for
another base.
basic local alignment search tool (BLAST)
A computer program that can identify
homologous genes that are found in a database.
basidiocarp A fungal fruiting body such as
a toadstool or mushroom (basidiomycete
fungi).
Basidiomycota A phylum of fungi in the Dikarya;
basidiomycetes bear spores on a fruiting body,
such as a mushroom or a toadstool.
basidiospore A spore of a basidiomycete fungus
borne externally on a basidium.
basidium A club-shaped fungal cell that bears
basidiospores on its surface (basidiomycete
fungi).
basophil A blood cell with granular cytoplasm
and irregular nucleus which stains strongly
with basophilic dyes.
Batesian mimicry Resemblance of one animal
(the mimic) to another to the benefit of
the mimic; named after the naturalist
H. W. Bates.
b-diversity A measure of the diversity among
different biological communities in different
habitats in spatially separate areas.
behavioural ecology A subdiscipline of
organismal ecology that focuses on how
the behaviour of an individual organism
contributes to its survival and reproductive
success, which in turn eventually affects the
population density of the species.
1144
Bergey classification A practical classification of
bacteria that aids species identification.
bicuspid A type of tooth with two cusps or
elevated points, as in the premolar teeth of
primates.
bidirectional replication In DNA replication,
the two DNA strands unwind, and DNA
replication proceeds outward from the origin
in opposite directions.
biennial A plant that does not reproduce during
the first year of life but may reproduce within
the following year.
bilateral symmetry The symmetry of an
organism where only one plane divides the
organism into two similar halves.
bilayer Of biomembranes, where phospholipids
form two layers.
bile A substance produced by the liver that
contains bicarbonate ions, cholesterol,
phospholipids, a number of organic wastes
and a group of substances collectively termed
bile salts.
binary fission The process of cell division
involving cleavage to create two equal-sized
cells, each containing one copy of the genetic
information and approximately half the
cytoplasm.
binomial system The system devised by
Linnaeus, whereby the name of each
kind of organism (each species) consists
of two words—genus name and specific
epithet.
biochemical unity of life All life on earth is
based on similar biochemical patterns and
processes of metabolism and synthesis, and
the expression and replication of genetic
information.
biochemistry The study of the chemistry of
living organisms.
biodiversity The number, relative abundance
and genetic diversity of organisms on earth;
see also species diversity.
biodiversity crisis The idea that there is
currently an elevated loss of species on
earth, far beyond the normal extinction rate
of species.
biodiversity hotspot A geographically defined
region with high species diversity and
endemism that are under threat.
bioenergetics The study of energy
transformations in biological systems.
biofilm An aggregation of microorganisms that
secrete adhesive mucilage, thereby gluing
themselves to surfaces.
biogeochemical cycle The movement of
material through an ecosystem, from
atmospheric and geologic stores through
food webs and back again.
biogeographic region A region inhabited by
unique forms of life, which reflects the
evolutionary history of life and earth.
bioinformatics A field of study that uses
computers to study biological information.
biological control The control of a pest species
by biological means, for example, introducing
a herbivore to consume and control a
plant pest.
biological electron carrier A molecule that
can act to accept (or donate) electrons, for
example NADP+.
biological evolution The phenomenon that
populations of organisms change over the
course of many generations. As a result, some
organisms become more successful at survival
and reproduction.
biological species concept A species is a group
of individuals whose members have the
potential to interbreed with one another in
nature to produce viable, fertile offspring,
but cannot successfully interbreed with
members of other species.
biology The study of life.
bioluminescence A phenomenon that results
from chemical reactions that give off light
rather than heat.
biomagnification The increase in the
concentration of a substance in living
organisms with each increase in trophic
level in a food web.
biomass ‘Living mass’; the amount, usually
expressed as weight, of organisms in a
particular area at a particular time.
biome On a global scale, ecological
communities with the same structure and
delineated by climate (for example grasslands
of the world).
biomembrane Biological membranes, formed
from phospholipids and glycolipids.
bioremediation The use of living organisms,
usually microbes or plants, to detoxify
polluted habitats such as dump sites or
oil spills.
biosphere The parts of the earth that are
accessible to living organisms (hydrosphere,
atmosphere and lithosphere).
biota The fauna and flora of a given habitat or
region.
biotechnology The use of microorganisms to
produce products (e.g. drugs) and services.
biotic Pertaining to organisms.
bipedalism Walking on two hind legs, for
example, in human locomotion.
bipinnate Compound leaves that are twice
divided; a leaf with small leaflets.
bipolar distribution A distribution pattern
where one species lives in the Arctic region
and a related species lives in the Antarctic.
biramous Appendages consisting of two parts
(e.g. in crustaceans).
birth In mammals, the process of delivering
offspring so that they are no longer carried
within the mother’s body.
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bisexual A flower containing both stamens
and carpels.
bivalent Homologous pairs of sister
chromatids associated with each other,
lying side by side.
bivalve A mollusc of the class Bivalvia, such as
mussels and oysters, that have two shells.
blade The photosynthetic parts of a brown alga,
such as kelp, which are attached to the stipe
(stem).
blastocoel The first cavity of the embryo;
appears during cleavage.
blastocyst A mammalian blastula.
blastoderm The layer of cells of the blastula.
blastodisc The layer of cells of the avian
embryo, forming a disc on the uncleaved
yolk mass.
blastomere The cell of an embryo during
cleavage.
blastopore A depression on the surface of the
gastrula marking the site at which inward cell
movement occurs.
blastula An embryo at the end of cleavage.
blood A liquid tissue, circulated around the
body to transport substances (nutrients,
gases, waste products, hormones and
cells).
blood–brain barrier The barrier between the
blood and the brain in mammals, which can
be crossed by CO2 but not H+, changing
the pH of the cerebral interstitial fluid and
regulating ventilation.
body mass index (BMI) A method of assessing
body fat and health risk that involves
calculating the ratio of weight compared to
height; weight in kilograms is divided by the
square of the height in metres.
Bohr effect The release of oxygen by
haemoglobin molecules in response to
elevated ambient levels of CO2.
bolting The rapid elongation of the plant stem,
often followed by flowering.
bone The relatively stiff, strong, living structural
material of the vertebrate skeleton, primarily
composed of an organic component (the
protein collagen), an inorganic component
(hydroxyapatite, a form of calcium
phosphate) and cells (osteocytes).
bone marrow The tissue in the centre of bones
that produces the cells of the blood.
book gill In some chelicerates; an abdominal
appendage, modified as a gill, that has
many leaf-like folds (lamellae); for gas
exchange.
book lung The gas-exchange organ of spiders
and scorpions; similar in structure to a book
gill but internal on the ventral side of the
abdomen.
bordered pits Pores with an overarching lip;
connecting water-conducting tracheids of
vascular plants.
Boreal region From or belonging to the north;
the biogeographical region of the world
extending from the Polar Sea southwards
and including North American and Eurasian
regions (also called Holarctic).
boundary layer The layer of stagnant water
around any object, the thickness of which
is inversely proportional to water
movement.
bovine spongiform encephalopathy Mad cow
disease caused by a prion; see prion.
Bowman’s capsule A sac at the beginning of
the tubular component of a nephron in the
mammalian kidney.
b-oxidation The pathway in which lipids are
processed for the release of energy in cells.
brain The structure in the head of animals that
controls sensory and motor functions of the
entire body.
branchial heart Accessory pumping structure,
for example, at the base of gills in
cephalopods.
brassinosteroid A class of plant hormones.
bronchiole A thin-walled, small tube that can
dilate or constrict to prevent foreign particles
from reaching delicate lung tissue.
bronchus (pl. bronchi) One of a pair of
respiratory tubes branching from the lower
end of the trachea (windpipe) into either
lung.
brown alga A multicellular marine seaweed
that is brown in colour due to presence
of the pigment fucoxanthin, the same as
chrysophytes, haptophytes and diatoms;
phylum Phaeophyta, which includes
giant kelps.
Bryophyta The phylum of mosses; a group of
small spore-producing, leafy, non-vascular
land plants.
bryozoan A moss animal or lophophorate.
buccal cavity An invagination in a ciliate that is
the site of ingesting prey.
buccal-force pump Used by most air-breathing
fish to force air into the gas bladder; also by
amphibians to ventilate their lungs. Involves
using the buccal (oral) cavity like a bellows to
inflate the gas bladder or lungs.
bud A miniature plant shoot having a dormant
shoot apical meristem.
budding A form of asexual reproduction
involving the development of a new
individual from outgrowths of the body
wall of the parent.
buffer A substance that minimises changes
in the pH of a solution by taking up or
releasing H+ ions when extraneous acids or
bases are added to the solution.
bulb An underground storage organ formed
from a swollen stem with roots on its lower
surface and fleshy leaves above (e.g. an
onion).
bundle sheath A layer of cells surrounding the
veins of the leaves of some plants; it provides
a link between photosynthetic mesophyll
cells and vascular tissue.
buoyancy The tendency of an object to float;
the vertical upward force of a fluid on a
floating or immersed body that is equal to
the weight of fluid displaced by the body.
Burgess Shale Famous Palaeozoic (Cambrian)
fossil sites; marine animal groups
represented.
bursicon (tanning hormone) A brain
neurosecretory hormone that produces
hardening and darkening of the adult cuticle
in insects.
C3 photosynthesis The process of carbon
fixation in most plants in which the threecarbon compound phosphoglyceric acid is
the first stable product.
C4 photosynthesis The process of carbon
fixation in which four-carbon compounds
(e.g. malate) are the first stable product;
it is found in tropical and subtropical grasses
and cereals.
cadherin Proteins that traverse the plasma
membrane and adhere to each other in the
space between adjacent cells.
caecotrophy The reingestion of special faecal
pellets (the contents of the caecum, which
are very rich in microbes) released at night,
allowing utilisation of this high-protein
source.
calcitonin A peptide hormone secreted by
the ultimobranchial glands in vertebrates;
it lowers plasma Ca++ levels by promoting
uptake into bone and increasing excretion by
the kidneys.
callase An enzyme, 1,3 b-glucanase, that
degrades the polysaccharide callose.
callose A polysaccharide; the main component
of the cell wall of pollen tubes.
calorie The amount of heat required to raise
the temperature of 1 gram of water 1 degree
Celsius.
Calvin–Benson cycle A series of lightindependent reactions in the stroma of
chloroplasts where carbon dioxide is
converted to sugars.
calyptra The tissue of the old gametophyte
neck that persists on the top of a moss
spore capsule.
calyx The sepals that form the outermost whorl
of a flower.
cambium A secondary (sheet) meristem in
vascular plants; vascular cambium increases
the girth of stems and roots.
cAMP response protein (CRP) See catabolite
activator protein (CAP).
cancer A disease caused by gene mutations that
lead to uncontrolled cell growth.
canopy The uppermost layer of tree foliage.
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capacitance Of a medium for a gas; the
increment of concentration per increment of
partial pressure for that gas.
capacitance vessel The large veins of an animal
that hold most of the blood.
capillary The smallest blood vessel, where
exchange of substances between blood and
extravascular fluid occurs.
capitulum The inflorescence of daisies;
small flowers clustered in a head with the
appearance of one large ‘flower’.
capsid The outermost protein covering of
a virus.
capsule 1. A dry simple fruit that opens by
valves on the top; fruit typical of eucalypts.
2. A very thick, gelatinous glycocalyx
produced by certain strains of bacteria
that invade animals’ bodies that may help
them avoid being destroyed by the animal’s
immune (defence) system.
carapace The dorsal protective covering over
the thorax or anterior trunk segments of
crustaceans; the dorsal, protective shield
in turtles.
carbohydrate The most abundant organic
compound in nature, composed of carbon,
hydrogen and oxygen; the basic unit is a
sugar molecule.
carbon cycle The movement of carbon through
ecosystems, with carbon dixoide being
withdrawn from the atmosphere during
photsynthesis and being returned by cellular
respiration.
carbon fixation The capture of atmospheric
carbon dioxide and its conversion into
carbohydrates; it occurs in the stroma of
chloroplasts in eukaryotes.
carcinogen An agent that increases the
likelihood of developing cancer, usually a
mutagen.
cardiac centre A centre in the brain that controls
the rate and strength of the heart beat.
cardiac muscle A type of muscle tissue found
only in hearts, in which physical and
electrical connections between individual
cells enable many of the cells to contract
simultaneously.
cardiac output The rate of blood flow
(mL/min) from one ventricle of the heart.
cardiovascular disease Diseases affecting the
heart and blood vessels.
cardiovascular system A system containing
three components: blood or haemolymph,
blood vessels and one or more hearts.
carnivore An animal that consumes animal flesh
or fluids.
carotenoid The orange, yellow, red or
brown fat-soluble pigment involved as an
accessory pigment in photosynthesis; also
found in flowers and fruits; carotenes and
xanthophylls.
1146
carotid body The peripheral chemoreceptor
found in mammals and birds near the
bifurcation of the common carotid artery;
it responds to changes in the partial pressure
of oxygen.
carpel The female reproductive organ of a
flowering plant; it encloses ovules; it ripens
to become a fruit.
carrageenan The product of red seaweeds
used as a stabilising agent in food such as
ice-cream.
carrier A membrane protein that is capable of
transporting solutes across the membrane;
it usually involves binding to the protein on
one side of the membrane and releasing on
the other side.
carrying capacity The equilibrium point in a
population when the number of births and
deaths balances.
cartilage A connective tissue in skeletons of
vertebrates. Cartilage forms much of the
skeleton of embryos, very young vertebrates
and some adult vertebrates, such as sharks
and their relatives.
Casparian strip The strip of suberin thickening
on the radial walls of the endodermis or
exodermis of plant roots; it regulates uptake
of water and solutes.
caspase An enzyme that is activated during
apoptosis.
catabolic The favouring of the breakdown of
tissue.
catabolism Metabolic reactions involving the
breakdown of molecules.
catabolite activator protein (CAP) An activator
protein also known as the cAMP receptor
protein (CRP). CAP is needed for activation
of the lac operon.
catabolite repression In bacteria, a process
whereby transcriptional regulation is
influenced by the presence of glucose.
catalase An enzyme within peroxisomes that
breaks down hydrogen peroxide to water and
oxygen gas.
catalysis The process by which the activation
energy of a reaction is lowered; it affects only
the rate of the reaction.
catalyst The substance that accelerates the rate
of a chemical reaction.
catalytic amino acid The active-site amino acids
in an enzyme involved in the making or
breaking of covalent bonds.
cataract An accumulation of protein in the lens
of the eye; causes blurring, poor night vision.
catarrhine Old World (Africa and Oriental
region) monkey, ape or human; characterised
by nostrils that are close together and
directed downward.
cation A positively charged ion (e.g. Na+).
cavitation The breaking of the water column in
xylem vessels under water stress.
cDNA library A collection of recombinant
vectors that have cDNA inserts.
cecum The first portion of the large intestine in
humans and other similarly sized animals and
mammals.
cell The simplest unit of a living organism.
cell adhesion The phenomenon in which cells
adhere to each other. Cell adhesion provides
one way to convey positional information
between neighbouring cells.
cell adhesion molecule (CAM) A membrane
protein found in animal cells that promotes
cell adhesion.
cell biology The study of individual cells and
their interactions with each other.
cell body A part of a neuron that contains the
cell nucleus and other organelles.
cell coat Also called the glycocalyx, the
carbohydrate-rich zone on the surface of
certain animal cells that shields the cell from
mechanical and physical damage.
cell communication The process through which
cells can detect and respond to signals in their
extracellular environment. In multicellular
organisms, cell communication is also needed
to co-ordinate cellular activities within the
whole organism.
cell cycle The period from the birth of a new
cell by cell division, through growth of the
cell and replication of its genetic material,
to the generation of two daughter cells by
cell division.
cell differentiation The process whereby a cell
ceases to proliferate and adopts a specialised
structure and/or role.
cell division The division of a cell to form two
daughter cells, each containing one copy of
the genetic information and approximately
half the cytoplasm.
cell elongation The enlargement of cells in one
direction.
cell junctions Specialised structures that adhere
cells to each other and to the ECM.
cell nucleus The membrane-bounded area of a
eukaryotic cell in which the genetic material
is found.
cell plate The region of new cell wall that
forms during cytokinesis in eukaryotic
walled cells.
cell proliferation The division of cells to create
an increased number of cells.
cell signalling A vital function of the plasma
membrane that involves cells sensing changes
in their environment and interacting with
each other.
cell theory A set of general principles that
defines living organisms on the basis that
they are made of cells.
cell wall A rigid extracellular matrix that
provides support for plant, fungal and
bacterial cells; plant cell walls are made
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primarily of cellulose, while cell walls of
bacteria, fungi and algae are made of chitin.
cell–cell adhesion molecule A membrane
protein found in animal cells that promotes
adhesion between one cell and another.
cell–matrix adhesion molecule A membrane
protein found in animal cells that promotes
adhesion between a cell and molecules in the
extracellular matrix.
cellular blastoderm In insect embryonic
development, the stage during which the
nuclei of the syncitial blastoderm become
separate cells through membrane formation.
cellular differentiation The process by which
different cells within a developing organism
acquire specialised forms and functions, due
to the expression of cell-specific genes.
cellular immunity The defence functions
carried out by cells, particularly by T cells,
rather than by antibodies.
cellular respiration A process by which living
cells obtain energy from organic molecules.
cellulase The enzyme by which cellulose is
digested.
cellulose A structural polysaccharide present
in cell walls of plants and some protists;
composed of a long chain of glucose
molecules.
Cenozoic The most recent era in the geologic time
scale, from 65 million years to present day.
centimorgan (cM) A measure of the extent of
linkage between two genes, expressed as the
percentage of recombination between the
two loci.
central cell In the female gametophyte of a
flowering plant, a large cell that contains two
nuclei; after double fertilisation it forms the
first cell of the nutritive endosperm tissue.
central dogma Refers to the steps of gene
expression at the molecular level. DNA
is transcribed into mRNA and mRNA is
translated into a polypeptide.
central nervous system The brain and spinal
cord of vertebrates.
central region The region of a plant apical
meristem that produces stem tissue.
central vacuole An organelle that often occupies
80% or more of the cell volume of plant cells
and stores a large amount of water enzymes
and inorganic ions.
central zone The area of a plant apical meristem
where undifferentiated stem cells are
maintained.
centric diatom A radially symmetrical type
of diatom.
centrioles A pair of structures within the
centrosome of animal cells. Most plant cells
and many protists lack centrioles.
centromere The region where the two sister
chromatids are tightly associated; the
centromere binds to the kinetochore.
centrosome A single structure often near
the cell nucleus of eukaryotic cells that
forms a nucleating site for the growth of
microtubules
cephalisation The localisation of sensory
structures at the anterior end of the body
of animals.
Cephalochordata A subphylum of chordates;
lancelets.
cephalopod A mollusc of the class
Cephalopoda, including octopuses.
cephalothorax A united head and thorax in
crustaceans and spiders.
cercaria (pl. cercariae) The fluke larva that
develops from a redia.
cerci A pair of appendages found on the last
segment of an insect abdomen.
cercozoa A group of protists that often form
amoeboid cells with peudopods connected to
one another; related to forams.
cerebellum The part of the hindbrain, along
with the pons, responsible for monitoring
and co-ordinating body movements.
cerebral cortex The surface layer of gray matter
that covers the cerebrum of the brain.
cerebral ganglia Ganglia at the anterior end
of an animal that have evolved to process
information from sense organs on the head;
in more advanced animals they form the
basis for the brain.
cerebrospinal fluid Fluid that surrounds the
exterior of the brain and spinal cord and
absorbs physical shocks to the brain
resulting from sudden movements or blows
to the head.
cerebrum A group of structures in the forebrain
that are responsible for the higher function of
conscious thought, planning and emotion in
vertebrates.
cervix A fibrous structure at the end of the
female vagina that forms the opening to
the uterus.
Cestoda A class of flatworms; tape worms.
cestode A parasitic flatworm of the class
Cestoda, which includes the tapeworms.
chaetae Bristles of chitin on each body segment
that help anchor annelid worms during
locomotion.
channel A transmembrane protein that
forms an open passageway for the direct
diffusion of ions or molecules across a
membrane.
character A visible characteristic, such as
the appearance of seeds, pods, flowers
and stems.
character displacement The tendency for
two species to diverge in morphology
and thus resource use because of
competition.
charaophyte A type of freshwater green alga
related to land plants.
checkpoint One of three critical regulatory
points found in the cell cycle of eukaryotic
cells. At these checkpoints, a variety of
proteins act as sensors to determine if a cell
is in the proper condition to divide.
checkpoint control A mechanism that senses
the non-completion of essential cell cycle
processes such as DNA replication and
repair, or spindle formation, and prevents
progression into the next cell cycle phase.
chelicera (pl. chelicerae) The first pair
of appendages behind the mouth of a
chelicerate animal that are used as fangs or
pincers.
Chelicerata A major lineage of arthropods that
includes pycnogonids, spiders, scorpions and
horseshoe crabs; see chelicera.
Chelonia The order of amniotes that includes
turtles; a body with protective armoured
shields and horny plates.
chemical bond An electrostatic attraction
between atoms or groups of atoms to form
a stable molecule.
chemical equilibrium In a chemical reaction,
occurs when the rate of formation of products
equals the rate of formation of reactants.
chemical fossil An organic compound
produced by organisms and preserved as
fossils.
chemical mutagen A chemical that causes
mutations.
chemical potential The usable energy of a
reaction, designated DG.
chemical synapse A synapse in which a
chemical called a neurotransmitter is
released from the nerve terminal and acts
as a signal from the presynaptic to the
postsynaptic cell.
chemiosmosis A chemical action between
substances separated by a semipermeable
membrane; for example, the synthesis of ATP
from ADP and phosphate is driven by the
passage of protons through the membranebound ATP synthase complex down an
electochemical gradient.
chemoautotroph An organism that uses reduced
inorganic substrates as sources of energy and
reduces carbon dioxide to organic carbon,
using water or hydrogen gas as a reductant;
certain forms of bacteria.
chemoheterotroph An organism that uses
organic substances as a source of both carbon
and energy (e.g. animals and certain forms of
bacteria).
chemoreceptor A type of receptor that binds to
a particular signal molecule, a ligand.
chemotroph An organism that gets its energy
chemically.
chert A black substance formed from gels of
silica precipiated on the surface of ancient
sea-floors; a source of Precambrian fossils.
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chiasma (pl. chiasmata) The attachment point
between chromosomes where crossing over
occurs.
chiral A chemical compound where it cannot be
superimposed on its mirror image.
chiral pairs Two chiral forms of a compound
(see chiral); e.g. D-glyceraldehyde and
L-glyceraldehyde.
chitin An insoluble nitrogenous polysaccharide,
similar to cellulose, that is the main
constituent of the walls of fungi and the
exoskeleton of arthropods.
chiton A mollusc of the class Polyplacophora.
chlorarachniophyte A type of marine
photosythetic cercozoan (protist); see cercozoa.
chlorenchyma A photosynthetic parenchyma
cell.
chlorofluorocarbon (CFC) A compound used
as an aerosol propellant and refrigerant;
chlorine in CFCs reacts with and breaks
down atmospheric ozone.
chlorophyll A light-absorbing, green pigment
involved in photosynthesis.
chloroplast An organelle (plastid) containing
membrane-bound light-absorbing pigments;
functions in photosynthesis.
chlorotic Having an unhealthy pale colour due to
a lack of chlorophyll in plant tissues.
choanocyte A collar cell; the flagellated cell
lining the internal cavity of a sponge.
choanoflagellate A marine, free-living,
heterotrophic unicellular organism (a protist);
similar to the collar cell of a sponge.
cholesterol A lipid, an amphipathic sterol,
abundant in animal membranes.
Chondrichthyes A class of chordates;
cartilaginous fishes including chimaeras,
sharks, skates and rays.
chordae tendinae The fibrous strings that
connect the edges of the heart valves to
the heart wall for added strength and
control.
Chordata A phylum of deuterostome animals
including tunicates, lancelets, lampreys and
vertebrates; it is characterised by a notochord
and an internal skeleton.
chorion An extraembryonic membrane in the
amniotic egg that, along with the allantois,
exchanges gases between the embryo and the
surrounding air.
chromatin The DNA-protein complex that
makes up eukaryotic chromosomes.
chromatophore A cell that contains pigment
granules and expands and contracts under
muscular control, allowing body colour
change.
chromist A diverse group of protists called
the ‘brown lineage’; characterised by having
chlorophyll a and c, with many being
heterokont flagellate; for example, see brown
alga and chrysophyte.
1148
chromophore The light-absorbing region of a
protein photoreceptor; it absorbs light of a
particular wavelength.
chromoplast A plastid containing carotenoid
pigments; it is responsible for red, orange or
yellow colours of some plant organs.
chromosome A structure containing a single DNA
molecule and associated proteins; in prokaryotic
cells and in the nucleus, mitochondria and
chloroplasts of eukaryotic cells; nuclear
chromosomes are visible during cell division.
chrysolaminarin A β-(1→3)-glucan, a product
of photosynthesis in chrysophytes (golden
flagellates).
chrysophyte A golden-brown photosynthetic
marine flagellate, unicellular or colonial; it is
characterised by the pigment ficoxanthin and
heterokont flagellation.
chylomicrons Large fat droplets coated with
amphipathic proteins that perform an
emulsifying function similar to that of bile
salts; they are formed in intestinal epithelial
cells from absorbed fats in the diet.
chyme A solution of food particles in the
stomach that contains water, salts, molecular
fragments of proteins, nucleic acids,
polysaccharides, droplets of fat and various
other small molecules.
chymotrypsin A protease involved in the
breakdown of proteins in the small intestine.
Chytridiomycota A phylum of microscopic
fungi in soil and water, consisting of single
cells or short chains.
ciguatera Food poisoning caused by toxins from
dinoflagellates and other marine organisms
moving through the food chain; often toxin
accumulates in large reef fish, which are eaten
by humans.
cilia Short, thin extensions of cytoplasm that
undergo vigorous bending movements from
their base, thus providing non-muscular
locomotion as a result of a powerstroke.
ciliate Unicellular alveolate (k. protista) with
two nuclei (micro- and macro-nuclei) and
numerous cilia on the surface.
cilium (pl. cilia) A cell appendage that functions
like flagella to facilitate cell movement; cilia
are shorter and more numerous on cells than
are flagella.
circulatory system A system that transports
necessary materials to all cells of an animal’s
body, and transports waste products
away from cells. The three basic types of
circulatory systems are gastrovascular cavities,
open systems and closed systems.
Circum-Antarctic current The sea current that
circulates cold water around Antarctica.
cis-acting A DNA segment that must be
adjacent to the gene(s) that it regulates. The
lac operator site is an example of a cis-acting
element.
cisternae Flattened, fluid-filled tubules within
the cell.
CITES The Convention on International Trade
in Endangered Species of Wild Fauna and
Flora.
citric acid cycle A cycle that results in the
breakdown of carbohydrates to carbon
dioxide; also known as the Krebs cycle.
clade (klayd) See monophyletic group.
cladistics Phylogenetic systematics; a
comparative method to discover the
phylogeny of organisms based on discovering
shared apomorphic characters; it is used in
classification of organisms.
cladode A photosynthetic stem in plants that
has leaves reduced or absent.
cladogenesis A pattern of speciation in which
an ancestral species is split into two or more
daughter species.
cladogram A phylogenetic tree based on a
cladistic approach.
clamp connection A feature of basidiomycete
fungal hyphae that ensures that the two
nuclei of the dikaryon remain together
following mitosis.
clasper An extension of the pelvic fin of a
chondrichthyan, used by the male to transfer
sperm to the female.
class A higher level taxon (grouping) used
in the classification of organisms; below
phyla.
classification A hierarchy of groups and
subgroups of organisms reflecting their
phylogenetic relationships; each group
(taxon) is given a name and rank—kingdom,
phylum, class, order, family, genus, species
(see also taxonomy).
clathrin A protein located just inside the plasma
membrane in eukaryotic cells, in indentations
called clathrin-coated pits.
cleavage In vertebrates, a rapid series of
successive cell divisions of a fertilised egg,
forming a hollow sphere of cells,
the blastula.
cleistogamy In flowering plants, a process that
ensures self-fertilisation; anthers open and
self-pollination occurs within unopened
flowers.
climacteric A period of increased respiration in
fruits that includes a set of changes resulting
in fruit ripening.
climate The prevailing weather pattern in a
given region.
clitellum The thickened region of the epidermis
of a euclitellate worm (e.g. an earthworm)
that secretes the cocoon in which eggs are
deposited.
clitoris Located at the anterior part of the labia
minora, erectile tissue that becomes engorged
with blood during sexual arousal and is very
sensitive to sexual stimulation.
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cloaca In some animals, the common exit
chamber from the digestive, reproductive,
and urinary system; in others, the cloaca may
also serve as a respiratory duct.
clonal deletion A process that explains why
individuals normally lack active lymphocytes
that respond to self components; T cells with
receptors capable of binding self proteins are
destroyed by apoptosis.
clonal inactivation A process that explains why
individuals normally lack active lymphocytes
that respond to self components; the process
occurs outside the thymus and causes
potentially self-reacting T cells to become
non-responsive.
clonal selection A process during an immune
response in which those lymphocytes
that encounter their specific antigen are
stimulated to proliferate, thus increasing the
number of cells reacting to that antigen.
clone A collection of identical individuals; all
the descendants derived from one individual
(an organism, cell or molecule); for example,
asexual reproduction of plants by cuttings,
bulbs or bacteria by fission; to make a copy
of DNA.
cloning Methods that produce many copies
of something. These can be copies of genes,
copies of genetically identical cells, or copies
of genetically identical organisms. The
cloning of mammals can be achieved by
fusing a somatic cell with an egg that has had
its nucleus removed. Plants can be cloned
simply by removing cells and growing them
in particular mixtures of hormones.
closed circulatory system A circulatory system
in which blood flows throughout an animal
entirely within a series of vessels and is kept
separate from the interstitial fluid.
closed conformation Tightly packed chromatin
that cannot be transcribed into RNA.
clubmoss A plant in the phylum Lycophyta; a
vascular land plant with a homosporous life
cycle; for example, Selaginella.
clumped The most common pattern of
dispersion within a population, in which
individuals are gathered in small groups.
cluster roots Also called proteoid roots; groups
of hairy rootlets that form dense mats near
the soil surface and enhance nutrient uptake
in nutrient-poor soils. Found in plants from
family Proteaceae and some legumes (family
Fabaceae).
Cnidaria The phylum of jellyfish, anemones
and corals.
cnidocyte A cell in a cnidarian that contains a
nematocyst.
cocci Spheres; one of the five major shapes of
prokaryotic cells.
coccolith An ornamented calcite plate or scale,
many of which cover a coccolithophoroid cell.
coccolithophorid A type of haptophyte,
a marine unicellular flagellate; it is
characterised by a wall covered with scales or
coccoliths; some chalk deposits made from
the skeletons of these cells.
cochlea A coiled structure containing the hair
cells and other structures that generate the
responses that travel via the auditory nerve to
the brain.
cocoon A container or capsule produced by
animals to house eggs or protect a developing
larva and pupa.
co-current flow Water flowing in the same
direction as the haemolymph, for example in
gills (see also countercurrent flow).
codes of nomenclature International rules that
govern the scientific naming of organisms;
for example, codes for animals, plants,
cultivated plants and bacteria.
coding sequence A sequence in DNA that
encodes amino acids incorporated into
polypeptides during protein synthesis.
coding strand The DNA strand opposite to the
template (or non-coding strand).
codominant Alleles whose phenotypes are
equally recognisable in the heterozygote.
codon A sequence of three nucleotide bases that
specifies a particular amino acid or a stop
codon; codons function during translation.
coelacanth A fleshy-finned or lobe-finned fish of
the group named Sarcopterygii, which includes
land vertebrates, the tetrapods; fleshy fins
are homologous to terapod limbs; one living
genus, Latimeria, and fossils.
coelenteron In cnidarians; a gastrovascular
cavity lined with endoderm.
coelom The body cavity of an animal, lined on
all sides by mesoderm.
coelomate protostome Annelids, molluscs
and arthropods that develop a body cavity
(coelom) in which lie the body organs; see
also protostome.
coelomoduct A tubular excretory organ that has
a ciliated, funnel-like opening in the coelomic
cavity to draw coelomic fluid into the tubule; it
develops from the interior of an animal towards
the outside, unlike nephridia.
coenocytic A term used to describe a cell or
non-septate hypha containing numerous
nuclei.
coenzyme A type of cofactor, required by an
enzyme to function as a catalyst; a nonprotein, complex organic molecule, often
with a vitamin as a building unit.
coevolution The evolution of two species in
relation to one another, such as flowers and
their animal pollinators, parasites and their
hosts.
cofactor An additional chemical component, such
as a metal ion or organic molecule, required by
certain enzymes in order to function.
cohesion Intramolecular attraction between like
molecules.
cohesion theory The viewpoint that the water
molecules in the xylem sap have sufficiently
strong attractive forces between them that the
sap rises as a continuous column (under the
suction developed by transpiring shoots).
Coleochaete A small green alga, related to land
plants, that grows on other aquatic plants; a
disc generally one cell thick, with protective
cells around its reproductive structures.
coleoptile The sheath that encloses the newly
emerged leaves of a grass seedling.
coleorhiza The sheath that encloses the newly
emerged root of a grass seedling.
collagen A structural protein of the extracellular
matrix that is the most abundant protein of
mammals; collagens associate into a strong
sheet-like meshwork in basement lamina and
form fibrils in interstitial matrices.
collecting duct The terminal portion of the
vertebrate nephron, which conveys fluid from
the distal convoluted tubule into the renal
pelvis.
collenchyma Living plant cells strengthened
with primary thickening either at the corners
or on the tangential walls; these have a
support function.
colloid 1. A gel-like substance in the follicles
of the thyroid gland. 2. In chemistry: A
dispersion of particles of one substance (the
dispersed phase) throughout a dispersing
medium made of another substance.
colloid osmotic pressure (oncotic
pressure) Osmotic pressure due to large
proteins, chiefly albumin, in the blood; it is
involved in the balance between filtration
and reabsorption of fluid in tissues.
colon A part of the large intestine consisting
of three relatively straight segments—the
ascending, transverse and descending
portions. The terminal portion of the
descending colon is S-shaped, forming the
sigmoid colon, which empties into the
rectum.
colony A group of cells derived from a single
initial cell; normally used to describe
bacterial or unicellular fungal (e.g. yeast)
clusters of cells, derived from a single cell,
growing on a nutrient agar plate.
column Of an orchid: structure in the flower
consisting of one or two stamens fused with
the stigma and style.
comb row Ciliary plates, made up of fused
cilia, characteristic of comb jellies
(ctenophores).
commensalism A symbiotic interaction between
two species where one benefits and the other
is unaffected; it is usually one organism
living with another for shelter or support
(e.g. epiphyte).
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communicating junction (gap junction)
A junction specialised for chemical and
electrical communication between cells.
community In an ecological sense, an
assemblage of populations of different
species, interacting with one another, living
in a particular area (e.g. pond or forest).
community dynamics How biological
communities vary through time and space.
community structure The species present, their
abundances and distributions in a biological
community.
companion cell A cell type of phloem; a transfer
cell involved in the loading of sucrose into
the sieve cells.
comparative morphology The comparison of
body form of organisms.
compartmentalisation A characteristic of
eukaryotic cells that is defined by many
organelles that separate the cell into different
regions. Cellular compartmentalisation allows a
cell to carry out specialised chemical reactions
in different places.
competent A cell that is capable of taking up
DNA during genetic transformation.
competition (intraspecific and interspecific)
Individuals of either one species (intraspecific
competition) or different species (interspecific
competition) striving for the same resource
that is in limited supply.
competitive exclusion principle When one
species outcompetes another for a limited
resource, resulting in the local extinction of
the other species.
complement The family of plasma proteins that
provides a means for extracellular killing of
microbes without prior phagocytosis.
complement system A series of about 20 serum
proteins that activate sequentially in a cascade
of reactions; triggering of the complement
cascade leads to activation of non-specific
defensive cells, facilitation of phagocytosis
and lysis of cells.
complementary DNA (cDNA) A DNA copy of
an RNA transcript.
complementary sequence A nucleotide
sequence that can form a base-paired
double helical structure with another
sequence.
complete metamorphosis In advanced insects
such as butterflies, a series of distinct forms
during development: egg, larva, pupa and
adult.
compound A molecule composed of two or
more different elements.
compound eyes Image-forming eyes in
arthropods and some annelids consisting of
several hundred to several thousand light
detectors called ommatidia.
compound leaf A leaf divided into leaflets, each
with its own stalk.
1150
concentration The amount of a solute dissolved
in a unit volume of solution.
concentration gradient The difference in
concentration of a solute between one region
and another, for example, on either side of a
membrane.
conceptacle A warty structure on the surface
of a brown alga that contains reproductive
structures.
condensation reaction A reaction involving
removal of water molecules in the assembly
of complex molecules from simpler ones.
condensation The reaction by which unit
molecules (monomers) react together to form
a larger molecule (polymer), for example
dehydration reactions in which water
molecules are eliminated.
conductance The amount of gas transferred per
unit of time across a membrane.
conduction The passage of electrical
information along the surface of a neuron;
conduction is particularly rapid along
an axon.
cone 1. In plants, the reproductive structure
of non-flowering seed plants such as cycads
and conifers. 2. In vertebrates, a type
of light- sensitive neuron in the retina
concerned with the perception of colour
and with the most acute discrimination
of detail.
conform To become the same as. Of proteins:
a shape change to fit an enzyme. Of
multicellular organisms: possessing an
internal environment that mimics the
external environment.
congestive heart failure The condition resulting
from the failure of the heart to pump blood
normally; this results in fluid build-up in the
lungs (congestion).
conidiophore Specialised fungal hypha that
forms a stalk and bears spores (conidia).
conidiospore A fungal spore, asexually produced
in conidia on conidiophores.
conidium (pl. conidia) An asexual fungal spore
formed on a conidiophore; for dispersal and
spread of the fungus.
Coniferophyta The phylum of conifers;
cone-bearing seed plants such as pines.
conjugation A type of genetic transfer
between bacteria that involves a direct
physical interaction between two bacterial
cells.
connective tissue The tissue that provides
structural, metabolic and defensive support
for other tissues; for example, blood, bone
and cartilage; the extracellular matrix is
usually more abundant than cells.
connexon A channel that forms gap junctions
consisting of six connexin proteins in one
cell aligned with six connexin proteins in an
adjacent cell.
consensus sequence In genome sequencing, the
overall sequence that is consistent with the
sequences of individual fragments; computer
programs are used to compare sequences and
generate a consensus sequence.
conservation biology The study that uses
principles and knowledge from molecular
biology, genetics and ecology to protect the
biological diversity of life at all levels.
conservation genetics A branch of biology
involving the application of genetic principles
to the conservation of threatened species;
it is concerned particularly with the loss of
variation through genetic drift.
conservative mechanism In this incorrect
model for DNA replication, both parental
strands of DNA remain together following
DNA replication. The original arrangement
of parental strands is completely conserved,
while the two newly made daughter strands
are also together following replication.
constitutive gene An unregulated gene that has
essentially constant levels of expression in all
conditions over time.
constitutive secretion The constant release of
material from a cell.
consumer An organism that derives its energy
by consuming other organisms.
contagious Spread by direct or indirect contact;
for example, viral and bacterial diseases.
continental drift The phenomenon whereby,
over the course of billions of years, the major
landmasses, known as the continents, have
shifted their positions, changed their shapes,
and in some cases have become separated
from each other.
continental shelf The extension of the margin
of a continent that extends under the sea.
continuous trait A trait that shows continuous
variation over a range of phenotypes.
contractile ring A ring of myosin and actin
filaments responsible for pinching in of the
membrane during cytokinesis in animal cells.
contractile vacuole An organelle of cells that
excretes fluid by a pulsating action, first
filling the vacuole with fluid then ejecting
the fluid from the cell.
control sample The sample in an experiment
that is treated just like an experimental
sample except that it is not subjected to one
particular variable. For example, the control
and experiment samples may be treated
identically except that the temperature may
vary for the experimental sample.
conus arteriosus The last chamber of a fish
heart, leading to the ventral aorta and then
to the gills.
convection The transfer of heat by the
movement of air or water next to the body.
convection requirement Ventilation for a given
O2 consumption rate.
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Convention on International Trade in
Endangered Species of Wild Fauna and
Flora (CITES) An international agreement
that bans trade in highly endangered species
or regulates and monitors trade in others that
might become endangered. One hundred and
seventy-three countries are signatories and
more than 30 000 species are listed.
convergent evolution Evolution whereby
organisms from different, distantly related
lineages come to resemble one another.
co-operative breeding When adults in a
group do not themselves reproduce, but
instead help raise the young of the breeding
individuals.
co-ordinately regulated Occurs when the same
DNA-binding regulatory protein acts on
equivalent enhancer and silencer sequences
in genes at different loci, often resulting in
regulatory cascades.
copulation Mating between sexes associated
with internal fertilisation.
coral bleaching Occurs when corals are
stressed and they expel their photosynthetic
partners (zooxanthellae), become white
and starve.
coralloid roots The coral-like upward growth
of roots of certain plants (alders, cycads and
she-oaks) following root hair infection by the
bacterium Frankia or cyanobacteria.
core promoter For a eukaryotic structural gene,
refers to the transcriptional start site and
TATA box.
corepressor A small effector molecule that binds
to a repressor protein to inhibit transcription.
cork (phellem) The outermost part of the bark;
a secondary tissue produced by the cork
cambium; the cells non-living at maturity,
with suberised walls and impermeable to the
passage of water and gases.
cork cambium (phellogen) A lateral meristem
producing cork (phellem) to the outside of
the plant and phelloderm to the inside.
corm An underground storage organ formed
from a swollen stem; it is similar to a bulb
except it is solid and lacks fleshy leaves
(e.g. crocus).
cornea A thin, clear layer on the front of the
vertebrate eye.
corolla The petals of a flower, which occur in
the whorl to the inside of the calyx and the
outside of the stamens.
corona The ciliated crown of members of the
phylum Rotifera.
coronary artery The artery that supplies blood
to the heart muscle.
corpus In the shoot apex of flowering plants,
the inner layers of the apical dome of cells
that contribute to stem formation.
corpus callosum The major tract that connects
the two hemispheres of the cerebrum.
corpus luteum A structure that is responsible
for secreting hormones that stimulate the
development of the uterus needed for
sustaining the embryo in the event of a
pregnancy. If pregnancy does not occur, the
corpus luteum degenerates.
correlation A meaningful relationship between
two variables.
cortex (adj. cortical) In plants, the outer region
of a stem or root.
cortical alveoli The vesicles beneath the plasma
membrane of alveolates (e.g. dinoflagellates
and ciliates).
cosmid A vector sequence designed to generate
recombinant DNA molecules that carry
larger genomic fragments.
cotranslational sorting The sorting process in
which the synthesis of certain eukaryotic
proteins begins in the cytosol and then
halts temporarily until the ribosome has
become bound to the ER membrane. After
this occurs, translation resumes and the
polypeptide is synthesised into the ER lumen
or ER membrane.
co-transport The coupling of the movement
of one molecule down its electrochemical
gradient to drive the movement of another
molecule against its electrochemical gradient;
a form of active transport.
cotyledon A leaf-like structure of flowering
plant embryos involved in food storage and
digestion (in most dicots) and in nutrient
transfer to the endosperm (in grasses). In
a plant with epigeal germination, the first
leaves to emerge are the pair of cotyledons.
countercurrent exchange A change in a solute
or gas concentration, or temperature, by the
passage of a fluid in opposite directions along
two closely opposed vessels, through passive
exchange; for example, heat retention in
the limbs of aquatic mammals and water
vapour recovery from expired air in mammals
and birds; compare countercurrent
multiplication.
countercurrent multiplication An increase
in a solute or gas concentration by the
passage of a fluid in opposite directions
along two closely opposed vessels, involving
an active or passive mechanism for
increasing the concentration; for example,
renal osmotic concentration in the
mammalian kidney by solute transport and
gas secretion into the teleost swimbladder
by the Root effect; compare countercurrent
exchange.
courtship behaviour Interactions between
members of the opposite sex that take place
before mating.
covalent bond The bond formed between atoms
due to sharing of electrons in their outermost
orbitals.
covalent modification Modifying the chemical
bond formed by the sharing of electrons
between two atoms.
coxa (pl. coxae) The segment of the leg that
attaches to the body of an insect or other
arthropod.
cranial nerve A nerve in the peripheral nervous
system that is directly connected to the brain;
cranial nerves are located in the head and
transmit incoming and outgoing information
between the peripheral nervous system and
the brain.
Craniata A subphylum of chordates; lampreys,
hagfishes and vertebrates.
craniate A chordate that has a brain encased in
a skull and possesses a neural crest.
cranium A protective bony or cartilaginous
housing that encases the brain of a craniate.
crassulacean acid metabolism (CAM)
A variation of the C4 pathway of
photosynthesis, in which C4 and Calvin–
Benson cycle reactions occur in the same
cells but at different times; CAM plants fix
CO2 at night and convert it to carbohydrate
during the day.
crenation The process of cell shrinkage
that occurs if animal cells are placed in a
hypertonic medium—water exits the cells via
osmosis and equalises solute concentrations
on both sides of the membrane.
cretinism The impairment of growth and
development, particularly of the nervous
system, as a result of lack of thyroid
hormones in children.
Creutzfeldt-Jakob disease A human disease
caused by a prion (see prion); it may be
caused by mad cow disease.
crinoid An extinct group of stalked
echinoderms common in the Ordovician
period.
Crinoidea A class of echinoderms; feather stars
and sea lilies.
cristae The folds of the inner membrane of
mitochondria.
critical thermal minimum Point below which
temperatures are lethal for a particular
organism.
crop A storage organ that is a dilation of the
lower esophagus; found in most birds and
many invertebrates, including insects and
some worms.
cross-bridges The temporary links formed
between the heads of myosin molecules and
the adjacent actin molecules that are required
for muscle contraction or the maintenance of
muscle force.
cross-current exchange The perpendicular
arrangement of the air capillaries to the
parabronchi in bird lungs, together with
the unidirectional flow of air through
the parabronchi and movement of air by
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diffusion only in the air capillaries, that
establishes a pattern of airflow with respect to
the blood flowing in the lung.
cross-fertilisation Fertilisation after crosspollination; synonymous with out-breeding.
cross-pollination The pollination of a carpel by
pollen from a different individual.
crozier The curled branch of a dikaryotic,
heterokaryotic hypha where mitosis occurs
and acsi form in ascomycete fungi.
Crustacea A subphylum of arthropods that
includes crabs and crayfish.
crustacean cardiactive peptide A peptide
hormone first identified in crustaceans; in
insects it facilitates commencement of the
moulting process.
crustose A flat, ‘crusty’ growth form (e.g. of
a lichen).
cryoprotectants A substance used to protect
cells or tissues from damage during freezing.
cryptic A deceptive defence mechanism by
which an animal is well camouflaged and
blends into the background substrate, thus
reducing the risk of predation.
cryptic female choice A form of female choice
that occurs after insemination, and in which
females favour the sperm of particular males.
cryptomonad A flagellate protist, usually
photosynthetic, that has a small anterior
invagination into which two flagella are
inserted; pigments similar to chromists and
red algae.
Ctenophora A phylum of animals; comb jellies
(ctenophores).
Cubozoa A class of cinidarians; box jellyfish.
cupula A gelatinous structure within the lateral
line organ that helps an organism to detect
changes in water movement.
cutaneous exchange An exchange of gases (or
other substances) across the general body
surface.
cuticle An outer water-resistant layer secreted
by epidermis.
cutin A polyester polymer produced at the surfaces
of plants; helps to prevent attack by pathogens.
cyanelle A plastid of glaucophytes (k. Protista),
which are unique in having a peptidoglycan
wall as in bacteria.
cyanobacteria (sing. cyanobacterium)
Photosynthetic prokaryotes that have
chlorophyll a and produce oxygen as a
by-product of photosynthesis, similar to
the chloroplasts of plants.
Cycadophyta The phylum of cycads, the earliest
group of seed plants living today.
cyclic AMP (cAMP) Cyclic adenosine
monophosphate; a small effector molecule
that is produced from ATP via an enzyme
known as adenylyl cyclase.
cyclic photophosphorylation The process of
production of ATP in plant photosynthesis
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in which electrons are recycled back to
photosystem II and are not used in the
production of NADPH (as in non-cyclic
photophosphorylation).
cyclin A protein subunit required for the
activity of cyclin-dependent kinases; protein
levels vary during a cell cycle.
cyclin-dependent kinase An enzyme that
phosphorylates other proteins and that
requires a cyclin for activity; it is responsible
for progression through the cell cycle.
cyclosporin An immunosuppressant used
in medicine and derived from a fungus
(Tolypocladium inflatum).
cyclostome Primitive eel-like fish in which the
major regions of the vertebrate brain are
present; the brain, divided into forebrain,
midbrain and hindbrain, is continuous with
the spinal cord.
cyst A one- to few-celled structure that often
has a thick, protective wall and can remain
dormant through periods of unfavourable
climate or low food availability.
cysticercus Tapeworm larva consisting of
a bladder-like structure and inverted
scolex.
cytochalasin Anti-actin agents derived from
certain fungi; they act by specifically
disrupting actin microfilaments.
cytochrome b/f complex A protein complex
on the membranes of chloroplasts that
accepts electrons from photosystem II
and passes them to photosystem I during
photosynthesis.
cytochrome c oxidase The final protein
complex in the electron transport chain of
cellular respiration that reduces molecular
oxygen to water.
cytogenetics The field of genetics that
involves the microscopic examination of
chromosomes.
cytokine A glycoprotein messenger molecule
secreted by cells of the immune system to
control the activity of other cells.
cytokines A family of proteins that function
in both non-specific and specific immune
defences by providing a chemical
communication network that synchronises
the components of the immune response.
cytokinesis The division of the cytoplasm of a
cell following mitosis or meiosis.
cytokinins Plant hormones promoting cell
division.
cytoplasm The cytosol and organelles of
eukaryotic cells, excluding the nucleus.
cytoplasmic determinants In animal
development, gene products whose spatial
distribution may determine such things as
embryonic axes.
cytoplasmic streaming A phenomenon in which
the cytoplasm circulates throughout the cell
to distribute resources efficiently in large
cells, such as algal or plant cells.
cytoproct The site where undigested material is
excreted in a ciliate.
cytosine (C) A pyrimidine base found in DNA
and RNA.
cytoskeleton A network of microtubules,
microfilaments and intermediate filaments
in eukaryotic cells; it is involved in functions
such as the maintenance and change in cell
shape, movement of organelles within the
cytoplasm and cell movement.
cytosol An aqueous solution of molecules with
a gel-like consistency within the cytoplasm of
eukaryotic cells.
cytotoxic cell (TC cells, killer T cells)
T cells that, when stimulated by antigen and
lymphokines produced by TH cells, directly
lyse or kill target cells recognised by TC cells
on the basis of their particular antigen.
Darwinian fitness The relative likelihood that a
genotype will contribute to the gene pool of
the next generation as compared with other
genotypes.
dauciform roots Specialised lateral roots that
have swollen to the shape of a carrot (Daucus
carota, hence dauciform); they are intensely
hairy and show a lack of mycorrhizal
infection.
daughter strand The newly made strand in
DNA replication.
day-neutral plant A plant not affected by day
length for flower initiation.
dead space An anatomical space in the
conducting airways in which air remains
after expiration.
deamination The removal of an amino group;
part of the degradation of proteins into
compounds that can enter the Krebs cycle.
death The end of life; the cessation of biological
functionality.
decomposer An organism, such as some fungi
and bacteria, that consumes and breaks down
organic matter for energy, releasing inorganic
nutrients.
deep-sea trench A site where the sea floor
descends back into the mantle of the earth in
a process called subduction.
defecation The expulsion of faeces that occurs
through the final portion of the digestive
canal, the anus; contractions of the rectum
and relaxation of associated sphincter muscles
expel the faeces.
defence mechanisms Adaptations that decrease
an individual’s vulnerability to predators or
other natural enemies.
defensins A group of antimicrobial peptides
that act by inserting themselves into, and
disrupting, cell membranes of bacteria
and fungi, and the envelope of some
viruses.
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defensive mutualism A mutually beneficial
interaction often involving an animal
defending a plant or a herbivore in return
for food or shelter.
deficiency The term used to describe when a
segment of chromosomal material is missing.
degenerate In the genetic code, this means that
more that one codon can specify the same
amino acid.
degrader Organisms that feed on dead
organisms and organic wastes.
dehydration reaction A reaction that involves
the removal of a water molecule, and the
formation of a covalent bond between two
separate molecules.
delayed implantation A reproductive cycle
in which a fertilised egg reaches the
uterus but does not implant until later,
when environmental conditions are more
favourable for the newly produced young.
delayed ovulation A reproductive cycle in
which the ovarian cycle in females is halted
before ovulation, and sperm are stored and
nourished in the female’s uterus over the
winter. Upon arousal from hibernation in
the spring, the female ovulates one or more
eggs, which are fertilised by the stored
sperm.
deletion A mutation that removes one or more
nucleotides from the DNA.
demographic stochasticity The process that
describes the random nature of births and
deaths in populations.
demography The study of birth rates, death
rates, age distributions and the sizes of
populations.
denatured An enzyme that has lost its
characteristic three-dimensional shape, for
example, by heat.
dendrite The branching process of neurons that
are generally short and receive information
from other cells.
dendritic cell A cell of the immune system that
has long, branching processes and is able to
break down foreign molecules and present
them to lymphocytes.
denitrification The conversion of nitrate to
nitrite and nitrite to molecular nitrogen;
carried out by certain types of bacteria in
ecosystems.
denitrifying bacteria Bacteria, mainly
chemoheterotrophs, that carry out anaerobic
respiration and use nitrite and nitrate as
electron acceptors, releasing nitrogen gases
back into the atmosphere.
density In the context of populations, the
numbers of organisms in a given unit area.
density-dependent population dynamics In
ecology, when the per capita birth and death
rates of a population depend on the size of
the population.
density-independent population dynamics In
ecology, when the per capita birth and death
rates of a population are independent of the
size of the population; also termed densityvague.
density-vague population dynamics See densityindependent population dynamics.
deoxyribonucleoside triphosphate A deoxyribose
sugar covalently attached through the 1’ C to
either adenine, cytosine, guanine or thymine,
and through the 5′ C to three phosphate
groups; it is polymerised to extend DNA
strands during DNA replication.
deoxyribose A five-carbon sugar found in DNA.
deoxyribose nucleic acid (DNA) A nucleic acid
that is the hereditary material of an organism,
stored as a coded sequence of nitrogenous
bases; it comprises two complementary
double helical strands of nucleotides made
up of a pentose sugar, phosphate group and
nitrogenous base.
dephosphorylation The removal of a phosphate
group, usually by a phosphatase enzyme.
Many proteins can be activated or inactivated
by dephosphorylation.
depolarised The decreased voltage difference
across a membrane; it brings membrane
potential closer to threshold potential and
therefore is excitatory.
deposit feeding A type of feeding in aquatic
animals where bottom sediments and detritus
are ingested.
dermal bone Bone that develops in the skin
without going through a cartilaginous phase.
dermal tissue The covering on various parts of
a plant.
desmosome Provides structural support and
cell adhesion by cross-linking between
cytoskeletons of neighbouring cells.
detritivore Animals that eat organic litter or
detritus (a type of degrader).
detritus food chain A food chain whose food
base is primarily mixed debris (detritus).
Deuteromycota A phylum of fungi, but a
polyphyletic assemblage of forms, called
Fungi Imperfecti, many of which have lost
the ability to reproduce sexually.
deuterostome An animal in which, during
development, the anus forms at the site
of the blastopore and the mouth forms
as a secondary opening; echinoderms and
chordates.
development A series of events leading to
the formation of an adult organism from a
zygote.
developmental genetics A field of study aimed
at understanding how gene expression
controls the process of development.
diaphragm a large muscle that subdivides
the thoracic cavity from the abdomen in
mammals
diapsid A vertebrate skull with two well-defined
temporal openings.
diarrhoetic shellfish poisoning (DSP) An illness
caused by toxins from certain dinoflagellates,
which move through marine food chains; it
causes gastrointestinal symptoms.
diastole A phase of the cardiac cycle involving
muscle relaxation and the filling of a heart
chamber.
diatom A unicellular, golden-brown alga with
a characteristic cell wall made of two valves
of silica.
diazotroph A bacterium that fixes nitrogen.
dicotyledon One of the major types of
flowering plant (class Magnoliopsida)
that typically has two embryonic leaves in
the seed.
dideoxy chain-termination method The most
common method of DNA sequencing that
utilises dideoxynucleotides as a reagent.
dideoxynucleoside triphosphate
(ddNTP) Nucleoside triphosphates lacking
both the 2′ and 3′ OH groups so that they
can be incorporated into a growing DNA
chain, but prevent addition of any further
nucleotides, permitting rapid DNA sequence
determination.
differential gene expression The phenomenon
in which the expression of genes is altered.
Differential gene expression allows cells to
adapt to environmental conditions, change
during development and differentiate into
particular cell types.
differentially regulated A gene which is only
needed in certain cell types or environments,
often expressed in response to particular
environmental conditions (as opposed to
constitutively regulated genes).
differentiate The term used to describe the
actual alteration of a cell’s morphology and
physiology.
diffusion For dissolved substances, occurs
when a solute moves from a region of
high concentration to a region of lower
concentration.
diffusion coefficient A measure of how easily
a gas will diffuse through a liquid or tissue;
takes into account the solubility of the gas in
the liquid and the size of the gas molecule.
digestion The process of breaking down
nutrients in food into smaller molecules that
can be directly used by cells.
digestive enzyme Proteins that catalyse the
breakdown of complex foodstuffs into simple
units that can be absorbed from the gut.
digestive system In a vertebrate, this system
consists of the alimentary canal plus several
associated structures, not all of which
are found in all vertebrates: the tongue,
teeth, salivary glands, liver, gallbladder and
pancreas.
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dihybrid cross A cross involving organisms that
are heterozygous at two different loci.
dikaryon A fungal cell containing two haploid
nuclei, one from each parent; it is usually
formed after the sexual fusion of parent
hyphae.
dikaryotic The occurrence of two genetically
distinct nuclei in the cells of fungal hyphae
after mating has occurred.
dingo A native dog of Australia, Canis lupus
dingo.
dinoflagellate A unicellular protist (whirling
alga) that has two flagella: one transverse
flagellum encircling the cell, which provides
spinning motion, and one posteriorly
directed flagellum that steers the cell;
it includes zooxanthellae and toxic species
that cause red tides.
dinosaur An extinct archosaurian; a name
meaning ‘terrible lizard’; the dominant
vertebrates of the Mesozoic era including
large animals up to 30 m long; see
Archosauria.
dioecious An organism in which sperm and
eggs are produced by separate individuals.
dipleurula A type of free-swimming larva,
bilaterally symmetrical and with winding
bands of cilia; for example, in echinoderms.
diploblastic In an animal: having two cell
layers.
diploid An organism that carries two sets of
chromosomes (2n), one set derived from each
parent.
diploid cell A cell that carries two sets of
chromosomes.
diplomonad A unicellular, heterotrophic
flagellate (a protist); it has two nuclei each
associated with a pair of flagella; a gut
parasite such as Giardia.
diplontic A life cycle where only the gametes
are haploid (e.g. humans).
diprotodont A marsupial with only one pair of
incisors in the lower jaw.
direct development Development in which an
animal is born with the general form of the
adult.
direct repair Refers to a DNA repair system in
which an enzyme finds an incorrect structure
in the DNA and directly converts it back to
the correct structure.
directional selection A pattern of natural
selection that favours individuals at one
extreme of a phenotypic distribution that
have greater reproductive success in a
particular environment.
directionality In a DNA or RNA strand, refers
to the orientation of the sugar molecules
within that strand. Can be 5′ to 3′ or
3′ to 5′.
disaccharide Two monosaccharide molecules
joined by a glycosidic bond.
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discontinuous trait A trait with clearly defined
phenotypic variants.
discovery-based science The collection and
analysis of data without the need for
a preconceived hypothesis. Also called
discovery science.
disease An impairment of normal physiological
function affecting all or part of an organism,
especially a specific pathological change
caused by infection, producing characteristic
symptoms.
dispersal Migration.
dispersion A pattern of spacing in which
individuals in a population are clustered
together or spread out to varying degrees.
disruptive selection A pattern of natural
selection that favours the survival of two
or more different genotypes that produce
different phenotypes.
dissociation constant An equilibrium constant
between a ligand and a protein, such as a
receptor or an enzyme.
distal convoluted tubule The part of the
vertebrate nephron located after the proximal
convoluted tubule (and loop of Henle in
mammals and birds), responsible primarily
for reabsorption of solutes but not water; it is
sometimes called the diluting segment.
distribution Where a species occurs
geographically.
disturbance Environmental fluctuations that
affect biological communities (e.g. cyclones
and fire).
diurnal Active during the day.
divergent evolution Evolution that leads to
descendants becoming different in form from
their common ancestor.
diversity Variety or variation; for example,
species diversity is a measurement of the
populations present in a community.
diving response Limits O2 use during a dive
to extend the time the animal’s O2 store
will last; preferentially maintains circulation
to the heart and brain but greatly reduces
circulation to other organs of the body.
DNA (deoxyribonucleic acid) The genetic
material that provides a blueprint for the
organisation, development and function of
living things.
DNA bar coding A genetic approach to species
identification that suggests standardising all
species identification on the analysis of a few
genes, and dividing groups into species on the
basis of variation in the DNA sequences of
these loci. Has limitations and cannot always
distinguish between species in certain groups
of organisms.
DNA helicase An enzyme that uses ATP and
separates DNA strands.
DNA hybridisation The process whereby two
DNA strands from different sources form
a double-stranded DNA molecule through
complementary base pairing; DNA strands
require complementary sequences to be able
to hybridise with each other.
DNA library A collection of vectors each
containing a particular fragment of
chromosomal DNA or cDNA.
DNA ligase An enzyme that catalyses the
formation of a covalent bond between
nucleotides in adjacent DNA fragments
to complete the replication process in the
lagging strand.
DNA methylase An enzyme that attaches
methyl groups to bases in DNA.
DNA methylation A process in which methyl
groups are attached to bases in DNA.
This usually inhibits gene transcription
by preventing the binding of activator
proteins or by promoting the compaction
of chromatin.
DNA polymerase An enzyme responsible for
covalently linking nucleotides together to
form DNA strands.
DNA primase An enzyme that synthesises a
primer for DNA replication.
DNA repair systems One of several systems to
reverse DNA damage before a permanent
mutation can occur.
DNA replication The mechanism by which
DNA can be copied.
DNA sequencing A method to determine the
base sequence of DNA.
DNA topoisomerase An enzyme that affects the
level of DNA supercoiling.
DNA-binding domain A region of a
DNA-binding regulatory protein that
specifically interacts with a sequence of DNA
in the enhancer or silencer cis-acting control
region of particular genes.
DNA-binding regulatory protein A protein
that binds to a specific DNA sequence in a
regulatory region to regulate transcription
(such as activator and repressor proteins).
DNase An enzyme that digests DNA.
dolipore septum A septum with a complex pore
separating adjacent cells in the hyphae of
basidiomycete fungi.
domain 1. A defined region of a protein with
a distinct structure and function. 2. One of
the three major categories of life: Bacteria,
Archaea and Eukarya.
domatia The specialised structures in plants
that house animals such as ants.
dominance relationship The physical
domination of one individual over another; it
is usually established by aggressive behaviour
and once established the relationship remains
stable without subsequent high levels of
aggression.
dominant A term that describes the displayed
trait in a heterozygote.
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dominant oncogenes A mutated version of a
normal cellular gene that is overexpressed,
misexpressed or produces an altered product
to cause tumour formation.
dominant species A species that has a large
effect in a community because of its high
abundance or high biomass.
donor DNA DNA that is to be cleaved into
segments, ligated into a vector and transformed
to produce cloned DNA segments.
dormant A state of a mature seed that does
not result in germination as a result of
rehydration but is broken more by periods
of cold or exposure to appropriate levels of
red light.
dorsal Refers to the upper side of an animal.
dorsal lip of the blastopore A region of the
blastopore that, in amphibian embryos,
initiates gastrulation and induces formation
of a dorsal–ventral axis.
dorsal nerve cord A hollow nerve cord above
the notochord of chordates and that controls
body movement.
dorsiventral Leaves in which the upper and
lower mesophyll sections have a distinctly
different anatomical arrangement of the cells.
dorsoventral axis One of the three axes along
which the adult body pattern is organised;
the others are the anteroposterior axis and
the right–left axis.
double fertilisation In angiosperms, the
process in which two different fertilisation
events occur, producing both a zygote and a
nutritive endosperm tissue.
double helix Two strands of DNA hydrogenbonded with each other. In a DNA double
helix, two DNA strands are twisted together
to form a structure that resembles a spiral
staircase.
drag A backward component of force acting
on a moving body produced by a fluid
resistance.
drupe Fleshy fruit, such as a plum, containing
a single seed enclosed in a hard stony layer
(endocarp).
dryland salinity The rise of a water table
bringing salt to the soil surface in dryland
regions.
duodenum In vertebrates, the upper portion of
the small intestine.
duplex A double-stranded (base-paired) stretch
of DNA.
duplication The term used to describe when a
section of a chromosome occurs two or more
times.
dwarfism A condition in which a person’s
growth is stunted, usually by a deficiency in
growth hormone during early growth.
dynamic instability The oscillation of a single
microtubule between growing and shortening
phases; important in many cellular activities
including the sorting of chromosomes during
cell division.
early endosome The early stage, prior to
fusion with a vesicle, of the membranebound compartment that processes material
taken up by endocytosis before transfer to
lysosomes for degradation.
ecdysis-triggering hormone (ETH) Peptite
hormone that stimulates excitability of
neurons, producing the eclosion hormone,
and unlocks the sequence of behaviours that
precede moulting.
ecdysone An insect hormone, secreted by the
ecdysial glands, that stimulates moulting,
growth and differentiation of adult tissues.
Ecdysozoa A clade of moulting animals that
encompasses primarily the arthropods and
nematodes.
echidna A spiny, egg-laying mammal from the
order Monotremata.
Echinodermata The phylum of deuterostome
animals that includes feather stars, sea lilies,
sea urchins, sea stars, brittle stars and sea
cucumbers.
Echinoidea The class of echinoderms that
includes sea urchins and sand dollars.
eclosion hormone A peptide hormone that
promotes moulting in insects.
ecological isolation When populations occur
in the same geographic area but do not
interbreed because they occupy separate
habitats.
ecological niche The unique set of habitat
resources that a species requires, as well as
its influence on the environment and other
species.
ecological pyramid Of numbers, biomass or
energy: a diagram showing the change in
energy, biomass or numbers of organisms at
successive trophic levels in an ecosystem.
ecological species concept A species concept
that considers a species within its native
environment. Each species occupies its own
ecological niche.
ecologically sustainable development
(ESD) Using, conserving and enhancing
resources so that ecological processes in
ecosystems are maintained.
ecology The study of interactions among
organisms and between organisms and their
environments.
ecosystem The biotic community of organisms
in an area as well as the abiotic environment
affecting that community.
ecosystem services The economic benefits
and essential services provided by natural
areas, including carbon trading, clean water
and maintenance of water cycles, habitat
for native species, soil formation, nutrient
cycling, pollination of crops and more
appealing landscapes.
ecotone The boundary between two different
ecological communities.
ecotype A locally adapted variant of an organism;
differing genetically from other ecotypes.
ectoderm The outermost germ layer of animal
embryos, giving rise to the outer body
covering and neural tissue.
ectomycorrhizae Beneficial interactions between
temperate forest trees and soil fungi whose
hyphae coat tree-root surfaces and grow into
the spaces between root cells.
ectoparasite A parasite that lives on the surface
of its host.
ectotherm An animal whose body temperature
changes with the environmental temperature.
edge effect A special physical condition that
exists at the boundary or ‘edge’ of an area.
effective population size The number of
individuals that contribute genes to future
populations, often smaller than the actual
population size.
effector A molecule that directly influences
cellular responses.
effector cell A component of the immune
response; these cells carry out the attack
response.
egg Also egg cell. The female gamete.
egg cell Also egg. The female gamete.
eicosanoids A diverse group of hormones
produced by a wide variety of tissues,
released in response to local stimuli and
acting on neighbouring cells at extremely low
concentrations.
El Niño An extreme weather event of dry
conditions when there is major disruption
of normal air and oceanic circulation in the
Pacific region.
elaioplast Plastid in which oil is stored.
elastin The structural protein of the
extracellular matrix that is unusual because
it remains in an unfolded, random coil
configuration.
elater An elongated water-absorbing cell with
helically arranged wall thickenings; as elaters
dry, they move and flick spores from spore
capsules of liverworts and hornworts.
electrical gradient The difference in electrical
voltage between two regions, for example
across a membrane.
electrical potential The tendency to donate or
accept electrons.
electrical synapse A synapse that directly passes
electric current from the presynaptic to the
postsynaptic cell via gap junctions.
electrocardiogram (ECG or EKG) A record of
the electrical impulses generated during the
cardiac cycle.
electrochemical gradient The combined effect
of both an electrical and chemical gradient;
determines the direction that an ion will
move.
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electrogenic pump A pump that generates an
electrical gradient across a membrane.
electromagnetic spectrum All possible
wavelengths of electromagnetic radiation,
from relatively short wavelengths (gamma
rays) to much longer wavelengths (radio
waves).
electron A subatomic particle that is negatively
charged; it orbits the nucleus of an atom.
electron microscope A microscope that uses an
electron beam for illumination.
electron transport pathway (electron transport
system) A group of membrane-bound
enzymes and cofactors that operate
sequentially in a highly organised manner.
element A substance made up of only one type
of atom with the same atomic number.
elephantiasis A grotesque swelling of lymphatic
tissue caused by the tropical nematode
parasite Wuchereria bancrofti.
elicitor A compound, produced by bacterial and
fungal pathogens, that promotes virulence.
elimination The loss of undigested and
unabsorbed food from the digestive tract (not
to be confused with excretion).
elongation stage The second step in
transcription or translation where
RNA strands or polypeptides are made,
respectively.
embryo The early stages of development in
a multicellular organism during which
the organisation of the organism is largely
formed.
embryo sac The female gametophyte of
flowering plants typically containing seven
cells and eight haploid nuclei.
embryogenesis The process by which embryos
develop from single-celled zygotes by mitotic
divisions.
embryonic induction The process during
embryogenesis whereby signals emitted
from one group of cells induce a change in
developmental fate of another group of cells.
embryonic stem cell (ES cell) A cell in the early
mammalian embryo that can differentiate
into almost every cell type of the body.
emigration Movement out of a population.
empirical thought Thought that relies on
observation to form an idea or hypothesis,
rather than trying to understand life from a
non-physical or spiritual point of view.
emulsification A process during digestion that
disrupts the large lipid droplets into many
tiny droplets, thereby increasing their total
surface area and exposure to lipase action.
enantiomer A type of stereoisomer that exists as
a mirror image of another molecule.
encephalisation The evolutionary process
whereby neurons aggregate towards the
anterior end of the body to form cerebral
ganglia and brains.
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endangered species Those species that are in
danger of extinction throughout all or a
significant portion of their range.
endarch xylem A pattern of primary xylem
development in which new xylem is added
to the outside of the protoxylem in stems
of plants.
endemic species A species that is unique to a
specific geographic region; it is assumed to
have evolved there.
endergonic A reaction in which the change in
free energy is positive; energy is needed for
the reaction to proceed.
endocarp The innermost layer of a fruit.
endocrine gland The gland of internal secretion;
it usually secretes into the circulatory system.
endocrine hormone A hormone released into
circulating blood (or haemolymph) and that
exerts its effects on distant cells.
endocrine system All the endocrine glands and
other organs with hormone-secreting cells.
endocytic pathway A pathway to take
substances into the cell; the reverse of the
secretory pathway.
endocytosis The process of invagination of
the plasma membrane to form a vesicle
containing extracellular material that is
transported into the cell.
endoderm In animals, the innermost germ
layer; it lines the archenteron and gives rise
to the lining of the lungs and the epithelial
mucosa of the gut and associated glands.
endodermis In plants, the layer of cells
immediately outside the pericycle of a root; it
regulates the uptake of water and solutes into
the central vascular cylinder by means of the
Casparian strip.
endolysosome The third developmental stage,
between late endsome and lysosome, of
the membrane-bound compartment that
processes material taken up by endocytosis
before transfer to lysosomes for degradation.
endomembrane system The membranes inside
a cell, including the endoplasmic reticulum,
Golgi apparatus, lysosome and vesicles.
endometrium The lining of the uterus in
mammals; thickens in response to secretion
of estrogens and progesterone and is sloughed
off in menstruation.
endomycorrhizae Partnerships between plants
and fungi in which the fungal hyphae grow
into the spaces between root cell walls and
plasma membranes.
endoparasite A parasite that lives internally in
its host.
endophyte A mutualistic fungus that lives
compatibly within the tissues of various types
of plants.
endoplasmic reticulum (ER) A network of
membranous sacs (cisternae) extending
thoughout the cytoplasm of a eukaryotic cell;
it is usually flat and sheet-like but can be
linked by tubular cisternae.
endopodite One of the two parts of a biramous
appendage of a crustacean.
endopterygotes Insects that have complete
metamorphosis; the developing wings are not
visible in juvenile stages (larvae and pupae).
endorphin One of a group of small
neuropeptides produced by the vertebrate
brain; like morphine, endorphins modulate
pain perception.
endoskeleton A skeleton that is inside the body,
as in the vertebrates.
endosome A membrane-bound compartment
that processes material taken up by endocytosis
before transfer to lysosomes for degradation.
endosperm Triploid nutritive tissue in the seeds
of angiosperms.
endospore A bacterial cell (spore) that is highly
resistant; it is virtually metabolically inactive
and able to survive high temperatures and
many chemicals, even disinfectants.
endosporic gametophyte A plant gametophyte
that grows within the confines of microspore
and megaspore walls.
endosymbiosis, primary and secondary An
organism living inside another; the theory of
the origin of chloroplasts and mitochondria
of eukaryotes from a host cell engulfing
a bacterium (primary endosymbiosis)
or another eukaryote (secondary
endosymbiosis).
endothelium A single-celled inner layer of a
blood vessel, which forms a smooth lining
in contact with the blood.
endotherm An animal that generates its own
internal heat.
endothermic A term to describe the ability of
an organism to generate and retain body heat
through its metabolism.
energy The capacity to do work; it exists in a
number of forms, including chemical, heat,
sound, electricity and light.
energy expenditure The amount of energy
an animal uses in a given period of time to
power all of its metabolic requirements.
energy quantum The energy required to move
an electron from one orbital to another.
enhancer A response element in eukaryotes that
increases the rate of transcription.
enteric nervous system The division of the
autonomic nervous system that controls the
functions of visceral organs.
enteroreceptor A sensory receptor inside the body
that senses the internal state of an animal.
enthalpy (H) The total energy of a system. In
a chemical reaction, the energy contained
in the chemical bonds of the molecule,
symbolised as H; in a cellular reaction, the
free energy is equal to the enthalpy of the
reactant molecules in the reaction.
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entomology The study of insects.
entropy The measure of disorder (randomness)
in a system; energy becomes lost as heat
in every energy conversion, resulting in
increased entropy.
Environment Protection and Biodiversity
Conservation Act 1999 Australian
Commonwealth legislation that provides
for the protection of envionments of
national significance and promotes
ecologically sustainable development and
the conservation of biodiversity.
environmental science The application of
ecology to real-world problems.
environment-specific adaptation The processes
and structures by which organisms adjust
to short-term or long-term changes in their
environment.
enzymatic digestion All digestion relies
on enzymes, but enzymatic digestion
distinguishes digestion resulting from
enzymes produced by the animal’s own cells
from digestion by microbial fermentation.
enzyme The biological catalyst, usually a
protein, that increases the rate of a reaction.
enzyme-linked receptor A receptor found in all
living species that typically has two important
domains: an extracellular domain, which binds
a signalling molecule; and an intracellular
domain, which has a catalytic function.
eosinophil A blood cell often associated with
asthma but also important in defence against
parasites.
epacrid Sclerophyllous heath plants in the
family Ericaceae.
Ephyra (pl. ephyrae) The small medusa
produced asexually in the life cycle of a
jellyfish.
epiboly The spreading and/or overgrowth
of one cell layer by another layer during
gastrulation.
epicotyl The growing meristem or shoot
of a germinating seed that lies above the
cotyledons.
epidermis The outer cellular layer of a
multicellular organism.
epigeal germination Germination in which the
cotyledons emerge above the ground.
epigenetic regulation Inherited states of activity
of a gene, independent of the genotype.
epigynous A flower with an inferior ovary that
is buried within the receptacle below the
perianth.
epinephrine A hormone secreted by the adrenal
glands; also known as adrenaline.
epiphyte A plant that grows on another plant
for support, but is not parasitic.
epiphytotic A fungal disease epidemic on
plants.
epistasis The masking of the phenotype of one
gene by the phenotype of a different gene.
epithelial tissue A sheet of densely packed cells
that covers the body or individual organs
or lines the walls of various cavities inside
the body.
epithelial to mesenchymal transition The
process by which an epithelial cell that has
adhered to neighbouring cells detaches and
adopts a migratory morphology.
epithelium The tissue that forms a continuous
layer covering internal or external surfaces of
most multicellular organisms.
epitope The portion of an antigenic molecule
that is recognised by an antibody or T-cell
receptor; a large protein may have hundreds
of different epitopes.
epsp (excitatory post synaptic potential) An
excitatory change in the membrane of a
postsynaptic neuron caused by chemical or
electrical signals from a presynaptic cell.
Epstein-Barr virus The cause of glandular fever.
equilibrium 1. In a chemical reaction, occurs
when the rate of the forward reaction is
balanced by the rate of the reverse reaction.
2. In a population, the situation in which the
population size stays the same.
equilibrium constant The description of
the equilibrium position of a chemical
reaction; Keq = concentration of product(s)/
concentration of reactant(s).
equilibrium model of island biogeography A
model to explain the process of succession
on new islands, which states that the number
of species on an island tends toward an
equilibrium number that is determined by
the balance between immigration rates and
extinction rates.
equilibrium potential In membrane physiology,
the membrane potential at which the flow of
an ion is at equilibrium—no net movement
in either direction.
erosion The loss of soil by the action of wind
or water.
erythrocruorin The respiratory pigment of
annelid worms; a type of haemoglobin.
erythrocyte A mature anucleate red blood cell;
it contains haemoglobin; in adult mammals
it is anucleated.
erythropoiesis The manufacture of blood cells
in the bone marrow.
erythropoietin (EPO) A hormone naturally
made by the liver and kidneys in response to
any situation where additional blood cells are
required, such as when animals lose blood
following an injury; when abused, as in
blood doping, the concentration of red blood
cells reaches such high levels that the blood
becomes much more concentrated.
esophagus The tubular structure that forms a
pathway from the throat to the stomach.
ester Formed from combining an acid and
alcohol with the loss of water.
estradiol The major estrogen in many animals,
including humans.
estrogens Steroid hormones produced by the
female ovaries that affect most aspects of
reproduction.
ethology Scientific studies of animal behaviour.
ethylene A plant hormone that is particularly
important in co-ordinating plant
developmental and stress responses.
etioplast A plastid that develops in darkness
and that, on exposure to light, develops into
a chloroplast.
Eucalyptus The eucalypts or gum trees, plants
from the family Myrtaceae; characterised by
pungent oil glands, sclerophylly and flowers
with showy anthers and dehiscent bud cap.
euchromatin Lightly staining regions in an
interphase nucleus; it consists of dispersed
strands of chromatin that are sites of active
gene transcription.
eudicots The core group of dicotyledonous
flowering plants that are a monophyletic
group.
euglenoid A single-celled marine or freshwater
flagellate; the photosynthetic forms probably
acquired their plastid from engulfing green
alga.
Eukarya The super kingdom (or domain) that
includes all eukaryote organisms—plants,
fungi and animals.
eukaryote Protists, fungi, animals and plants;
cellular organism with membrane-bound
organelles such as a nucleus, mitochondria
and chloroplasts.
eukaryotic cell A cell with a nucleus and other
membrane-bound organelles; compare
prokaryotic cell.
Eumetazoa The subkingdom of animals, except
sponges.
euphotic zone A fairly narrow zone close to the
surface of an aquatic environment, where
light is sufficient to allow photosynthesis to
exceed respiration.
euphyll A leaf with branched veins.
euryhaline Able to tolerate a broad range of
salinities.
eurytherm A non-regulating ectotherm; its
body temperature varies widely and it spends
no time or energy on maintaining thermal
homeostasis.
eusociality A social grouping in which
individuals co-operate in raising young;
essentially sterile workers care for the
young of reproductively active individuals
(reproductive division of labour).
Eustachian tube A connection from the middle
ear to the pharynx that maintains the pressure
in the middle ear at atmospheric pressure.
eutelic The condition of having a set number
of cells, for example in nematodes, after
hatching, mitosis does not occur.
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Eutheria One of the three subclasses of
mammals; the so-called placental mammals.
eutrophication The process by which elevated
nutrient levels lead to an overgrowth of
algae or aquatic plants and the subsequent
depletion of water oxygen levels.
evaporation The loss of water molecules as a
vapour from a surface.
evapotranspiration The combined process of
transpiration of water from plants or direct
loss by evaporation from soil.
evapotranspiration rate The rate at which water
moves into the atmosphere through the
processes of evaporation from the soil and
transpiration of plants.
evolution The process of change and divergence
in populations and taxa.
evolutionarily stable strategy A strategy (or
behaviour) that, if adopted by most members
of a population of interacting individuals,
cannot be bettered (in terms of reproductive
success) by another strategy or behaviour.
evolutionary developmental biology
(evo-devo) A field of biology that compares
the development of different organisms in an
attempt to understand ancestral relationships
between organisms and the developmental
mechanisms that bring about evolutionary
change.
evolutionary species concept A species
is derived from a single lineage that is
distinct from other lineages and has its own
evolutionary tendencies and historical fate.
evolutionary tree A representation of the
branching evolutionary links between species
over time.
exarch xylem The pattern of xylem
development in roots in which the xylem
forms from the outside, filling the centre of
the root.
excitable cell The term used to describe neurons
and muscle cells, because they have the
capacity to generate electrical signals.
excitation-contraction coupling The sequence
of events by which an action potential in the
plasma membrane of a muscle fibre leads to
cross-bridge activity.
excretion The loss of ions, solutes, metabolic
waste products or water from body fluids;
not to be confused with elimination.
exergonic A reaction when the change in free
energy is negative; energy is released in the
reaction.
exine The outer patterned layer of pollen
grains.
exocarp The outermost layer of a fruit, for
example the skin of a peach.
exocrine gland A gland of external secretion.
exocytosis A process in which material inside
the cell is packaged into vesicles and excreted
into the extracellular medium.
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exodermis The layer of suberised cells at the
junction of epidermis and cortex of certain
roots; it regulates uptake of water, solutes and
ions into the cortex.
exon A portion of RNA that is found in the
mature RNA molecule after splicing is
finished.
exonuclease An enzyme that cuts DNA by
removing bases sequentially from the ends
of DNA strands by hydrolysing terminal
phosphodiester bonds.
exopodite One of the two parts of a biramous
appendage of a crustacean.
exopterygote An insect that has an incomplete
metamorphosis; developing wings are visible
in nymphs.
exoskeleton The external hard body covering of
some animals.
exotic species Species moved from a native
location to another location, usually by
humans.
expansin A protein that occurs in the plant cell
wall and fosters cell enlargement.
experiment A test of one or more hypotheses.
Hypotheses make contrasting predictions
that can be tested experimentally in control
and test experiments where a single variable
is altered.
experimental control In an experiment, a
variable to be tested is altered in one trial
and compared to a second trial (the control)
where the variable is left unaltered.
experimental sample The sample in an experiment
that is subjected to some type of variation that
does not occur for the control sample.
exponential growth J-shaped, rapid population
growth that occurs when the per capita
growth rate remains above zero.
expressivity The degree to which an allele is
expressed phenotypically in an individual.
exteroreceptor A sensory receptor that senses
the world outside the body of an animal.
extinction The end of the existence of a species
or group of species.
extracellular compartment That part of the
body fluid that is located outside of the cells;
it includes fluid located between cells, the
plasma component of the blood, intra-ocular
fluid and fluid in the gut.
extracellular fluid The fluid in an organism’s
body that is outside of the cells.
extracellular matrix (ECM) A network of
material that is secreted from cells and
forms a complex meshwork outside of cells.
The ECM provides strength, support and
organisation.
extranuclear inheritance In eukaryotes, the
transmission of genes that are located outside
the cell nucleus.
extremophile An organism that occurs primarily
in extreme habitats.
F1 progeny The progeny of a cross between two
pure-breeding individuals.
F2 progeny The progeny of a cross between F1
progeny.
Fabaceae One of the legume families;
characterised by ‘pea’ flowers, fruits
resembling pea pods, and symbiotic root
nodules containing nitrogen-fixing bacteria.
facilitated diffusion A method of passive
transport that involves the aid of a transport
protein.
facilitation A mechanism for succession in
which a species facilitates or makes the
environment more suitable for subsequent
species.
facultative aerobe A microorganism that can use
oxygen in aerobic respiration, obtain energy
via anaerobic fermentation, or use inorganic
chemical reactions to obtain energy.
facultative mutualism An interaction that is
beneficial but not essential to the survival
and reproduction of either species.
family A higher level taxon (grouping) used in
classification of organisms; above genus.
fast fibre A muscle fibre containing myosin
with high ATPase activity.
fatty acid A hydrocarbon chain with a carboxyl
group at one end; a component of many
lipids.
feather A modified scale characteristic of birds;
feathered wings allow flight.
fecundity The probability of giving birth.
female choice A mechanism of sexual selection,
in which females show a preference for males
according to secondary sexual characteristics.
feral animal A domestic animal or animal
introduced to control a pest or for recreation
that has gone wild.
fermentation The anaerobic production of
alcohol, lactic acid or similar molecules from
carbohydrates by the glycolytic pathway.
fern See Filicophyta.
fertilisation The specific interaction between
an egg and sperm leading to formation of a
zygote.
fibre Cellulose and pectins of plant cell walls,
which are not easily digested and which form
bulk in the diet; an elongated and tapered
sclerenchyma cell with a secondary wall, with
or without lignin.
fibrin Protein strands created from the plasma
protein fibrinogen that form the meshwork
over a wound to initiate a blood clot.
fibrinogen A soluble blood plasma protein that
is converted to insoluble fibrin strands by the
action of the enzyme thrombin; involved in
blood clotting.
fibroblast A flat, irregularly branching cell of
connective tissue that secretes structurally
strong proteins into the matrix between
the cells.
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fibronectin An adhesive protein of the
extracellular matrix occurring in interstitial
matrices; it has a high relative molecular
mass (about 460 kD) and two polypeptide
chains.
fibrous protein Proteins that have a high
proportion of hydrophobic R-groups, are
insoluble in water and have high tensile
strength, for example keratins and silks.
fibrous root system The root system of
monocots, which consists of multiple
adventitious roots that grow from the
stem base.
filament The elongate portion of a flower’s
stamen; contains vascular tissue that delivers
nutrients from parental sporophytes to
anthers.
Filicophyta The phylum of ferns; the most
diverse group of living vascular land plants
that reproduce by spores.
filter feeder Animals that obtain food by
straining suspended matter from a volume
of water, usually by passing the water over
specialised structures such as the baleen plates
of some whales.
filtrate In the process of filtration, the material
that passes through the filter and enters the
excretory organ either for further processing
or excretion.
filtration The passive removal of water and
small solutes from the blood.
first law of thermodynamics Energy can be
neither created nor destroyed; energy can
be transformed from one form to another
but the total energy of the universe remains
constant.
fitness The genetic contribution of an
individual to succeeding generations; relative
fitness refers to the fitness of an individual
relative to other individuals in a population.
flagella (sing. flagellum) Long, thin extensions
of cytoplasm that result in non-muscular
locomotion when a wave of bending travels
from the tip of a long flagellum to its base,
or base to tip, forcing water in the opposite
direction.
flagellate A protist that uses one or more
flagella to move in water or cause water
motions useful in feeding.
flame cell A cell that exists primarily to
maintain osmotic balance between an
organism’s body and surrounding fluids;
present in flatworms.
flatus Intestinal gas, which is a mixture of
nitrogen and carbon dioxide, with small
amounts of hydrogen, methane and hydrogen
sulfide.
flavonoid A type of phenolic secondary
metabolite that provides plants with
protection from UV damage or colours
organs such as flower petals.
fleshy-finned fishes Coelacanths and
lungfishes.
flexor A muscle that bends a limb at a
joint.
florigen A hypothetical plant hormone that
promotes flowering.
flower The sexual reproductive structure of
angiosperms; comprises four whorls or
layers—sepals, petals, stamens (male organs)
and carpels (female organs).
flow-through system A form of ventilation in
fish in which water moves unidirectionally
such that the gills are constantly in contact
with fresh, oxygenated water.
fluid mosaic Describes cell membranes; fluidity
referring to the lateral movement of lipid
molecules, and mosaic referring to the
irregular arrangement of proteins.
foetus The maturing embryo, after the eighth
week of gestation in humans.
foliose Leaf-like, describing some types of
lichen.
follicle A structure within the ovary where each
ovum undergoes growth and development
before it is ovulated.
follicle cell Also known as granulosa cells;
somatic cells that surround the maturing
oocyte and serve a protective and nutritive
function.
follicle-stimulating hormone (FSH) A
gonadotropin that stimulates follicle
development.
food chain A sequence of organisms from
producer to consumers along which energy
flows in an ecosystem; usually with three or
four trophic levels.
food web A number of interacting food chains
in an ecosystem.
foot In molluscs, a muscular structure usually
used for movement.
foraging theory The evaluation of the costs
and benefits associated with particular
foraging behaviours, in order to predict
which behaviours might be expected under
different circumstances.
foram A marine protist that produces a
calcaerous shell (a test); the shells house
endosymbiotic algae.
foregut fermentation The digestion of
foodstuffs (largely cellulose) by symbiotic
microorganisms located anterior to the true
stomach.
fossil Recognisable remains of past life on
earth.
fossil cast A fossil mould filled by material
such as silica or phosphate, forming a
three-dimensional cast of the remains of an
organism.
fossil fuel A fuel formed in the earth from
protist, plant or animal remains, such as coal,
petroleum and natural gas.
fossil mould An impression in a rock of the
remains of an organism that has been
dissolved away.
founder effect A small group of individuals
separates from a larger population and
establishes a colony in a new location;
genetic drift is common due to the small
population size.
fovea A small area on the retina directly behind
the lens that is responsible for the sharpness
with which we and other animals see in
daylight.
frameshift mutation A mutation that removes
or adds a number of nucleotides not equal to
multiples of three (i.e. mutations that disrupt
the normal sequence of codons).
free energy (G) The usable energy in a chemical
system.
free radical A molecule containing an atom
with a single, unpaired electron in its outer
shell. A free radical is unstable and interacts
with other molecules by ‘stealing’ electrons
from their atoms.
frond A fern leaf.
fructose A monsaccharide (sugar) that is a
structural isomer of glucose.
frugivore A fruit-eating animal.
fruit The mature ovary of a flowering plant;
contains seeds; may be dry or fleshy; simple
(from one carpel), aggregate (from a cluster
of separate carpels on one flower) or multiple
(from a cluster of many carpels from different
flowers).
frustule Valve or silica dish, two of which make
up the cell wall of a diatom; the two valves
of a cell are interconnected by silica hoops
(girdle bands) to create frustules.
fruticose A type of lichen with loosely attached
leaf-like structures.
fucoxanthin An accessory photosynthetic
pigment found in the chloroplasts of
chrysophytes (golden flagellates).
fuel molecule A molecule such as a carbohydrate
and fat with energy-rich chemical bonds that
are broken down to give energy.
functional genomics Genomic methods aimed
at studying the expression of a genome.
functional group A group of atoms with
chemical features that are functionally
important. Each functional group exhibits
the same properties in all molecules in which
it occurs.
fundamental niche That region of the
environment within which a species can
persist indefinitely; defined by all the abiotic
and biotic factors that impinge on the
survival and reproduction of the species.
Fungi Name of one of the four eukaryotic
kingdoms of the super kingdom Eukarya.
A fungus (pl. fungi) is a eukaryote with cell
walls, lacks chlorophyll and absorbs its food
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directly from a substrate (e.g. moulds, yeasts,
mushrooms).
fungi imperfecti The artificial phylum
Deuteromycota of the kingdom Fungi that
includes those organisms not known to
reproduce sexually and thus not able to be
classified in the natural phyla.
fusiform initial A type of meristematic
cell produced by the vascular cambium
differentiating into secondary xylem and
secondary phloem cells.
G1 phase The first gap phase; the period
between completion of mitosis and the onset
of DNA replication (S phase).
G2 phase The second gap phase; the period
between the completion of DNA replication
(S phase) and the onset of mitosis.
gait A characteristic pattern of locomotion.
Gal4p A sequence-specific DNA-binding
protein that binds to upstream activator
sequences to increase transcription of five
genes in yeast in the presence of galactose.
Gal80p A protein that represses transcrition
of the five genes expressed in yeast in the
presence of galactose by binding directly to
the Gal4p protein, masking its activation
domain and thus preventing it from acting as
an activator of transcription.
gallbladder A small sac underneath the liver
that is a storage site for bile and allows the
release of large amounts of bile to be precisely
timed to the consumption of fats.
game theory A mathematic technique used
to examine the relative costs and benefits
of different behaviours, when the outcome
of the behaviour depends upon what other
individuals are doing; it is commonly used in
economics.
gametangia A structure that produces gametes,
for example in fungi.
gametes The mature male and female germ cells
that fuse to form the zygote.
gametic life cycle In this type of life cycle, all
cells except the gametes are diploid, and
gametes are produced by meiosis.
gametocyte A cell type in the life cycle of
the malarial parasite Plasmodium; a stage
ingested by mosquitoes feeding on host
blood; it develops into sperm and eggs in the
mosquito’s gut.
gametophyte The haploid stage of a plant life
cycle that produces gametes.
ganglion (pl. ganglia) An organised group of
neurons.
gap gene A gene expressed in and required for
formation of a group of segments.
gap junction A type of junction between
animal cells that provides a passageway for
intercellular transport.
gas bladder An outgrowth of the alimentary
tract found in some fishes that originally
1160
evolved for buoyancy but can also be used for
gas exchange.
gas exchange The process of moving oxygen
and carbon dioxide in opposite directions
between cells and blood, and between blood
and the environment.
gastrin A non-neural endocrine hormone that is
released by the stomach lining and stimulates
the release of HCl.
gastropod A mollusc of the class Gastropoda,
including snails and nudibranchs.
gastrovascular cavity A body cavity with a
single opening to the outside; it functions
as both a digestive system and circulatory
system.
gastrula The embryonic stage during which
gastrulation is proceeding.
gastrulation The large-scale reorganisation
and movement of cells, following cleavage,
during which the three germ layers form and
bilateral symmetry appears.
gated channel A channel that can open to allow
the diffusion of solutes and close to prohibit
diffusion.
gating The release of a hormone only at a
particular time in the circadian cycle.
gel electrophoresis A technique used to
separate macromolecules by using an electric
field that causes them to pass through a gel
matrix.
gemma (pl. gemmae) An asexual reproductive
structure (propagules) of liverworts; formed
in gemmae cups on the surface of thalloid
forms; dispersed by splashes of rainwater.
gene amplification An increase in the copy
number of a gene.
gene cloning The process of making multiple
copies of a gene.
gene duplication The duplication of a segment of
DNA within a genome; it enables organisms
to produce different (but related) forms of a
protein, each adapted to function efficiently in
particular circumstances.
gene expression Gene function both at the level
of traits and at the molecular level.
gene interaction A situation in which a single
trait is controlled by two or more genes.
gene knockout An organism in which both
copies of a functional gene have been replaced
with non-functional copies. Experimentally,
this can occur via gene replacement.
gene pool All of the genes in a population.
gene silencing The ability of one gene to
silence the effect of another via small RNA
molecules called microRNAs.
general transcription factors (GTFs) Five
different proteins that play a role in initiating
transcription at the core promoter of
structural genes in eukaryotes.
generation of diversity The process of
differentiation of lymphocytes in the bone
marrow or thymus leading to the ability of
each lymphocyte to recognise a different
antigen.
generation time The term used to measure
growth rates in bacteria; the time taken for a
bacterial population of individually dividing
cells to double in number.
generative cell The male reproductive cell of
a pollen grain, formed by asymmetric cell
division of the microspore; lies entirely
within the vegetative cell; the progenitor of
the sperm cells.
genes The heredity units that contribute to the
characteristics or traits of an organism. At
the molecular level, a gene is composed of
organised sequences of DNA.
genetic drift The random change in a
population’s allele frequencies from one
generation to the next that is attributable
to chance. It occurs more quickly in small
populations.
genetic engineering The direct manipulation of
genes for practical purposes.
genetic marker A genetic-based phenotype
that enables a population to be characterised
genetically.
genetic modification Changing the genetic
make-up of an organism through the
introduction of new genetic material.
genetic mosaic An individual with somatic
regions that are genetically different from
each other.
genetic transfer The process by which genetic
material is transferred from one bacterial cell
to another.
genetically modified organisms (GMOs) See
transgenic.
genome The complete genetic composition of a
cell or a species.
genomic equivalence The cells of the same
organism having the same genes, although
they may be differentially expressed in
different tissues.
genomic library A collection of cloned genomic
DNA fragments; normally consist of a large
number of random genomic fragments,
each cloned into a vector and propagated
separately as individual plaques or colonies,
each carrying one of the random genomic
fragments.
genomics Techniques that are used in the
molecular analysis of the entire genome of a
species.
genotype The genetic composition of an
individual organism.
genotypic frequency The frequencies of
particular genotypes in a population
genus (pl. genera) A taxon (grouping) above the
level of species; designated by the first word
of the species binomial (which is a Latin
name).
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germ cell The line of cells that gives rise to
gametes.
germ layers The three basic tissue layers formed
during gastrulation—endoderm, mesoderm
and ectoderm.
germ line Cells that give rise to gametes such as
egg and sperm cells.
germ plasm Cytoplasmic determinants that
help define and specify the primordial germ
cells in the gastrula stage.
germination The beginning or resumption
of growth by a spore, seed, bud or other
structure.
gestation The state of being pregnant, when the
adult carries within its body the developing
embryo(s).
ghrelin A peptide hormone produced largely in
the stomach; its concentration in the blood
rises before meals, stimulating appetite, and
decreases after meals.
gibberellic acid GA3, the best studied of the
gibberellins, a class of plant hormones.
gibberellins A class of plant hormones
promoting stem elongation and seed
germination; composed of small molecules
each containing 19 or 20 carbon atoms;
synthesised in the shoot and germinating
seeds.
gigantism A condition in which a person’s
growth is excessive, usually caused by an
excess of growth hormone during early
growth.
gills Specialised filamentous organs in aquatic
animals that aid in obtaining oxygen and
eliminating carbon dioxide.
Ginkgophyta A phylum of seed plants that
includes only one living genus, Ginkgo.
girdle A site of the transverse flagellum that
encircles a dinoflagellate cell.
gizzard In the stomach of a bird, the muscular
structure with a rough inner lining capable of
grinding food into smaller fragments.
glacial period A cold, dry period associated
with low sea level (e.g. during the Pleistocene
period); glaciers formed in parts of the world.
glaucophyte A photosynthetic flagellate with
a plastid, termed a cyanelle, which has a
peptidoglycan wall, as do bacteria.
glial cell A supporting cell of the nervous
system; provides insulation and mechanical
and nutritional support for neurons, and
guides their development and repair.
gliding A form of unpowered flight; muscles
in gliding animals may be active to maintain
a particular posture (such as outstretched
wings), but they do not generate work.
global cycle The cycling of material through
ecosystems on a global (world) scale;
involving the atmosphere (e.g. carbon).
global warming An increase in the average
temperature of the earth’s atmosphere,
occurring because of an increase in
greenhouse gases.
globular protein One type of protein shape
created by the folding of the backbone so
that the hydrophobic side chains are buried
in the interior of the molecule away from
contact with water.
glochidium A type of veliger larva of some
bivalve molluscs that can grip onto and
parasitise fish.
Glomeromycota Fungal phylum characterised
by coenocytic hyphae and a distinct
branching form of mycorrhizae (mutualistic
relationships with plant roots) called
arbuscular mycorrhizae.
glomerulus A spherical tuft of capillaries
associated with the vertebrate nephron;
filtration of fluid from the glomerular
capillaries forms the primary filtrate.
glomus cell A cell containing a variety of
neurotransmitters that is found in the carotid
body and is in close proximity to capillaries
and nerve endings; it primarily responds to
changes in the partial pressure of oxygen,
hence assisting in the control of ventilation.
glucagon A hormone released by alpha
pancreatic islet cells; it causes increased blood
glucose levels due to breakdown of glycogen
and synthesis of glucose from amino acids.
glucocorticoid A steroid hormone produced
in the adrenal cortex that promotes
gluconeogenesis.
gluconeogenesis A synthesis of glucose from
non-carbohydrate sources such as amino acids.
glucose A carbohydrate (monosaccharide) that
is the initial sugar product of photosynthesis;
a building block of cellulose.
glycemic index (GI) A classification of
foods based on their ability to raise blood
glucose; the measurement is made on a
portion containing an identical amount of
carbohydrate and standardised against white
bread or pure glucose.
glycocalyx 1. An outer viscous covering
surrounding a bacterium. The glycocalyx,
which is secreted by the bacterium, traps
water and helps protect bacteria from drying
out. 2. A carbohydrate covering that is found
outside of animal cells.
glycogen A polysaccharide that serves as the
principal storage form of carbohydrate in
animals.
glycogenolysis A mechanism for maintaining
blood glucose levels; stored glycogen can be
broken back down into molecules of glucose
by hydrolysis.
glycolipid A lipid with a short chain of sugar
residues; in membranes, it occurs on the
non-cytosolic side.
glycolysis An anaerobic catabolism of glucose to
pyruvic acid, producing two molecules of ATP.
glycophyte A plant species that cannot grow on
salt-affected soil.
glycoprotein A chain of sugar molecules
attached to protein; it occurs on the noncytosolic side of plasma membranes.
glycosaminoglycans Large polysaccharide
molecules composed of repeating
dissaccharide units, usually linked to a
protein core; they are a major component of
the extracellular matrix and are responsible
for gel hydration.
glycosidic linkage The linkage between two
monsaccharides in their cyclic form to make
a disaccharide.
glycosylation The attachment of carbohydrate
to a protein or lipid, producing a
glycoprotein or glycolipid.
glyoxysome A microbody that contains the
five enzymes of the glyoxylate pathway; it is
involved in the breakdown of fats.
Gnathostomata Jawed craniates; see vertebrate.
Gnetophyta The phylum of seed plants that
includes only three living genera thought
to be related to the angiosperms; includes
Gnetum, Ephedra and Welwitschia.
Go A stage in which cells exit the cell cycle and
postpone making the decision to divide.
goitre An enlargement of the thyroid gland
caused by lack of iodine in the diet.
Golgi apparatus Stacks of four to ten
disc-shaped cisternae functioning in synthesis
of polysaccharides, glycosylation of proteins
and the sorting of molecules for storage or
secretion.
gonad The testis or ovary.
gonadotropins Hormones secreted by the
anterior pituitary gland that are the same
in both sexes; gonadotropins influence the
ability of the testes and ovaries to produce
the sex steroids.
gonangium (pl. gonangia) An individual in a
colony of hydrozoan polyps that functions in
reproduction.
Gondwana The past supercontinent uniting all
southern landmasses.
G-protein (guanosine triphosphate-binding
protein) An intermediate molecule in many
cellular signalling pathways; they alter the
activity of ion channels or intracellular
enzymes.
G-protein-linked receptor A receptor that when
activated forms a complex with, and acts
through, a G-protein to modify ion channel
or intracellular enzyme activity.
Gram stain A stain made from crystal violet
and iodine that is used to identify particular
types of bacteria; Gram-positive species
stain purple and Gram-negative stain
pink.
Gram-negative bacteria Stained pink in the
Gram stain test for peptidoglycan; cell walls
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contain less peptidoglycan than Grampositive bacteria, and are surrounded by a
second membrane.
Gram-positive bacteria Stained purple in the
Gram stain test for peptidoglycan; cell walls
display a thick layer of peptidoglycan exterior
to the membrane.
grana Stacks of thylakoids that form part of the
internal membrane system of chloroplasts.
granulocyte A leucocyte produced in bone
marrow that migrates to sites of infection,
where it engulfs and kills foreign organisms.
granum (pl. grana) A structure composed
of stacked tubules within the thylakoid
membrane of chloroplasts.
graptolite Extinct colonial animals with skeletons
of chiton, known from the Cambrian.
gravid Filled with eggs.
gravitropism The response of a root or shoot to
the pull of gravity.
grazing food chain A food chain directly
dependent on plants.
green alga Chlorophyte; a unicellular, colonial
and multicellular alga of fresh and marine
water; green due to presence of chlorophyll a
and b pigments, as with land plants.
green bacteria A type of photoheterotrophic
bacteria that uses sunlight for energy but
ready-made organic compounds as building
blocks; comprise a characteristic set of
photosynthetic pigments different from plants.
greenhouse effect The natural warming of the
earth by heat trapped due to the presence
of certain heat-absorbing gases in the
atmosphere.
groove In the DNA double helix, an
indentation where the atoms of the bases
make contact with the surrounding water.
gross primary production (GPP) The measure
of biomass production by photosynthetic
organisms; equivalent to the carbon fixed
during photosynthesis.
ground meristem A type of primary plant tissue
meristem that gives rise to ground tissue.
ground tissue Most of the body of a plant,
which has a variety of functions, including
photosynthesis, storage of carbohydrates and
support. Ground tissue can be subdivided
into three types: parenchyma, collenchyma
and sclerenchyma.
group selection The premise that natural
selection produces outcomes beneficial for
the whole group or species rather than for
individuals.
growth An irreversible increase in the dry mass
of an organism.
growth curve The growth pattern of a bacterial
population, with growth graphed against
time; usually sigmoidal, displaying a lag
phase, an exponential growth phase and a
stationary phase.
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growth hormone (GH) A hormone produced
in vertebrates by the anterior pituitary gland;
GH acts on the liver to produce insulin-like
growth factor-1 (IGF-1).
guanine (G) A purine base found in DNA and
RNA.
guanotely The pattern of nitrogenous waste
excretion where excess nitrogen (mainly from
digested protein) is excreted as the purine,
guanine.
guard cell A specialised kidney-shaped plant cell
that allows epidermal pores (stomata) to close
when conditions are too dry, and open under
moist conditions, allowing the entry of CO2,
needed for photosynthesis.
gullet An anterior depression where flagella
emerge in euglenoid cells; food may be
ingested through the gullet in some species.
guttation The exudation of liquid water from
leaves due to root pressure.
gymnosperm A plant that produces seeds that
are exposed rather than enclosed in fruits.
gynoecium The carpels of a flower.
habitat corridor The linking of adjacent
fragments of vegetation and species
populations.
habitat fragmentation The loss of natural
habitat that separates and isolates (fragments)
species populations.
habitat selection theory The idea that all
individuals of a species attempt to live in
places that maximise their chances of survival
and reproduction (their evolutionary fitness).
habitat The environment of an organism; the
place where it is usually found.
haemagglutinin The type of protein; for
example a surface protein in the lipid outer
membrane of the ‘flu’ virus.
haematocrit The volume fraction of whole
blood occupied by the blood cells.
haematopoietic stem cell Progenitor cells (stem
cells) in the bone marrow that give rise to all
blood cell types.
haemocoel Large spaces in the body that are
filled with blood.
haemocyanin Copper-containing respiratory
pigment found in arachnids.
haemoglobin An oxygen-binding protein found
within the cytosol of red blood cells.
haemolymph Blood and interstitial fluid
combined in one fluid compartment in many
invertebrates.
hair In mammals, the thread-like outgrowth
of the skin derived from scales; in plants, a
trichome.
Haldane effect The more deoxygenated that
haemoglobin becomes, the better it binds
with CO2, and with H+.
half-life 1. In the case of organic molecules in a
cell, refers to the time it takes for 50% of the
molecules to be broken down. 2. In the case
of radioisotopes, the time it takes for half the
molecules to decay and emit radiation.
halophile A bacterium (in super kingdom
Archaea) that survive in highly saline
environments (salt-loving).
halophyte A plant adapted to saline
environments.
haltere A mobile knobbed rod of flies; a
modified wing that assists with balance
during flight.
haplodiploidy A genetic system in which
females develop from fertilised eggs and are
diploid but males develop from unfertilised
eggs and are haploid.
haplodiplontic A life cycle where meiosis
produces haploid spores that give rise to
gametes subsequently following one or more
mitotic divisions (e.g. flowering plants).
haploid A cell possessing only one set of
chromosomes (n), as in egg or sperm.
haploid-dominant species Species in which the
haploid organism is the prevalent organism
in the life cycle. Examples include fungi and
some protists.
Haplorhini Tarsiers, monkeys, apes and
humans; primates that lack a rhinarium
(nose pad), which is replaced by a nose with
nostrils that are usally rounded in shape.
haptonema A thread-like extension between the
two flagella of a haptophyte (protist), which
can bend or coil, capturing prey.
haptophyte A flagellated, photosynthetic
protistan cell that is extremely abundant
in oceans.
Hardy–Weinberg equilibrium A mathematical
concept that relates allele and genotype
frequencies; the equation (p2 + 2pq + q2 = 1)
predicts an equilibrium if the population
size is very large, mating is random, the
populations do not migrate, no natural
selection occurs and no new mutations
are formed.
Hardy–Weinberg frequencies The frequencies
of genotypes expected for given allele
frequencies in an infinitely large
interbreeding population in which there is
random mating but no migration, mutation
or selection.
Hartig net A mycorrhizal fungus mycelium that
grows between root cortical cells facilitating
nutrient transfer.
haustoria The end of a fungal hypha that
absorbs nutrients from a host cell.
heartwood A mature secondary xylem, in which
the rays have degenerated, and the vessels and
tracheids are filled with secondary organic
compounds that make the wood hard and
durable.
heat A measure of the random motion of
molecules; the greater the heat, the greater the
motion. Heat is one form of kinetic energy.
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heat of fusion The amount of heat energy
that must be withdrawn or released from
a substance to cause it to change from the
liquid to the solid state.
heat of vaporisation The heat required to
vaporise 1 mole of any substance at its
boiling point under standard pressure.
heat shock protein A protein that helps to
protect other proteins from heat damage and
refold them to their functional state.
helicase Any of a group of enzymes that unwind
the two DNA strands in the double helix to
facilitate DNA replication.
heliotherm An ectotherm that gains the
majority of its heat from solar radiation.
helper cells (TH cells) Regulatory T cells that
produce and secrete lymphokines.
Hemichordata The phylum of chordates that
includes acorn worms and pterobranchs.
hemidesmosome The junction formed by crosslinking between the cytoskeleton of a cell and
the extracellular matrix; it provides structural
support by anchoring cells to the matrix.
hemiparasite A parasitic organism that generally
photosynthesises, but lacks a root system to
draw water and thus depends on its hosts for
that function.
hemizygous A gene present in only one copy in
a diploid organism, for example, a gene on
the X chromosomes of a male.
Hepatophyta The phylum of liverworts among
the non-vascular land plants; includes thalloid
and leafy forms.
herbaceous plant A plant that produces little or
no wood and is composed mostly of primary
vascular tissues.
herbicides Chemical or biological agents that
kill plants or inhibit their growth.
herbivore An animal that consumes algae or
plants as food.
hermaphrodite A type of animal in which
both male and female reproductive organs
occur within the same individual (also called
monoecious); a type of plant in which the
flowers contain both male and female organs.
hermaphroditism Having both male and female
sex organs in the one individual.
herpes simplex virus A virus that infect the lips
and genital regions of humans.
heterochromatin The densely staining regions
in an interphase nucleus; it consists of
aggregated strands of chromatin that are
inactive in gene transcription.
heterochrony An alteration in the timing
of developmental events due to a genetic
change; for example, a mutation that delays
flowering in plants.
heterocyst The specialised cell of a
cyanobacterium; the colourless, thick-walled
cell that is a site for nitrogen fixation and
may be involved in asexual reproduction.
heterodimer The structure that results when
two different proteins come together.
heterokaryon The multinucleate vegetative cell
of a fungus where the nuclei are genetically
different.
heterokont A protist characterised by one
smooth flagellum directed posteriorly and
one hairy flagellum directed anteriorly.
heterosporous In vascular plants: having spores
of two kinds, namely, microspores and
megaspores.
heterospory Having two types of spore;
heterosporous plants develop separate male
and female gametophytes.
heterothallic Sexually incompatible, for example
in fungi.
heterotherm An animal that has a body
temperature that varies with the
environment.
heterotroph An organism that consumes other
organisms as food; unable to synthesise
organic molecules from inorganic compounds.
heterotrophic The obtaining of chemical
energy by consuming other organisms
or by degrading their organic molecules;
heterotrophic organisms include animals
and fungi.
heterozygote advantage A phenomenon in
which a heterozygote has a higher Darwinian
fitness compared to the corresponding
homozygotes.
heterozygous An individual with two different
alleles of the same gene.
heterozygous Having two different alleles of the
same gene; the term is usually applied to one
or more specific loci, as in ‘heterozygous with
respect to the PFlocus’ (that is, the genotype
is W/w).
hexacanth A six-hooked larva of a tape worm
that hatches from the egg; it is able to
penetrate the gut wall of a host animal; it is
also called an onchosphere.
hexapods Arthropods of the class Hexapoda;
insects.
hibernation The state of torpor in an animal
over months.
high-energy bond In ATP, bonds that can be
used to release energy to drive reactions.
highly repetitive sequence A DNA sequence
found tens of thousands or even millions of
times throughout a genome.
hindgut fermentation The digestion of
foodstuffs (largely cellulose) by symbiotic
microorganisms located posterior to the
true stomach.
hirudin An anticoagulant protein produced
by leeches; it specifically inhibits the boodclotting factor thrombin.
histone A basic protein that interacts strongly
with DNA; it aids the formation of
nucleosomes in nuclei of eukaryotic cells.
holdfast The part of the thallus of a brown alga
that attches it to the substrate.
holoblastic cleavage A complete type of cell
cleavage in certain animals in which the
entire zygote is bisected into two equal-sized
blastomeres.
holomorph The anamorphic (asexual) and
teleomorphic (sexual) forms of a fungus;
entire forms of a fungus.
holoparasite A parasitic organism that lacks
chlorophyll and is totally dependent on the
host plant for its water and nutrients.
Holothuroidea A class of echinoderms; sea
cucumbers.
homeobox A 180-bp sequence within the
coding sequence of homeotic genes.
homeodomain A region of a protein that
functions in binding to the DNA.
homeostasis The maintenance of a relatively
constant internal environment.
homeotherm An animal that maintains its body
temperature within a narrow range.
homeotic A term that describes changes in
which one body part is replaced by another.
homeotic (Hox) gene A gene that is expressed
in a restricted region of the anterior–posterior
axis and determines the developmental fates
of segments along the anterior–posterior axis.
hominid Human-like; refers to fossil great
apes that are related and in the same family
(Hominidae) as modern humans.
Hominidae The family of great apes that
includes modern humans and fossil hominid
relatives.
hominoid Hominid-like; refers to great apes.
Homo A genus of hominids including modern
humans (Homo sapiens) and several extinct
species of primitive humans, including
H. habilis and H. erectus.
homodimer The structure that results when two
identical proteins come together.
homokaryon A multinucleate vegetative cell in
fungi where the nuclei are genetically all the
same.
homologous Structures that have the same basic
plan but not necessarily the same function;
used as evidence of phylogenetic relationship
between organisms.
homologous chromosomes (homologues) A
pair of similar chromosomes in a diploid
individual; one homologue is inherited from
each parent.
homologous genes Genes that share sequence
similarity; presumably due to descent from a
common ancestral gene.
homoplasy In cladistics, a shared character state
that has not been inherited from a common
ancestor exhibiting that state; may result from
convergent evolution or evolutionary reversal.
The wings of birds and of bats, which are
convergent structures, are examples.
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homosporous In some plants, production
of only one type of spore rather than
differentiated types. Compare with
heterosporous.
homospory A haplodiplontic life cycle where all
gametes are produced by one type of spore
(e.g. most ferns).
homothallic Sexually compatible, for example
in fungi.
homozygous Organisms in which the alleles are
the same, for example YY or yy.
horizontal gene transfer The transfer of genes
between different species.
hormone A chemical messenger secreted by
cells of an organism in response to specific
stimuli; hormones modify the activity of
cells as a result of interaction with specific
receptors.
hornwort See Anthocerophyta.
host The prey organism in a parasitic association.
host cell A cell that is infected by a virus,
fungus or bacterium.
host plant resistance The ability of plants to
prevent herbivory.
host range The number of species and cell types
that a virus or bacterium can infect.
hot spot The immobile point at the surface of
the earth’s mantle where a column of hot,
upwelling asthenosphere rises, which may
form islands.
human immunodeficiency lentivirus (HIV)
See AIDS.
humoral immunity Immunity that depends on
antibodies.
humus A collective term for the organic
constituents of soil.
Hyaluronan A water-soluble, linear
polysaccharide in connective tissue; it forms
clear gels.
hybrid zone An area where two populations can
interbreed.
hybridisation A situation in which two
individuals with different characteristics are
mated or crossed to each other; the offspring
are referred to as hybrids.
hydathode A pore-like structure on the tip or
margin of leaves of rainforest plants that
permits water to be extruded when required
by high root pressure.
hydranth An individual in a colony of
hydrozoan polyps that functions in feeding.
hydrated Surrounded by molecules of water.
hydration shell A ‘cloud’ of water molecules
surrounding a dissolved substance, such as
sucrose or Na+ and Cl– ions.
hydrofoil A structure that generates a lifting
force when moving through a fluid.
hydrogen bond Electrostatic attraction between
a hydrogen atom of a polar molecule and
an electronegative atom of another polar
molecule.
1164
hydrogen ion An ion that forms in the
dissociation of water, H+.
hydrolase An enzyme that involves water in the
breakdown of complex molecules to simple
molecules.
hydrolysis The process in which reactions utilise
water to break apart other molecules.
hydronium ion In solution, hydrogen ions form
mostly as H3O+.
hydrophilic ‘Water-loving’—generally, ions
and molecules that contain polar covalent
bonds will dissolve in water and are said to
be hydrophilic.
hydrophobic ‘Water-fearing’—molecules that
are not attracted to water molecules. Such
molecules are composed predominantly
of carbon and hydrogen and are relatively
insoluble in water. Because carbon–carbon
and carbon–hydrogen bonds are non-polar,
the atoms in such compounds are electrically
neutral.
hydroponics The cultivation of plants with,
instead of soil, only a solution of mineral
salts around the roots.
hydrostatic pressure The pressure exerted by a
liquid, such as blood; a misnomer because
the pressure can be exerted by liquids other
than water, and the liquids can be moving.
hydrostatic skeleton A fluid-filled body cavity,
surrounded by muscles, that gives support
and shape to the body of organisms.
hydroxyl ion A negatively charged ion that
forms with the dissociation of water (OH–).
Hydrozoa A class of cnidarians that has polyps as
the dominant life stage; for example Hydra.
hylids A Gondwanan family of frogs (family
Hylidae), including Australia’s largest frog
fauna.
hyperglycaemic Blood glucose raising.
hypermutation A process, primarily involving
numerous C to T point mutations, that is
crucial to enabling lymphocytes to produce a
diverse array of immunoglobulins capable of
recognising many different antigens.
hyperosmotic A solution that has a higher
osmotic concentration than another.
hyperpolarised Increased voltage difference
across a membrane; this moves membrane
potential further from the threshold potential
and therefore is inhibitory.
hypersaline Salt solutions that have a higher salt
concentration than sea water.
hypersensitive response (HR) A plant’s local
defensive response to pathogen attack.
hyperthermia An unregulated increase in body
temperature (as opposed to hypothermia, an
unregulated decline in body temperature).
hypertonic A solution with a higher
concentration of solutes than the cell. A cell
in a hypertonic solution tends to lose water
by osmosis.
hyperventilation An increase in the convective
requirement for ventilation; that is, an
increase in ventilation with respect to the rate
of oxygen consumption.
hypha (pl. hyphae) A microscopic tube of
cytoplasm bounded by a tough, waterproof
cell wall; forms fungal mycelia.
hypocotyl The part of the axis of a germinating
seed below the point of attachment of the
cotyledons.
hypodermic impregnation Sexual reproduction
in which a bundle of sperm is injected
directly across the body wall; for example
during copulation of some polycheate worms.
hypogeal germination Germination in which
the cotyledons stay below the ground.
hypoglycaemic Blood glucose lowering.
hypogynous A flower with a superior ovary,
that is, attached to or above the receptacle.
hypo-osmotic A solution that has a lower
osmotic concentration than another.
hypopharynx Tongue-like projection behind the
mouth of an insect.
hypophysis An alternative name for the
pituitary gland.
hypostome The projection of a cnidarian polyp
where the mouth is situated.
hypothalamo-hypophyseal portal system Portal
blood vessels carrying blood between the
capillaries of the median eminence and the
capillaries of the anterior pituitary gland.
hypothalamus In vertebrates, the region in the
midbrain surrounding the third ventricle; it
receives information regarding the wellbeing
of an animal and provides central neural and
hormonal control of many functions.
hypothermia An unregulated decline in body
temperature.
hypothesis A possible explanation for an
observed phenonemon.
hypothesis testing Also known as the scientific
method, a strategy for testing the validity of
a hypothesis.
hypotonic When the solute concentration
outside the cell is lower relative to the inside
of the cell.
hypoxia Lower than normal levels of oxygen.
I band A light band that lies between the
A bands of two adjacent sarcomeres.
icosahedron A basic type of structure of virus
particles where the protein shell has an
arrangement of 20 sides.
ideal free distribution The distribution
of animals between two resource sites;
individuals are free to choose between the
sites, and the distribution is ideal because
each individual goes to the place that
provides the highest returns.
ideal population A population that satisfies
the Hardy–Weinberg population
characteristics—an infinitely large
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interbreeding population in which there is
random mating but no migration, mutation
or selection.
imago The sexually developed adult stage of an
insect life cycle.
immigration Movement into a population.
immune response In vertebrates, a defensive
reaction of the body to invasion by a foreign
substance or organism. See antibody and
B cell.
immune system The cells and organs within
an animal’s body that contribute to immune
defenses.
immunisation See vaccination.
immunity The ability of an animal to ward off
internal threats, including the invasion of
potentially harmful microorganisms such as
bacteria, the presence of foreign molecules
such as the products of microorganisms,
and the presence of abnormal cells such as
cancer cells.
immunodeficiency A deficiency in the immune
response leading to repeated infections; it
may affect B cells, T cells, phagocytic cells,
antibody or complement.
immunogen Antigen that can stimulate an
immune response.
immunogenic Able to stimulate an immune
response.
immunoglobulin (Ig) A Y-shaped protein with
two heavy chains and two light chains that
provide immunity to foreign substances;
antibodies are a type of immunoglobulin.
immunological memory The retention of
stimulated B cells or T cells, called memory
cells; allowing a rapid immunological
response to a subsequent interaction with
the same antigen (secondary immune
response).
immunological tolerance The acquired ability
to ignore an immunogen; the induction
of immunological tolerance to grafts is an
immunologist’s dream.
immunopathology Tissue damage induced as a
side effect of the immune response.
imprinting (genetic) A ‘marking’ of a gene
during gametogenesis that alters the activity
of the gene in the offspring; it can produce
different phenotypes in organisms with the
same genotype; (behavioural) occurs when a
newborn animal recognises the first moving
object it sees (usually its mother) and follows
it for the next few weeks; occurs usually
during a limited period, the sensitive
period.
in vitro Literally, ‘in glass’. An approach to
studying a process in living cells that involves
isolating and purifying cellular components
outside the cell.
in vivo Meaning, ‘in life’. An approach to
studying a process in living cells.
inactivation gate A string of amino acids that
juts out from a channel protein into the
cytosol.
inbreeding The preferential mating of related
individuals.
inbreeding depression The phenomenon
whereby inbreeding produces homozygotes
that are less fit, thereby decreasing the
reproductive success of a population.
inclusive fitness A concept in evolutionary
biology that considers selection at the
level of the gene, thereby incorporating
the reproductive benefits an individual
might accrue as a result of its behavioural
interactions with relatives.
incomplete dominance The situation in which
a heterozygote exhibits a phenotype that
is intermediate between the homozygous
phenotypes of the two alleles.
incomplete metamorphosis In more primitive
insects, the gradual change in successive
instars during development; compare
complete metamorphosis.
indeterminate growth Growth in which plant
shoot apical meristems continuously produce
new stem tissues and leaves as long as
conditions remain favourable.
indicator species A species whose status
provides information on the overall health
of an ecosystem.
indirect development Development that
involves a larval stage followed by
metamorphosis into the adult stage.
individual selection The proposal that adaptive
traits generally are selected for because they
benefit the survival and reproduction of the
individual rather than the group.
individualistic model A view of the nature
of a community that considers it to be an
assemblage of species coexisting primarily
because of similarities in their physiological
requirements and tolerances.
indole-3-acetic acid (IAA) Plant hormone; the
main auxin occurring naturally in plants.
induced fit The change in shape of a protein
when a substrate is bound to it.
induced mutation A mutation brought about
by environmental agents that enter the cell
and then alter the structure of DNA.
inducer In transcription, a small effector molecule
that increases the rate of transcription.
inducible gene A gene that is only expressed
in response to particular environmental
conditions.
inducible operon In this type of operon, the
presence of a small effector molecule causes
transcription to occur.
induction The process during embryogenesis
whereby signals emitted from one group of
cells induce a change in developmental fate
of another group of cells.
indusium The protective leaf-like structure
covering sori of some ferns.
industrial nitrogen fixation The human activity
of producing nitrogen fertilisers.
infection Invasion of the body by pathogenic
microorganisms.
inflammation The accumulation of phagocytic
cells at the site of infection or other
irritation.
inflorescence A cluster of flowers.
influenza orthomyxovirus The virus that causes
the disease commonly called ‘flu’ (influenza);
it infects the lungs and respiratory tract of
humans.
infundibular stalk The structure that is
physically connected to a multilobed
endocrine gland sitting directly below the
hypothalamus, called the pituitary gland.
ingroup A monophyletic group in a cladogram
of interest.
initiation stage In transcription or translation,
the first step that initiates the process.
initiator tRNA A specific tRNA that recognises
the start codon AUG in mRNA and binds
to it.
innate defence mechanisms Ancient, nonspecific defences, including physical and
chemical barriers to invasion, and innate
immunity (cellular non-specific defences that
are part of the immune system).
innate immunity The inborn immune response
that pre-exists encounter with an infection;
compare acquired immunity.
inner cell mass A group of cells at one end
of the mammalian blastocyst that contain
embryonic stem cells and give rise to the
embryo proper.
inorganic chemistry The study of the nature of
atoms and molecules, with the exception
of those that contain rings or chains of
carbon.
inorganic phosphate A phosphate molecule
that is not a part of an organic molecule;
inorganic phosphate groups are added and
removed in the formation and breakdown of
ATP and in many other cellular reactions.
insertion A mutation that inserts one or more
nucleotides into the DNA.
instar The juvenile stage between moults during
the development of insects.
insulin A hormone released by alpha cells of the
islets of Langerhans; it binds to membrane
receptors and increases the membrane
permeability to glucose and amino acids;
it leads to increased storage of glucose and
increased fat production.
insulin-dependent diabetes mellitus (Type
I diabetes) Characterised by high blood
glucose concentrations and excretion of
blood in the urine; caused by the destruction
of the insulin-secreting cells in the pancreas.
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insulin-like growth factor-I (IGF-I) A hormone
that, in mammals, stimulates the elongation
of bones, especially during puberty, when
mammals become reproductively mature.
insulin-like growth factor-II (IGF-II) A factor
released from liver cells into the bloodstream
in response to circulating growth hormone;
mediates the growth-promoting effects of
growth hormone.
integral membrane protein A protein that
cannot be released from the membrane unless
it is dissolved with an organic solvent or
detergent—in other words, you would have
to disrupt the integrity of the membrane to
remove it.
integrated pest management The co-ordinated
use of various control techniques, and
integrating control with other activities, to
manage pest species.
integration The adding and subtracting of
electrical signals arriving on the membrane of
a neuron, resulting in an increase or decrease
in its resting potential.
integrin Transmembrane glycoproteins
that connect the actin filaments to the
extracellular matrix.
integument The layer of cells surrounding
megasporangium in seed plants; see also
ovule.
intercellular signalling The passing of chemical
or electrical signals between different cells of
a multicellular organism for communication
and to control the activities of cells.
interferon A group of proteins secreted by some
virus-infected cells that assist uninfected cells
to resist infection by that virus.
intergenerational equity The idea that future
generations should enjoy an environment
that is at least as healthy, diverse and
productive as the one we presently
experience.
interglacial periods Warm geological periods
with associated high sea level that alternate
with glacial periods; see glacial periods.
Intergovernmental Panel on Climate Change
(IPCC) An organisation established by the
World Meteorological Organisation and
the United Nations Environment Program
in 1988 to study the problem of potential
global climate change.
interleukin A cytokine that acts between
leukocytes (white blood cells).
intermediate disturbance hypothesis The
idea that a biological community has
maximum species diversity when there is
some (intermediate level) of disturbance such
that no long-lived species is able to become
dominant.
intermediate filament An element of the
cytoskeleton of eukaryotic cells that is
intermediate in size between microtubules
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and microfilaments; 8–10 nm in diameter;
it provides mechanical support for the cell.
interneuron A neuron that transmits
information from one neuron to another.
internode The portion of stem between
successive nodes (site of leaf attachment).
interphase The period during which a cell is
not undergoing mitosis; it comprises the
G1 phase, S phase and G2 phase.
interspecies hybrid The offspring resulting from
two species mating.
interspecific competition The term used to
describe competition between individuals of
different species.
interstitial Refers to the fluid between cells.
interstitial matrix A type of extracellular matrix
prominent in connective tissues.
intervertebral disc Part of the skeleton of a
tetrapod; the disc between adjacent vertebrae.
intracellular compartment That part of the
body fluid that is located inside the cells of
the body.
intracellular fluid The fluid inside cells.
intracellular receptor A signal receptor that
binds a ligand inside a cell, such as the
receptors for NO, steroid hormones,
vitamin D and thyroid hormones.
intranuclear spindle A spindle that forms
within an intact nuclear envelope during
nuclear division.
intraspecific competition The term used to
describe competition between individuals of
the same species.
introduced species A species brought by
humans from a native location to another
location.
intron A segment of RNA that is removed from
the primary transcript by splicing and is not
present in the mature RNA.
inulin A water-soluble fructose polymer; plant
storage polysaccharide; in dahlia tubers and
artichokes.
invasive The term used to describe introduced
species that spread on their own, often
outcompeting native species for space and
resources.
invasive cell A cell that can invade healthy
tissues.
inversion A change in the direction of the
genetic material along a single chromosome.
involucral bract Small, leaf-like structures that
surround a daisy head; see capitulum.
involution A process whereby groups of cells
roll under their neighbours and move
inwards, for example, during gastrulation
of frog embryos.
ion An atom that loses or gains electrons,
becoming positively or negatively charged.
ion channel The fastest enzyme known; it
permits passive transport through cellular
membranes; it is highly selective for
particular ions; it is opened by a change in
voltage across a membrane, or by binding
with specific signal molecules.
ion electrochemical gradient A dual gradient
for an ion that is composed of both an
electrical gradient and chemical gradient for
that ion.
ionic bond The bond formed when ions of
opposite charge are attracted to each other.
ionic compound The chemical compound
composed of anions and cations, bonded
together by opposite charges.
ionoconform To have the same ionic
concentrations in the body fluids as occur in
the external medium.
ionoregulate To maintain body fluid ionic
concentrations different from those of the
external medium.
ionotropic receptor A ligand-gated ion channel
that opens in response to binding of a
neurotransmitter molecule.
ipsp (inhibitory post synaptic potential)
An inhibitory change in the membrane of a
postsynaptic neuron caused by chemical or
electrical signals from a presynaptic cell.
irrigation salinity Increased soil salinity due to
a rise in the water table as a result of applying
irrigation water to the land.
islets of Langerhans Spherical clusters of
endocrine cells that are scattered in great
numbers throughout the endocrine pancreas.
isobilateral Leaves in which palisade mesophyll
extends from the upper surface to the lower
surface.
isomer A variant of a molecule, with the same
chemical composition but atoms arranged
differently.
isomerase An enzyme that causes structural or
geometric changes in a substrate molecule.
iso-osmotic Two solutions with the same
osmotic concentration.
isotonic Fluids that have the same potential
to move water by osmosis across a
semipermeable membrane, depending on the
solute concentrations and the permeability of
the membrane.
isotope An element that exists in multiple
forms that differ in the number of neutrons
they contain.
jet propulsion The ejection of a stream of
particles (fluid or gas) in one direction that
produces a force that propels the object
expelling those particles in the opposite
direction.
joint The part of an animal’s body that (usually)
permits movement between adjacent,
rigid skeletal elements; the form of these
articulations is quite varied, with some (e.g.
hip joints) allowing large ranges of movement
and others (e.g. teeth within tooth sockets)
providing virtually none.
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jumping A rapid acceleration of a body
resulting in loss of contact with the substrate;
in land animals, the force is usually generated
by the muscle-powered extension of limbs or
parts of the body.
juvenile hormone An insect hormone
secreted by the corpora allata; it stimulates
development of nymphal structures.
juxtaglomerular apparatus An association of
structures (macula densa cells of the distal
convoluted tubule, and juxtaglomerular
cells of the afferent and efferent glomerular
arterioles) in the mammalian kidney
nephron; it controls glomerular blood flow
by secreting renin.
K strategist Species that are long-term
competitors; they are likely to encounter
competition when population size reaches
the carrying capacity (K) of the environment.
K/T event An ancient cataclysm at the
Cretaceous/Tertiary boundary that involved
at least one large meteorite or comet that
crashed into the earth near the present-day
Yucatan Peninsula in Mexico, about
65 million years ago.
karyogamy Nuclear fusion, for example in
fungi.
kelp Giant marine brown alga, for example
Australian bull kelp Durvillea potatorum.
keratins Fibrous proteins; long sequences
with α–helical secondary structure; in hair,
feathers and skin.
ketones Small compounds generated from
carbohydrates, fatty acids or amino acids.
Ketones are made in the liver and released
into the blood to provide an important
energy source for many tissues, including the
brain, during prolonged fasting.
keystone species A species within a community
that has a role out of proportion to its
abundance.
kidney The excretory organ of vertebrates; the
nephron is the functional excretory unit of
the kidney.
kin selection Selection that takes into
account relatives as well as direct
descendants.
kinesis The movement of the bones of the jaw
or skull that facilitates feeding.
kinetic energy Energy of movement, as in
running water.
kinetochore The two protein discs of a
centromere, into which microtubules are
inserted.
kinetochore fibre A microtubule fibre that is
attached to the kinetochore.
kinetoplast A large mass of DNA composed
of thousands of catenated DNA minicircles (linked as in a chain) present in the
mitochondrion of certain flagellate parasites
(e.g. trypanosomes).
Koch’s postulates A series of steps used to
determine whether a particular organism
causes a specific disease.
Krebs cycle See citric acid cycle.
kuru A human disease caused by a prion; see
prion.
Kyoto Protocol A legally binding international
agreement adopted in 1997 that commits
developed countries to reduce their collective
emissions of greenhouse gases by at least
5% compared to 1990 levels by the period
2008–12.
La Niña An extreme weather event of wet
conditions when there is major disruption
of normal air and oceanic circulation in the
Pacific region; it alternates with El Niño.
labellum A petal modified as the lip of an
orchid flower.
labium An insect mouthpart derived from the
fused pair of second maxillae.
labrum An anterior exoskeleton plate of an
insect head that covers the mouthparts.
lac operon An operon in the genome of E. coli
that contains the genes for the enzymes that
allow it to metabolise lactose.
lac repressor A repressor protein that regulates
the lac operon.
lactose A disaccharide found in milk of
mammals; it is formed by the linking of the
hydroxyl at carbon 1 of b-D-galactose to the
hydroxyl at carbon 4 of D-glucose.
lagging strand A DNA strand that grows in an
overall 3′ to 5′ direction but is synthesised
discontinuously in short fragments (5′ to 3′)
that are later joined by DNA ligase.
lamellae Plate-like structures in the internal
gills of fish that branch from structures called
filaments.
lamellibranch Bivalve mollusc, referring to the
gills that are expanded as sheets or lamellae.
laminarin The storage product of brown algae;
a β-(1→3)-glucan.
laminin An adhesive protein of the extracellular
matrix, occurring in basement lamina.
land degradation The decline in the quality of
the land due to soil acidification, erosion,
salinity, etc.
larva A free-living organism that is
morphologically very different from the
embryo and adult.
larynx The area beyond the throat where the
vocal cords lie.
late endosome The second developmental
stage, following fusion with a vesicle and
acidification of the lumen, of the membranebound compartment that processes material
taken up by endocytosis before transfer to
lysosomes for degradation.
latent The term used to describe when a
prophage or provirus remains inactive for a
long time.
lateral meristems In vascular plants, the
meristems that give rise to secondary tissue;
the vascular cambium and cork cambium.
laterite A weathered land surface with ironstone
gravel occurring at the surface or subsurface
overlying a layer of bleached white clay.
Laurasia The past supercontinent uniting all
northern land masses.
law of independent assortment The alleles of
different genes assort independently of each
other during gamete formation.
law of segregation The phenomenon that
the two copies of a gene segregate from
each other during gamete formation and
transmission from parent to offspring.
leaching The dissolution and removal of
inorganic ions as water percolates through
materials such as soil.
leading strand A DNA strand that is
synthesised continuously in a 5′ to 3′
direction.
leaf A lateral outgrowth from the stem; it is
usually green and the main photosynthesis
organ of a plant.
leaf abscission The dropping off of leaves
normally following the formation of an
abscission layer.
leaf primordia Small bumps that occur at the
sides of a shoot apical meristem and develop
into young leaves.
leaflet 1. Half of a phospholipid bilayer.
2. A portion of a compound leaf.
learning Any change in an individual’s
behaviour that is due to its experience.
leech An annelid worm of the class Hirudinea
that has a sucker at each end of its body
and feeds on the blood or tissues of other
animals.
legume A member of the pea (bean) family.
Also their distinctive fruits, dry pods that
develop from one carpel and open down
both sides when seeds are mature.
leishmania A type of kinetoplast, a unicellular
flagellate parasite that causes the disease
Leishmaniasis, which is an infection of white
blood cells; a disease transmitted by sand flies
occurring in, for example, Africa and South
America.
Leishmaniasis A disease transmitted by sand
files that causes an infection of white blood
cells; see leishmania.
lentic Referring to a freshwater habitat
characterised by standing water.
lenticel A special site for gas exchange in the
periderm (outer layer of bark) of woody
plants; the raised area of cells with extensive
intercellular spaces.
Lepidosaura The evolutionary lineage of
amniotes that includes the New Zealand
tuatara, snakes, lizards, skinks and geckos;
with teeth fused to the edges of the jaws.
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leptin A protein produced by adipocytes (white
fat cells) by the Ob gene and which is
involved in signalling the level of fat stores to
the brain and other body organs.
leucocyte A class of nucleated white blood cells
that protect the body against invasion and
collect cellular debris.
leucoplast In plant cells, a colourless plastid in
which starch grains are stored; usually found
in cells not exposed to light.
levans A water-soluble fructose polymer; plant
storage polysaccharide; it is in stems of
grasses.
lichen The mutualistic relationship between a
fungus and a green alga or cyanobacterium.
life cycle The sequence of events that
characterise the steps of development of the
individuals of a given species.
life table A table consisting of probabilities of
survival and expected number of offspring
varying with age of individuals in a
population.
lift The component of force acting at right
angles to the direction of motion of a
hydrofoil.
ligand A molecule or part of a molecule that
binds selectively to one or more specific sites
on a larger molecule; a signal molecule that is
capable of interacting with a receptor.
ligand-gated channel A membrane ion channel
that opens or closes in response to the
binding of specific signalling molecules.
ligase An enzyme that joins (ligates) two
molecules to form C–C, C–S, C–O or C–N
bonds coupled with the hydrolysis of ATP or
similar triphosphate.
ligation The covalent joining of DNA
fragments by DNA ligase.
lignin The main component of secondary
walls and wood of plants; composed of
phenylpropanoid units, which provide a rigid
matrix for cellulose fibres.
lignophytes Modern and fossil seed plants, and
seedless ancestors that produced wood.
lignotuber Masses of dormant buds that form
a swelling at the base of a stem, for example
in eucalypts; the organ allows vegetative
reproduction.
ligulate floret A type of flower (ray floret)
on the outside of a daisy head that has
elongated, fused petals strongly developed to
one side.
limiting resource An environmental
requirement of an organism that is in limited
supply (e.g. food, nest site, etc.).
linkage The transmission of alleles of different
genes located on the same chromosome at
a frequency greater than that expected for
independent assortment.
lipase The major digestive fat-digesting enzyme
from the pancreas.
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lipid A molecule composed predominantly
of hydrogen and carbon atoms. Lipids are
non-polar and therefore very insoluble in
water. They include fats, phospholipids and
steroids.
lipid bilayer A double layer of lipid molecules
that forms the basic structure of cell
membranes.
lipopolysaccharides Lipids having covalently
bound carbohydrates. Major components of
the thin outer envelope that encloses the cell
walls of Gram-negative bacteria.
liposome A vesicle surrounded by a lipid bilayer.
Lissamphibia Modern amphibians, for example
salamanders, frogs, toads and caecilians.
lithophyte A plant that grows among rocks.
liver A large organ that secretes bile, stores
glycogen, detoxifies certain poisons and plays
an important part in the metabolism of food.
liverwort See Hepatophyta.
lobe-finned fish Fish in which the fins are part
of the body; they are supported by skeletal
extensions of the pectoral and pelvic areas
that are moved by muscles residing in
the fins.
local cycle The cycling of material through
ecosystems at a local scale; involving the soil
(e.g. phosphorus).
locus The position of a gene on a chromosome;
a locus may be occupied by any one of the
alleles of a gene.
long-day plant A plant that must be exposed to
light periods longer than some critical length
for flowering to take place.
loop of Henle The part of the mammalian
nephron (and some avian nephrons) that lies
between the proximal and distal convoluted
tubules; it enables the osmoconcentration
of urine.
lophophore The feeding structure of ciliated
tentacles containing extensions of the coelom
in lophophorates.
lorica The external vase-shaped shell of some
chrysophyte protists.
lotic Referring to a freshwater habitat
characterised by running water.
lumen The internal space of an organelle.
lungfish A fleshy-finned or lobe-finned fish, of
the group named Sarcopterygii that includes
land vertebrates, the tetrapods; the swim
bladder functions as a lung; one species
endemic to Australia (Neoceratodus forsteri).
lungs Invaginated gas-exchange surfaces
connected to air outside the body via narrow
airways; in mammals, the initial portion of
the airway before it branches dichotomously
into the bronchi leading to the lungs is
known as the trachea.
luteovirus Viruses that cause various plant
diseases, for example of cereals; transmitted
by aphids.
lyase Enzymes that act on substrates with C–C,
C–O and C–N bonds; for example their
activities involve reactions that eliminate
water.
lycophyll A relatively small leaf having a single
unbranched vein; the type of leaf produced
by lycophytes.
Lycophyta The phylum of clubmosses and
quillworts, non-seed vascular plants.
lymph A transparent fluid formed by filtration
of liquid from capillaries into the interstitial
space; it is collected by primary lymphatic
vessels and returned to the blood; it contains
white blood cells that attack invading
organisms; it transports proteins and fats into
the blood.
lymph node A nodule occurring at intervals
along a lymphatic vessel where T cells and
B cells encounter antigen to induce an
immune response.
lymphatic capillary Fine vessels, originating in
the tissues, that collect interstitial fluid and
channel it to larger lymphatic vessels.
lymphatic system A system of vessels along
with a group of organs and tissues where
most leukocytes reside. The lymphatic vessels
collect excess interstitial fluid and return it to
the blood.
lymphatic vessel A vessel that drains the tissues
of lymphocytes, ultimately leading them back
into the blood.
lymphocyte (lymphoid cell) Small round cells
that are the predominant cells in immune
organs; it is responsible for the immune
response; the two principal classes are T cells
and B cells.
lymphokine Cytokines secreted by helper
T cells that control the development and
function of other T and B cells, as well as of
accessory cells such as macrophages.
lysis The disintegration of a cell.
lysosome The membrane-bound component of
a cell containing hydrolytic enzymes involved
in the breakdown and recycling of many
types of molecules.
lytic cycle The growth cycle of a bacteriophage
in which the production and release of new
viruses lyses the host cell.
M line A narrow, dark band in the centre of the
H zone that corresponds to proteins that link
together the central regions of adjacent thick
filaments.
M phase A phase of nuclear division (mitosis)
in the cell cycle.
macroevolution The evolution at or above the
species level.
macronucleus The larger of the two types
of nuclei in ciliates; it develops from the
micronucleus and contains multiple copies of
genes on short pieces of DNA; it divides by
simply pinching approximately in half.
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macronutrient A nutrient element required
by organisms in large amounts for healthy
growth.
macrophage A type of phagocyte capable of
engulfing viruses and bacteria; strategically
located where it will encounter invaders.
madreporite A porous disc; opening of the
water vascular system of echinoderms.
magnetic reversal The reversal of the earth’s
magnetic field, from normal (present day) to
reversed polarity (north becomes south and
south becomes north).
Magnoliophyta The phylum of flowering plants
(angiosperms).
major groove A groove that spirals around
the DNA double helix. The major groove
provides a location where a protein can bind
to a particular sequence of bases and affect
the expression of a gene.
major histocompatibility complex (MHC)
molecule It presents (shows) antigen to
T cells, without which T cells will not
respond to antigen; it is fundamental to
identification of ‘self ’ and in graft rejection.
malaria A disease caused by the apicomplexan
parasite, Plasmodium (k. Protista); it involves
both a vertebrate host and a blood-sucking
insect.
male–male competition A mechanism of
sexual selection in which males compete over
mating opportunities with females.
Malpighian tubule A blind-ended excretory
tubule of arthropods; urine is formed by
active K+ secretion into the tubule and
passive solute and water influx; the urine is
emptied into the hindgut.
Mammalia A class of vertebrates that is
characterised by the presence of hair and
suckling young.
mandible (pl. mandibles) The first pair of
appendages in mandibulate arthropods, used
for feeding; for example in insects, modified
for grinding and chewing (jaw-like) or
piercing and sucking (stylets).
Mandibulata A major lineage of arthropods that
includes myriapods, crustaceans and insects;
see mandible.
manganese cluster A site where the oxidation of
water occurs in photosystem II.
mantle (of molluscs) The dorsal fold of the
body wall with a cavity beneath it; it secretes
the shell.
mantle (of mycorrhiza) The thick sheath of a
mycorrhizal fungal mycelium surrounding a
root, replacing epidermis and root hairs.
mantle cavity The space below the mantle of a
mollusc that opens to the outside; it houses
respiratory gills and receives the excretory,
reproductive and alimentary openings.
manubrium The projection in a medusa
(e.g. a jellyfish) on which the mouth is borne.
mapping The process of determining the
relative locations of genes or other DNA
segments along a chromosome.
mark-release-recapture method A method used
to estimate the abundance of a mobile animal
population.
marsupial A metatherian mammal, for
example a possum, koala or kangaroo; it
characteristically has a marsupium (pouch)
for carriage and concealment of developing
young.
marsupium See marsupial.
mass extinction The extinction of many species
at one time in the fossil record marking the
end of a geological period, for example the
Permian.
mass number The combined number of
protons and neutrons in a nucleus.
mass-flow transport The movement of a fluid
(usually water) in bulk as a cohering mass
(the movement is caused by a difference of
hydrostatic pressure).
mass-specific food intake The amount of food
eaten by an animal divided by its mass.
mass-specific metabolic rate The metabolic rate
of the animal divided by its mass.
mast cell The strongly staining basophilic cell in
the tissues, associated with allergies but also
important in guiding inflammation.
mastax The circular muscular pharynx in the
mouth of rotifers.
masting The synchronous production of many
progeny by all individuals in a population
to satiate predators and thereby allow some
progeny to survive.
mate-guarding behaviour Male behaviour that
ensures the female he mates with does not
mate with another male.
maternal-effect genes Genes that are
transcribed from the genome of the mother
and whose products are deposited in the
egg but act after formation of the zygote
(i.e. after fertilisation).
mating system The number of partners each sex
may have during its lifetime or during the
mating season.
maturation of behaviour Behaviour that
changes as a result of an individual’s age,
independent of the individual’s experience.
maturation The process of development to a
final developmental stage or adulthood.
maturational effect Those factors that influence
the maturation of behaviour.
maxilla (pl. maxillae) A second pair of
appendages; mouthparts in insects.
maxillary gland A blind excretory sac at the
base of each of the second pair of maxillae in
crustaceans.
maximal metabolic rate The greatest possible
metabolic rate (energy expenditure) an
organism can achieve, usually associated with
increased activity; generally five to 10 times
the basal metabolic rate.
maximum sustainable yield (MSY) The
harvesting of a population at a rate that
allows the population size to be maintained
indefinitely.
mechanically gated channel A membrane ion
channel that opens or closes in response
to physical disturbance, usually mediated
by small changes in pressure across the
membrane.
mechanoreceptor A type of receptor that
detects stimuli such as mechanical pressure
or stretch.
mechanosensitive channel A channel that is
sensitive to changes in membrane tension.
median eminence The part of the
neurosecretory portion of the pituitary gland
lying beneath the hypothalamus.
mediator A large protein complex that plays
a role in initiating transcription at the core
promoter of structural genes in eukaryotes.
medusa The free-floating (pelagic), bell-shaped
form of a cnidarian, with its mouth pointing
downwards; jellyfish.
megafauna Various large animals that became
extinct in Australia.
megagametophyte Haploid stage (gametophyte)
that develops from the germination of a
megaspore, the female spore, in the life cycle
of some ferns, some clubmosses and all seed
plants; in seed plants contained within an
ovule.
megakaryocyte A large cell in the bone marrow
that produces blood platelets.
megaphyll Large leaves, typical of ferns
compared with earlier vascular plants such as
lycophytes; see microphylls.
megasporangium The female sporangium of
plants in which megaspores develop.
megaspore Haploid spore of some ferns, some
clubmosses and seed plants that germinates
into a female gametophyte, which bears
archgonia with egg cells.
megasporocyte In a heterosporous plant,
a diploid cell that undergoes meiosis
to produce a megaspore; also known as
megaspore mother cell.
meiosis Two specialised nuclear divisions
(meisois I and II) that result in a halving of
the number of chromosomes of a diploid
(2n) germ cell to produce haploid (n)
daughter cells (gametes).
melatonin A hormone secreted by the pineal
gland in the absence of light, influencing
diurnal and seasonal rhythms in a variety of
vertebrates.
membrane potential The difference between
the electric charges inside and outside the
cell; also called a potential difference
(or voltage).
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membrane receptor A signal receptor present
as an integral protein in the cell membrane,
such as GPCRs, chemically gated ion
channels in neurons and RTKs.
membrane transporter A transmembrane protein
that accelerates the movement of particular
solute molecules across the membrane.
memnospore The sexual spore of a fungus; it
allows survival during harsh conditions.
memory cell A long-lasting B or T cell formed
after antigen stimulation; it is the basis of
immunological memory.
Mendelian inheritance The inheritance
patterns of genes that segregate and assort
independently.
meristem In plants, an organised tissue that
includes actively dividing cells and a reservoir
of stem cells.
meroblastic cleavage An incomplete type of
cell cleavage, in which only the region of the
egg containing cytoplasm at the animal pole
undergoes cell division.
merozoite The stage in the life cycle of the
malarial parasite Plasmodium that invades the
red blood cells of the vertebrate.
mesocarp The middle layer of a fruit, for
example the thick edible layer in peaches.
mesoderm One of the three germ layers, lying
between the ectoderm and the endoderm;
it gives rise to many of the internal organs,
connective tissues and internal epithelia.
mesoglea The intermediate gelatinous layer,
between the ectoderm and endoderm, in a
cnidarian (e.g. a jellyfish).
mesophile An organism that grows best
between 10°C and 30°C.
mesophyl The middle layer of a sponge
consisting of a gelatinous protein matrix
containing amoeboid cells, collagen fibres
and skeletal elements.
mesophyll The internal tissue of a plant leaf
whose cells carry out photosynthesis.
mesophyte A plant that lives where water is in
adequate supply; compare xerophyte.
Mesozoic The geologic era from 245 to
65 million years BP.
messenger A molecule that transmits messages
from many types of activated sensors to
effector molecules.
messenger RNA (mRNA) RNA that contains
the information to specify a polypeptide with
a particular amino acid sequence; its job is
to carry information from the DNA to the
ribosome.
metabolic depression A reduction in metabolic
rate; it may be regulated or unregulated.
metabolic diversity The great range of
metabolic systems displayed by bacteria, such
as anaerobic capabilities, nitrogen fixing and
denitrifying, and fermentations, which are
commercially significant.
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metabolic enzyme A protein that accelerates
chemical reactions within the cell.
metabolic pathway The chemical reactions in
cells that occur in a step-by-step sequence.
metabolic rate The total energy expenditure of
an organism per unit of time.
metabolism All the chemical processes occurring
within the cells of a living organism.
metaboly The sinuous gyration or crawling
motion of euglenoids.
metacentric A chromosome in which the
centromere is near the middle.
metacercaria The stage of parasitic flukes that
encysts on vegetation; it is ingested by a
vertebrate host.
metameric segmentation A body plan in
animals in which there is linear repetition
of functional units, which are added at the
posterior end (e.g. in annelids).
metamorphosis The process where there is
a marked change in body form during
development; see incomplete and complete
metamorphosis.
metanephridium (pl. metanephridia) The
tubular excretory organ with a ciliated,
funnel-like opening in the coelomic cavity
that draws coelomic fluid into the tubule.
metaphase A phase of mitosis in which
chromosomes are arranged equatorially on
the mitotic spindle.
metaphase plate The equatorial region of a
metaphase cell in which the chromosomes
are assembled.
metarteriole The smaller arterioles, surrounded
by discontinuous muscle.
metastasis The process by which cancer cells
move from their point of origin to other
locations in the body; also, a population of
cancer cells in a secondary location, the result
of movement from the primary tumour.
metastatic cell A cancer cell that can migrate to
other parts of the body.
Metatheria Marsupials; one of the subclasses of
mammals.
metaxylem Xylem tissue that forms outside the
protoxylem in stems of plants; it has larger
and thicker walled cells than protoxylem,
with reticulate secondary thickening.
Metazoa An alternative name for kingdom
Animalia (all multicellular).
methanogen (methanogenic bacteria) A type of
bacterium that uses hydrogen gas and carbon
dioxide to generate energy and make sugars.
methanotroph An aerobic bacterium that
consumes methane.
methylation The addition of a methyl group to
bases (primarily cytosine) in DNA. Cytosine
methylation is correlated with DNA that is
not expressed.
methyl-directed mismatch repair A DNA repair
system that involves the participation of
several proteins that detect the mismatch and
specifically remove a segment from the newly
made strand.
MHC restriction The phenomenon whereby
T cells are restricted to recognising antigen
presented by the MHC they have been
programmed by in the thymus.
micelle A spherical structure formed when
phospholipids are added to water; it forms
because fatty acid tails of phospholipids are
hydrophobic.
microbial fermentation A form of digestion in
which symbiotic microorganisms in the gut
produce enzymes that degrade foodstuffs into
absorbable units.
microbody An organelle in eukaryotic cells
involved in removal of compounds generated
within a cell; it is spherical in shape and
surrounded by a single membrane; it often
contains crystalline inclusions.
microcirculation Circulation and exchange in
arterioles, capillaries and venules.
microclimate Local variations of the climate
within a given area.
microevolution A gradual change in gene pools,
often over long evolutionary time periods.
microfibrils Regular aggregates of cellose
molecules associated through hydrogen bonds
and van der Waals forces; embedded in a
matrix containing polysaccharides, pectins
and proteins in cell walls.
microfilament A fine fibre composed of
filamentous chains of actin molecules; part of
the cytoskeleton.
microgametophyte Haploid stage
(gametophyte) that develops from the
germination of a microspore, the male
spore, in the life cycle of some ferns, some
clubmosses and all seed plants; in seed plants,
microgametophytes develop as pollen.
micronucleus The smaller of the two types
of nuclei in ciliates; diploid with normal
chromosomes and divides mitotically.
micronutrient A nutrient that is required in
only small amounts.
microphyll A small simple leaf, typical of
lycophytes.
micropyle The small opening in an ovule
that allows the pollen tube to enter during
fertilisation.
microRNAs (miRNAs) Small RNA molecules,
typically 22 nucleotides in length, that silence
the expression of specific mRNAs, either by
inhibiting translation or by promoting the
degradation of mRNAs.
microsatellite A segment of DNA that consists
of short repeated sequences; such segments
are often polymorphic and can be used in
DNA fingerprinting.
microsporangium The male sporangium of
plants in which microspores develop.
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microspore A haploid spore of some ferns, some
clubmosses and seed plants that germinates
into a male gametophyte, which produces
sperm.
microsporidia Unicellular water-borne obligate
parasites of protists and animals; they can
cause illness, are not known to reproduce
sexually and lack typical mitochondria.
microsporocyte A diploid male spore-forming
cell of plants that undergoes meiosis to form
microspores and pollen grains.
microtubule A type of hollow protein filament
composed of tubulin proteins that is part of
the cytoskeleton and is important for cell
organisation, shape and movement.
microvilli Small projections in the surface
membranes of epithelial cells in the small
intestine.
midbody A dense body of actin, microtubules
and other components that forms between
daughter cells in the late stages of animal
cytokinesis after contractile ring constriction.
middle lamella A thin layer between the
primary wall of a plant cell; it is rich in
pectins.
migration Long-range seasonal movement
among animals in order to feed or breed.
milieu intérieur The internal environment
provided by the extracellular fluids (in a
multicellular animal).
mimicry The resemblance of an organism (the
mimic) to another organism (the model).
Mimosaceae The acacia (wattle) family.
mineral element An inorganic element required
by organisms.
mineralisation The general process by
which phosphorus, nitrogen, CO2, and
other minerals are released from organic
compounds.
mineralocorticoid A steroid hormone secreted
by the adrenal cortex; it is involved in Na+
and K+ balance.
minimum viable population size (MVP) The
population size above which a species needs
to be maintained to ensure its long-term
survival.
minor groove A smaller groove that spirals
around the DNA double helix.
miracidium The free-swimming ciliated stage of
parasitic flukes.
missense mutation A mutation that changes
the nucleotide sequence of a codon so that it
encodes a different amino acid.
mitochondrial genome The chromosome found
in mitochondria.
mitochondrial matrix A compartment inside
the inner membrane of a mitochondrion.
mitochondrion (pl. mitochondria) A DNAcontaining organelle of eukaryotic cells;
it is surrounded by a highly permeable
double membrane; it contains circular DNA
molecules, RNA and small ribosomes; the
site of cellular respiration.
mitogen Stimulates mitosis of cells and thus has
important functions in regulating growth and
differentiation.
mitosis The process of a chromosome
condensation, nuclear envelope breakdown,
sister chromatid separation and formation of
two daughter nuclei in eukaryotes.
mitotic spindle An elaborate cytoskeletal
structure that causes chromosomes to move
towards the equator at metaphase of mitosis,
and the chromatids to separate and move
towards the poles at anaphase.
mitral valve The valve between the left atrium
and left ventricle of the mammalian heart.
mixotroph An organism that is able to use
autotrophy as well as phagotrophy or
osmotrophy to obtain organic nutrients.
molality Solute concentration of a solution,
measured as the number of moles of a solute
per kilogram of water.
molarity Solute concentration of a solution,
measured as the number of moles of a solute
per litre of solution.
mole The amount of a substance that contains
the same number of atoms or molecules as
there are in 12 grams of carbon-12
(6.022 ⫻ 1023, Avogadro’s number).
molecular biology A field of study spawned
largely by genetic technology that looks at the
structure and function of the molecules of life.
molecular clock An estimate of evolutionary
time based on the rate of change of DNA or
protein sequences over time.
molecular cloning A commonly used technique
for increasing the number of copies of a piece
of DNA.
molecular homologies Similarities that indicate
that living species evolved from a common
ancestor or interrelated group of common
ancestors.
molecular mass The sum of the atomic masses
of all the atoms in a molecule.
molecule The stable association of two or more
atoms due to the sharing of electrons in their
outer orbitals.
Mollusca The phylum of snails, octopuses and
their relatives; diverse group of unsegmented,
soft-bodied animals that usually produce a
hard shell.
Monera A kingdom name used in the past for
all prokaryotes (bacteria).
monocarpic senescence Plants that flower, fruit
and die.
monocistronic mRNAs A transcription unit
that contains a single open reading frame.
monocistronic Of mRNAs: a transcription unit
that contains a single open reading frame.
Monocotyledon One of the major types
of flowering plant (class Liliopsida) that
typically has one embryonic leaf in the seed;
generally with flower parts in threes and
parallel leaf veins.
monocyte A type of phagocyte that circulates
in the blood for only a few days, after
which it takes up permanent residence as a
macrophage in different organs.
monoecious A type of animal in which both
male and female reproductive organs occur
within the same individual (also called
hermaphrodite); a type of plant in which
male and female organs are present in
different flowers on the same plant.
monogamy A mating system in which one male
mates with one female, and most individuals
have mates.
monogenean A class of platyhelminths;
ectoparasitic flukes.
monohybrid cross A cross involving organisms
that are heterozygous at a single locus.
monomorphic gene A gene that exists
predominantly as a single allele in a
population.
monophyletic group A group of species, a
taxon, consisting of the most recent common
ancestor and all of its descendants.
monoplacophorans Limpet-like molluscs, first
known from 520 million years ago, with
15 living species found so far; typically live
at great ocean depths.
monosaccharide Sweet-tasting simple sugar
that cannot be broken down into smaller
sugar molecules; five-carbon pentoses and
six-carbon hexoses are most common.
monosynaptic reflex A reflex movement that
occurs when a sensory receptor excites a
muscle across one synapse; only two cells
and one synapse are involved so the response
is very rapid.
Monotremata The group of egg-laying
mammals; living examples are the echidna
and platypus.
morphogen A molecule that induces pattern
formation during development.
morphogenesis The generation of a new shape
during development.
morphogenetic field A group of embryonic
cells that ultimately produce a specific body
structure.
morphology The form and structure of an
organism.
mosaic An individual in which some cells
throughout the body show genetic
differences. For example, in female mammals,
about half of the somatic cells will express
one X-linked allele, while the rest of the
somatic cells will express the other allele.
mosaic development Development in which the
fate of blastomeres is predetermined and not
influenced by interactions with surrounding
cells.
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moss See Bryophyta.
motor neuron A neuron that innervates a
muscle and causes it to contract.
motor unit A neuroanatomical unit comprising
a single motor neurone and the muscle fibres
it acts upon; all the fibres act in unison, with
an ‘all or nothing response’ when stimulated;
smaller motor units permit finer control of
force production and movement.
moulting The process of discarding the
exoskeleton and secreting a new one;
characteristic of insects and other arthropods.
M-phase-promoting factor A specific cyclin
and cyclin-dependent protein complex whose
activity drives a cell from the G2 phase into
the M phase.
mucigel A gooey plant substance that lubricates
roots, aiding in their passage through the soil;
helps in water and mineral absorption, prevents
root drying and provides an environment
hospitable to beneficial microbes.
multicellular Consisting of more than one cell,
with cells attached to each other; cells able
to communicate with each other by chemical
signalling, and some cells able to specialise.
multiple alleles Refers the occurrence of a
gene that exists as three or more alleles in a
population.
multiple fruits Fruits that develop from a
cluster of many carpels from different flowers
(e.g. pineapple).
multipotent A term used to describe a stem cell
that can differentiate into several cell types,
but far fewer than pluripotent cells.
Murray Valley encephalitis A disease caused
by an arbovirus (arthropod-borne) related to
yellow fever; it is found in the river systems
and tropical north of Australia; it causes fever
but sometimes death.
muscle A grouping of muscle tissue bound
together by a succession of connective tissue
layers.
muscle tissue Clusters of cells that are
specialised to contract, generating the
mechanical forces that produce body
movement, exert pressure on a fluid-filled
cavity, or decrease the diameter of a tube.
mutagen An environmental factor (e.g.
radiation or chemicals) that induces
mutations in DNA.
mutation A change in the nucleotide sequence
of a DNA molecule resulting most
commonly from errors in DNA synthesis or
from chemical- or radiation-induced damage
to DNA.
mutualism The symbiotic interaction between
two species where both benefit from the
association (e.g. lichen).
mutualist Living in partnership with another
organism, where both organisms benefit from
the relationship.
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Mycelia Sterilia Fungi that produce no spores.
mycelium The body of a fungus, generally
growing as filamentous hyphae.
Mycophycota The phylum of lichens.
mycorrhiza (pl. mycorrhizae) A mutualistic
association between certain types of fungi
and the roots of plants; it enhances nutrient
uptake by the plant; it includes arbuscular,
orchid, epacrid and ectomycorrhizae.
mycorrhizal fungi Fungi growing around or
inside roots, where they receive organic
nutrients from the plant and may improve the
uptake of mineral nutrients by the plant.
mycotoxin A poison produced by a fungus.
myelin (myelin sheath) An insulation layer
around some axons in vertebrates; it is
formed by wrapping the axon in many layers
of glial cell membrane.
myobatrachids A family of frogs found in
Australia that has Gondwanan affinities.
myocardial infarction (MI) The death of
cardiac muscle cells, which can occur if a
region of the heart is deprived of blood for
an extended time.
myocardium The three-layered muscle
composing the heart wall.
myofilament A contractile microfilament,
composed largely of actin and myosin,
within muscle.
myogenic The initiation of the heart beat
within cardiac muscle itself, for example, in
vertebrate hearts.
myoplasm A yellow cytoplasm in the sea squirt
that segregates into particular cells of the
blastula which, in turn, give rise to muscle.
myosin A protein that commonly interacts
with actin filaments to generate cytoplasmic
movements or changes in cell shape; it is
organised into thick filaments in muscle
cells.
myotome A block of muscle on each side of the
body, as invertebrate chordates.
myriapod A millipede or centipede, subphylum
Myriapoda, with many legs.
myrmecotrophy A mutualistic relationship
where ants live within special chambers
formed in certain plants.
Myrtaceae A Southern Hemisphere plant family
that includes eucalypts, bottlebrushes, lilly
pilly and paperbarks.
myxamoeba The amoeba of slime moulds.
myxoedema A condition associated with the
accumulation of water as a result of lack of
thyroid hormones in adult humans.
myxomatosis A disease of European rabbits that
is caused by myxoma poxvirus; the virus was
used in the past as a biological control of
rabbits in Australia.
NAD+ Nicotinamide adenine dinucleotide;
a dinucleotide that functions as an energy
intermediate molecule.
NADP+ Nicotinamide adenine dinucleotide
phosphate; a dinucleotide that functions
as an energy intermediate molecule in
chloroplasts. It combines with two electrons
and H+ to form NADPH.
Native Flora and Fauna Guarantee Act
1987 Victorian legislation that aims to
protect all species of wattle, banksia, grevillea,
orchid and many other species of plant and
animal.
natural killer (NK) cell A lymphocyte lacking
the antigen receptors of T and B cells but
having a major role in defence against viruses
by lysis of infected cells and production of
interferon.
natural selection The process of differential
rates of reproduction and survival of
offspring, proposed by Darwin as a basis
for evolution; members of a population that
possess particular heritable characteristics
will survive and leave more offspring than
members with less favourable characteristics.
nauplius (pl. nauplii) The first larva of a
crustacean; consists only of a head and a
telson.
nautiloid An early type of extinct mollusc
(cephalopod) related to the living Nautilus.
Neanderthal Fossils of the genus Homo,
dating from 130 000 years ago, orginally
found in the Neander Valley of Germany;
with a brain as large as modern humans,
and toolmakers.
negative feedback control A control system
where the response produced to a particular
stimulus reduces the size of the original
disturbance; it leads to homeostasis.
negative feedback regulation A homeostatic
control mechanism whereby an increase
in some substance or activity inhibits the
process leading to the increase; also known as
feedback inhibition.
negative pressure filling The mechanism by
which reptiles, birds and mammals ventilate
their lungs.
negative regulation The regulation of a gene
such that expression of the gene is repressed
by the presence of a particular molecule.
nekton Free-swimming animals in the open
ocean that can swim against the current to
locate food.
nematocyst The stinging organelle of cnidarians
(e.g. jellyfish), which functions in defence
and capture of prey; nematocysts are also
called cnidae.
Nematoda The phylum of roundworms.
nematode An unsegmented worm that has
a tough outer cuticle; includes free-living
forms and disease-causing parasites, such as
hookworm and filaria.
Nemertea The phylum of proboscis worms
(nemerteans).
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neo-Darwinian theory of evolution A fusion
of Darwin’s theory of evolution by natural
selection with Mendel’s laws of heredity and
the mathematical principles of population
genetics.
Neognathae A major group of modern birds;
see also Paleognathae.
Neotropical region A biogeographical region
of the world including South America and
lower Central America.
nephridium (pl. nephridia) The tubular
excretory organ of invertebrates that develops
from the body surface into the coelomic
cavity; it is either a protonephridium or a
metanephridium, depending on the structure
of the coelomic end.
nephron The tubular excretory unit of the
vertebrate kidney; it is derived from a
coelomoduct but lacks a ciliated, funnel-like
opening.
nephrostome The funnel-shaped opening
that leads to the nephridium, which is the
excretory organ of molluscs.
nerve A structure found in the peripheral
nervous system that is composed of multiple
neurons; it makes contact with structures
outside the central nervous system and
transmits signals that enter or leave the CNS.
nerve impulse A way that neurons
communicate, involving changes in the
amount of electric charge across a cell’s
plasma membrane.
nervous system Groups of cells that sense
internal and environmental changes and
transmit signals that enable an animal to
respond in an appropriate way.
net primary productivity The portion of total
(gross) primary productivity that remains
after the respiratory losses of primary
producers are accounted for.
net reproductive rate The population growth
rate per generation.
neural crest cells In vertebrates, a group of cells
formed as the neural tube detaches from the
overlying ectoderm; they migrate individually
throughout the embryo and ultimately
differentiate into many different cell types,
such as peripheral nerves and much of
the face.
neural fold Folds of tissue that rise upwards on
either side of the neural groove.
neural groove A groove in the ectoderm that is
the first step in formation of the neural tube.
neural plate The region of dorsal ectoderm in
vertebrate embryos that forms the neural
tube.
neural stem cell A stem cell that can proliferate
and differentiate to give rise to all neural
tissue types.
neural tube The structure in vertebrate embryos
from which the spinal cord and brain form.
neuraminidase A type of protein; for example a
surface protein in the lipid outer membrane
of the ‘flu’ virus.
neurogenic Initiation of the heart beat by
nerves leading to the heart muscle, for
example, in many invertebrate hearts.
neuroglia Non-conducting nerve cells that
are intimately associated with neurons and
appear to provide nutritional support.
neurohaemal organ An organ in which axons
release their neurosecretions into the blood.
neuron A cell specialised for receiving, conducting
and transmitting information to other cells; the
basic unit of the nervous system.
neurosecretion Secretion of hormones by
nerve cells.
neurotransmitter A water-soluble signal
molecule released from nerve endings at
a synapse with an effector cell; it acts on
receptors located on other nerve cells, muscle
cells or glands.
neurula An embryonic stage in vertebrates
during which the neural tube forms.
neurulation The process of neural tube
formation.
neutral mutation A mutation that does not
affect the function of the encoded protein.
neutral variation Variation that does not favour
any particular genotype.
neutron A subatomic particle that is uncharged;
part of the nucleus of an atom.
neutrophil A small phagocytic cell in blood
and sites of inflammation, has an irregular
nucleus and neutral staining granules full of
digestive enzymes in the cytoplasm; it is also
referred to as a polymorph or granulocyte.
Newtonian model The classic model for cooling
that explains endothermic regulation of
metabolic heat production to balance heat
produced by cellular metabolism with heat
lost to the environment; the greater the
temperature difference between the animal
and the ambient air, the greater the rate
of heat loss and the greater the animal’s
metabolic rate.
niche The physical distribution and ecological
role of an organism.
nitrification The conversion by soil bacteria of
NH3 or NH4+ to nitrate (NO3–), a form of
nitrogen commonly used by plants.
nitrifying bacteria Chemoautotrophic bacteria
that use ammonium ions as an energy
substrate and reductant; produce nitrite and
nitrate; for example Nitrobacter; important in
the nitrogen cycle.
nitrogen cycle The movement of nitrogen
through ecosystems; involving soil
microrganisms that fix or release nitrogen.
nitrogen fixation The conversion of gaseous,
atmospheric nitrogen by certain bacteria to
ammonia, nitrites and nitrates.
nitrogen mineralisation The conversion
by microorganisms of organic nitrogen
compounds in soil to simple ions in solution.
nitrogenase An enzyme that catalyses the
reduction of molecular nitrogen (N2) to
ammonium ions; see nitrogen-fixing bacteria.
nitrogen-fixing bacteria Bacteria able to reduce
(fix) molecular nitrogen (N2) to ammonium
ions; for example Rhizobium associated with
plant roots; the important first stage of the
nitrogen cycle.
nitrogenous base One of the parts of a
nucleotide in DNA and RNA; pyramidines
with a single ring of carbon and nitrogen
atoms (uracil, cytosine and thymine) and
purines with a double ring (adenine and
guanine).
nitrogenous wastes Molecules that include
nitrogen from amino groups; these wastes
are toxic at high concentrations and must be
eliminated from the body.
nocturnal Active primarily at night.
node of Ranvier A small bare region of axon
between Schwann cells that form myelin
sheaths; it allows saltatory conduction of
action potentials from node to node.
node The region of a plant stem from which
one or more leaves, branches or buds emerge.
nodule A small swelling on a plant root that
contains nitrogen-fixing bacteria.
nodulin One of several plant proteins that
foster root nodule development.
non-cellulosic polysaccharides A porous,
hydrated, gel-like matrix of polysaccharides
other than cellulose, within which cellulose
microfibrils lie in a plant cell wall.
non-coding DNA DNA sequences that do
not encode amino acids incorporated into
polypeptides during protein synthesis.
non-covalent bond A diverse group of forces
that involve electrical charge interactions
between atoms or molecules; it is weaker
than covalent bonds; it includes hydrogen
bonds, ionic bonds and van der Waals forces.
non-cyclic photophosphorylation The process
of production of ATP in plant photosynthesis
in which electrons flow one way from water
through photosystems II and I and are
not recycled back to them but end up in
NADPH.
non-insulin-dependent diabetes mellitus
(Type II or maturity onset diabetes)
A disease caused by a relative lack of insulin
and usually not identified until middle age;
it is often associated with obesity and can be
controlled by diet.
non-polar molecule A molecule composed
predominantly of non-polar bonds.
non-random mating The phenomenon that
individuals choose their mates based on their
genotypes or phenotypes.
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non-recombinant An offspring whose
combination of traits has not changed from
the parental generation.
nonsense mutation A mutation that changes
the nucleotide sequence of a codon so that it
becomes a stop codon and causes premature
termination of translation.
non-vascular plant A plant that does not
produce lignified vascular tissue, such as a
modern bryophyte or extinct pretracheophyte
polysporangiophytes.
norepinephrine A neurotransmitter; also known
as noradrenaline.
normal (or resident) bacterial flora Bacteria
that reside in or on the body; may be
beneficial or harmless.
Nothofagus Southern beech trees, with living
species on land that was from Gondwana and
including fossils found in Antarctica.
notochord A cylindrical rod situated below the
nerve cord and above the gut in all chordates;
it provides support and lateral flexibility to
the body.
nuclear division (mitosis) The process of a
chromosome condensation, nuclear envelope
breakdown, sister chromatid separation
and formation of two daughter nuclei in
eukaryotes.
nuclear envelope A double membrane
surrounding the nucleus in eukaryotic cells.
nuclear genome The chromosomes found in the
nucleus of the cell.
nuclear pore A channel in the nuclear envelope
that allows movement of certain molecules
between the nucleus and cytoplasm.
nucleic acid A molecule consisting of long
chains of nucleotides joined through
phosphodiester bonds; the DNA and RNA
of living organisms.
nucleoid The circular DNA molecule of
prokaryotes compressed with the aid of folding
proteins and RNA; it is located in the cytosol.
nucleolus A prominent region in the nucleus of
non-dividing cells where ribosome assembly
occurs.
nucleomorph A remnant of an endosymbiont’s
nucleus found in the cells of a number
of protistan groups; it is evidence of their
plastids being acquired by secondary
endosymbiosis.
nucleosome A particle about 10 nm in diameter
comprising nucleosome core particle (histone
proteins) and associated wrapped DNA;
chromatin consists of DNA wrapped to form
long strings of nucleosomes.
nucleosome core particle A complex of eight
histone proteins that form a core around
which the double helix of DNA is coiled to
form a nucleosome.
nucleotide Five-carbon sugar, a phosphate
group and a nitrogenous base; nucleotides
1174
are linked together by phosphodiester bonds
between the sugar and phosphate groups to
form nucleic acids.
nucleotide excision repair (NER) A common type
of DNA repair system that removes (excises) a
region of the DNA where the damage occurs.
This system can fix many different types of
DNA damage, including UV-induced damage,
chemically modified bases, missing bases and
various types of cross-links.
nucleus The principal membrane-bound
compartment of the eukaryotic cell; control
centre of the cell; it contains chromosomal
DNA.
nutrient Any substance taken up by a living
organism that is needed for survival, growth,
development, repair or reproduction.
nymph A juvenile stage of development in
insects that is similar to the adult but lacks
wings and is not sexually mature.
obesity An excess of adipose (fat) tissue in
relation to gender, age and height.
obligatory mutualism An interaction in which
neither species can live without the other.
oceanic ridge The site where lava upwells from
part of the earth’s mantle.
ocellus (pl. ocelli) The simple eye of adult
arthropods.
oedema The build-up of fluid in tissues when
filtration exceeds reabsorption and lymph
flow; it is usually associated with vascular
disease.
oestrogen A steroid hormone including
oestradiol, oestrone and oestriol; it is
produced by ovarian follicles.
Okazaki fragment A short segment of DNA
synthesised from an RNA primer on the
lagging strand; the Okazaki fragment is
synthesised until the previous primerOkazaki fragment is encountered.
oligochaete Annelids of the subclass
Oligochaeta; for example earthworms.
oligotrophic The term used to describe a
young lake that starts off clear and with
little plant life.
omnivore An animal that has the ability to
eat and survive on both plant and animal
products.
onchosphere The six-hooked larva that hatches
from the egg of a tapeworm.
oncogene A tumour-causing form of a gene.
one-way digestive tract A digestive system in
which food enters via a mouth and exits
via an anus; contrast with simple digestive
cavities in which food enters and leaves via
the same opening.
ontogeny The development of an organism.
onychophoran A velvet worm, phylum
Onychophora, with a segmented body and
short unjointed limbs and that breathes by
tracheae.
oocyst The zygote in a sporozoan life cycle.
It is surrounded by a tough cyst to prevent
dehydration or other damage.
oocyte A female germ cell undergoing meiosis
within the ovary.
oogenesis The process by which eggs form from
primordial germ cells.
oogonia Diploid female germ cells undergoing
mitosis; in animals, within the ovary; in
protists, within oogonia (female reproductive
structure).
oogonium The female reproductive organ (e.g. in
a brown alga) that produces eggs (ova).
oomycete Water mould or downy mildew
classified in the k. Protista; for example
Phytophthora cinnamomi, which causes jarrah
die-back; oomycetes have cell walls made of
cellulose and form coenocytic hyphae.
open circulatory system A circulatory system
in which hemolymph, which is not different
than the interstitial fluid, flows throughout
the body and is not confined to special
vessels.
open flow ventilation Unidirectional
ventilation.
open reading frame A nucleotide sequence
that can encode a polypeptide (i.e. that has a
string of codons in frame that encodes amino
acids).
operator A DNA sequence in bacteria that is
recognised by activator or repressor proteins
that regulate the level of gene transcription.
operculum A cap-shaped covering, for example,
apical portion of a moss spore capsule or
covering formed from fused sepals or petals
of a eucalypt flower bud.
operon In bacteria, a region of the chromosome
that contains a transcription unit and the
associated regulatory genes and sequences
required for regulated transcription of the
unit.
Ophiuroidea A class of echinoderms; brittle
stars.
opisthosoma In arthropods, the abdomen,
posterior region of the body behind the
prosoma.
opportunistic A term used to describe animals
that have a strong preference for one type
of food but can adjust their diet if the need
arises.
opportunistic infection Caused by
microorganisms that are harmless to a healthy
person but debilitate a person whose immune
system has been weakened.
optimal body temperature The temperature at
which an animal’s body and its biochemical
processes function optimally.
optimal foraging The concept that in a given
circumstance, an animal seeks to obtain
the most energy possible with the least
expenditure of energy.
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oral Refers to the region of an animal where the
mouth is located; refers to the top side of a
radial animal.
oral arm The arm-like extension of the
manubrium of many medusae (jellyfish).
oral disc The top end of an anthozoan (anemone
or coral) polyp that is ringed with tentacles.
oral surface The surface on which the mouth is
found; used as a reference when describing the
body structure of echinoderms because of their
adult radial symmetry.
orbital The zones of space in which electrons
exist at any one moment around the nucleus
of an atom.
Orchidaceae The orchid family.
order A higher level taxon (grouping) used in
classification of organisms; between class and
family.
organ Two or more types of tissue combined to
perform a common function. For example,
the heart is composed of several types of
tissues, including muscle, nervous and
connective tissue.
organelle A subcellular structure or membranebounded compartment with its own unique
structure and function.
organic molecule A molecule composed
principally of six elements—hydrogen,
carbon and oxygen, together with nitrogen,
phosphorus and sulfur.
organiser A group of cells that secrete a
morphogen.
organism A living thing that maintains an
internal order that is separated from the
environment.
organogenesis The process of organ formation.
origin The start sequence of DNA replication;
there is a single origin in the circular DNA
molecules of prokaryotes, but multiple
origins in eukaryote chromosomes.
origin of replication A site within a
chromosome that serves as a starting point
for DNA replication.
orthologous genes Two genes in different
species that are derived from the same gene
in a common ancestor.
osculum (pl. oscula) The opening in the wall of
a sponge through which water leaves.
osmoconcentration The physiological process
whereby water is resorbed from the urine by
excretory tubules, making the urine more
osmotically concentrated than body fluids.
osmoconform To have the same osmotic
concentration of body fluids and the external
medium.
osmolality Solute concentration of a solution,
measured as the number of moles of all
dissolved solutes per kilogram of water.
osmolarity Solute concentration of a solution,
measured as the number of moles of all
dissolved solutes per litre of solution.
osmolyte Any solute molecule that is dissolved
in solution and contributes to its total
concentration of osmotically active particles.
osmoregulate To maintain an osmotic
concentration of body fluids different from
that of the external medium.
osmosis The movement of water from a region
of low osmotic concentration (high water
concentration, high water potential) to one
of high osmotic concentration (low water
concentration, low water potential) as a result
of the random thermal motion of water
molecules through a selectively permeable
membrane.
osmotic adjustment The process by which a
plant increases the solute concentration of its
cytosol.
osmotic potential A measure of the reduction in
the free energy of water due to the presence
of dissolved solutes (e.g. salt or sugar).
osmotic pressure The hydrostatic pressure
required to stop the net flow of water across a
membrane due to osmosis.
osmotroph An organism that relies on
osmotrophy (uptake of small organic
molecules via osmosis) as a form of nutrition.
ossicle A crystal of calcium carbonate; it is first
formed within a cell and enlarging to a plate
beneath the skin; forming the skeleton of
echinoderms.
osteoblast A bone-forming cell.
osteocyte A mature osteoblast.
osteoporosis The depletion of bone calcium in
women as a result of decreasing oestrogen
levels after menopause.
ostium (pl. ostia) The pore in the wall of a
sponge through which water enters during
filter feeding.
ostracoderm An extinct jawless fish from the
Ordovician and Devonian periods that was
an early vertebrate.
otoliths Granules of calcium carbonate found
in the gelatinous substance that embeds hair
cells in the ear.
outcrossing Breeding with individuals other
than oneself or one’s close relatives.
ovary (pl. ovaries) The female reproductive
organ, in which germ cells undergo
differentiation into eggs, and hormones are
produced to regulate reproductive function;
in flowering plants, the basal part of the
carpel that contains ovules and develops into
the fruit after fertilisation.
oviparity The release of eggs that are fertilised
externally.
ovulation The release of a mature egg cell from
the ovary.
ovule Megagametophyte is retained within
the megasporangium, which is further
surrounded and protected by one or more
layers of cells, the integuments; following
fertilisation, the whole structure (the ovule)
develops into a seed.
ovuliferous scale A leaf-like structure (thought
to be a reduced shoot) that bears ovules; it is
aggregated into female cones.
ovum (pl. ova) Female gamete; egg cell.
oxidation–reduction (redox) potential The
measure of the tendency to donate or accept
electrons, expressed in volts or millivolts
(symbol E0′).
oxidation–reduction reaction A chemical
reaction involving the transfer of electrons;
losing an electron is oxidation and gaining an
electron is reduction.
oxidative phosphorylation A process during which
NADH and FADH, are oxidised to make more
ATP via the phosphorylation of ADP.
oxidative respiration A cellular respiration
pathway that uses oxygen, allowing extraction
of more ATP from the breakdown of glucose
than anaerobic respiration.
oxidised When an atom or molecule gains an
electron.
oxidoreductase An enzyme that catalyses
reactions that transfer electrons, usually in the
form of hydrogen ions or hydrogen atoms.
oxygen equilibrium curve Also called the
oxygen dissociation curve; the relationship
between the partial pressure of oxygen (PO2)
and total O2 content.
oxygen-carrying capacity The maximum
amount of oxygen that can be carried by a
fluid; in many animals the oxygen-carrying
capacity of body fluids is improved by the
addition of respiratory pigments.
oxygenic photosynthesis The type of
photosynthesis in algae and land plants in
which oxygen is the product of the overall
reaction to fix carbon dioxide as sugars.
oxytocin A posterior pituitary hormone that
influences reproductive functions.
ozone ‘hole’ The thinning of the ozone layer
due to depletion of ozone; for example, over
Antarctica.
ozone O3, a stratospheric layer of the earth’s
atmosphere responsible for filtering out
ultraviolet radiation supplied by the sun.
pacemaker See sinoatrial node.
paedomorphosis The retention of juvenile traits
in an adult organism.
paired mouthparts Modifications of the mouth
that allow two parts to work against each
other to enhance the ability to grip or crush
or pierce food.
pair-rule genes Genes expressed in a twosegment periodicity and required for
formation of segments.
Palaeotropical region The biogeographical
region of the world including Africa
(Ethiopian region), India and South-East
Asia (Oriental region).
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Palaeozoic ‘Ancient life’; the geological era 570
to 245 million years BP.
Paleognathae The oldest group of birds,
including the tinamous and ratites of the
Southern Hemisphere (the name means
having a primitive jaw); see also Neognathae.
paleontologist A scientist who studies fossils.
palisade mesophyll The elongated cells of the
leaf mesophyll arranged perpendicularly to
the leaf surface.
palisade parenchyma Photosynthetic ground
tissue in a plant that consists of closely
packed, elongate cells adapted to absorb
sunlight efficiently.
pancreas An organ with exocrine (releasing
digestive juices into the duodenum) and
endocrine (releasing insulin from islet cells)
functions.
pancreozymin/cholecystokinin A hormone
released from gastrointestinal mucosa
into local blood vessels; it optimises the
composition of digestive juices released by
the pancreas and gall bladder.
Pangaea The supercontinent uniting
all northern (Laurasia) and southern
(Gondwana) landmasses that formed, and
then fragmented, during the Mesozoic.
papilla A small projection of tissue.
parabasalid Flagellate (k. Protista) with a
single nucleus and involved in commensal
or parasitic relationships with animals; it
typically has a parabasilid body, a large
Golgi-type membrane complex near the basal
body.
parabronchi A series of parallel air tubes that
make up the lungs and are the regions of gas
exchange in birds.
paracellular absorption Uptake of water-soluble
nutrients from the gut via a channel in the
tight junctions of adjoining cells by diffusion.
paracrine hormones Animal hormones that
usually act over very short distances,
travelling by diffusion through extracellular
fluid.
parallel evolution The independent gaining
(in evolution) of similar features in related
organisms.
paralogous genes Two genes within one genome
that are derived by duplication from the same
gene.
paralytic shellfish poisoning (PSP) An illness
caused by toxins from certain dinoflagellates
that move through marine food chains; it
affects the nervous system of humans.
paramylon A β-(1→3)-glucan storage product
in euglenoids that forms solid granules in the
cytoplasm.
Paranthropus A group of early hominids that
were more robust than Australopithecus.
parapatric speciation Divergence in
populations, initially in geographic isolation
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and subsequently when the populations again
come in contact, leading to formation of new
species.
paraphyletic group A group or taxon of
organisms that excludes some of the
descendants of a common ancestor;
non-monophyletic.
paraphysis (pl. paraphyses) Sterile hair (e.g.
associated with reproductive structures in
mosses).
parapodium (pl. parapodia) A lateral,
paired appendage on the body segments of
polychaetes; it functions in gas exchange and
locomotion.
parasite An organism that lives and feeds on
or in another organism, the host, which is
usually larger than the parasite.
parasitism The symbiotic interaction between
two species where one benefits and the other
is harmed.
parasitoid An insect that is free living as an
adult but parasitic as a larva.
parasympathetic nervous system That part of
the autonomic nervous system arising in the
brain stem and sacral spinal cord.
parathyroid hormone A peptide hormone
released by the parathyroid gland; it increases
Ca++ concentration in the plasma by a variety
of means.
Parazoa The subkingdom of animals that
includes sponges; compare Eumetazoa.
parenchyma Large living cells that form the
ground tissue of plants, comprising large,
thin-walled cells, a large central vacuole and
a peripheral nucleus and cytoplasm.
parental care Any kind of parental behaviour,
including preparation of nests, care of eggs
inside the body or provisioning of young
after birth.
parental strand The original strand in DNA
replication.
parthenogenesis A mode of reproduction,
found in some sexually reproducing
organisms, in which the male plays no
role.
partial pressure The individual pressure of each
gas in the air; the sum of these pressures is
known as atmospheric pressure.
passive diffusion Diffusion that occurs through
a membrane without the aid of a transport
protein.
passive immunity A type of acquired immunity
that confers protection against disease
through the direct transfer of antibodies from
one individual to another.
passive transport The diffusion of a solute
across a membrane in a process that is
energetically favourable and does not require
an input of energy.
pathogen A microorganism that causes disease
symptoms in its host.
pathogen-associated molecular pattern The
molecules on the surface of pathogens that
trigger the innate immune system.
pathogenicity A property of a bacterial species
that relates to its ability to cause disease.
pattern formation The process of generating the
pattern of different tissues in the developing
embryo.
pectin A type of polysaccharide found in plant
cell walls; the backbone of the molecule
formed from galacturonic acid (an acidic
monosaccharide); good gel-forming
molecules.
pedicel A narrow, waist-like point of attachment
in a spider or insect body.
pedipalp (pl. pedipalps) A second pair
of appendages of the cephalothorax in
arachnids; it is modified for various
functions; it corresponds to insect mandibles.
peduncle The tip of a flower stalk.
pelagic realm The marine biogeographic region
including the surface water of open oceans
and planktonic organisms.
pellicle Proteinaceous strips arranged in a spiral,
providing support, and giving shape to,
euglenoid cells.
penetrance The proportion of individuals of a
particular genotype that show a phenotypic
effect.
penis The intromittent organ of males used
for transferring sperm during internal
fertilisation in many animals.
pennate, of diatom A bilaterally symmetrical
type of diatom.
pentaradial symmetry The five-part
radial symmetry characteristic of adult
echinoderms.
PEP carboxylase An enzyme in C4 plants that
adds CO2 to phosphoenolpyruvate (PEP)
to produce the four-carbon compound
oxaloacetate.
peptide bond A bond formed when the acidic
carboxyl group (–COOH) of an amino
acid attaches to the amino group (–NH2)
of another, with the release of a molecule
of water.
peptidoglycan A type of molecule found in
the cell walls of bacteria and the plastids
(cyanelles) of glaucophytes, a group of
protists.
peptidyl transferase In translation, the enzyme
responsible for catalysing the formation of a
peptide bond between each new amino acid
and the previous amino acid in a growing
polypeptide chain.
perennial A plant that lives for more than two
years, often producing seeds each year after it
reaches reproductive maturity.
perfect flower A flower that has both stamens
and carpels.
perfusion Internal convection.
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perianth The term that refers to flower petals
and sepals collectively.
pericarp A fruit wall, comprised of exocarp,
mesocarp and endocarp.
pericycle The layer of cells in a root that
surrounds the vascular cylinder; the site of
initiation of lateral roots.
periderm A protective outer tissue that replaces
the epidermis in secondary stems and roots
of woody plants; corky tissue.
peridinin A xanthophyll; an accessory
photosynthetic pigment found in
dinoflagellates.
peripheral nervous system A peripheral
part of the autonomic nervous system in
vertebrates, which comprises ganglia and
connecting nerves, and is classified into three
subsystems—sympathetic, parasympathetic
and enteric divisions.
peripheral zone The area of a plant that
contains dividing cells that will eventually
differentiate into plant structures.
peristalsis An involuntary wave of contraction
of the circular muscles (and their subsequent
relaxation) that propels material along a
tubular organ (e.g. the oesophagus).
peristaltic locomotion The movement of
an animal by alternate constriction and
widening of the body; it depends on a
fluid-filled body cavity, as in annelids.
peristome Specialised rows of teeth-like
structures around the top of a moss
sporangium; it shelters spores.
peristomium The first segment of an annelid.
peroxisome A type of microbody in eukaryotic
cells that contains numerous enzymes; it is
involved in the production and degradation
of peroxides and oxidation of amino acids
and uric acid.
pest An animal, often an introduced species,
that is troublesome in ecosystems.
petal In a flower, one of the inner floral leaves;
often brightly coloured to attract pollinators.
petiole The stalk of a leaf.
pH The concentration of hydrogen ions (H+) in
solution; it is measured on a logarithmic scale
ranging from 0 to 14.
phaetophyte See brown alga.
phagocytosis A method of ingestion of food by
endocytosis utilised by unicellular organisms,
such as sponges, whereby a food particle is
engulfed in a membrane-bound food vacuole.
pharyngeal slit (pl. pharyngeal slits) Paired
openings appearing in the pharynx of
hemichordates and chordates at some stage
of development.
Pharyngotremata The group of deuterostome
animals characterised by having pharyngeal
slits that evolved different functions; includes
acorn worms, tunicates, lancelets and
vertebrates.
pharynx The area at the back of the throat
where inhaled air from the mouth and nose
converges.
phellogen See cork cambium.
phenetic A classification system based on
similarities between organisms without regard
to their evolutionary relationships.
phenotype The characteristics of an organism that
are the result of the expression of its genes.
phenotypic frequency The frequency of
particular phenotypes (e.g. height, weight
and colour) in populations.
phenotypic plasticity The phenomenon in
which individual members of the same plant
species that experience different environmental
conditions may display considerable variation
in structure or behaviour.
pheromone A hormone released into the
external environment for chemical
communication between individuals.
phloem The transport tissue of vascular plants;
it comprises several cell types, including
sucrose-transporting sieve cells, companion
cells and sclerenchyma fibres.
phosphatase Any of a number of enzymes that
removes a phosphate group from a protein,
reversing the action of a kinase.
phosphodiester bond The linkage between
two sugars in the backbone of a nucleic acid
molecule; the phosphate group connects the
pentose sugars through a pair of ester bonds.
phosphodiesterase An enzyme that breaks down
cAMP into AMP.
phospholipid A class of lipids that are similar
in structure to triglycerides, but the third
hydroxyl group of glycerol is linked to a
phosphate group instead of a fatty acid.
They are a key component of biological
membranes.
phospholipid bilayer The basic framework of
the cellular membrane, consisting of two
layers of lipids.
phosphoric acid Also orthophosphoric acid.
A chemical buffer that maintains a particular
pH in a solution.
phosphorus cycle The movement of phosphorus
through ecosystems; a local cycle primarily
involving the soil.
phosphorylation The addition of a phosphate
group to a compound; it often results in the
formation of a high-energy bond (e.g. ATP
from ADP).
photoautotroph An organism that gets its
energy from sunlight and uses carbon dioxide
as a carbon source (e.g. photosynthetic
plants, algae and cyanobacteria).
photodormant Seed dormancy that can be
broken by light.
photoheterotroph An organism that uses
sunlight for energy but organic compounds
(rather than carbon dioxide as in plants) as
ready-made building blocks for growth and
development (e.g. purple and green bacteria).
photon The elementary particle of
electromagnetic radiation (light).
photoperiodism A plant’s ability to measure and
respond to night length, and indirectly, day
length, as a way of detecting seasonal change.
photoreceptor A type of receptor that
detects light by absorbing it at a particular
wavelength.
photorespiration A light-activated type of
respiration found in the chloroplasts of plants
in which Rubisco uses O2 as a substrate
to oxygenate RuBP and produce CO2 as a
product of oxygenation.
photosynthesis The process whereby light energy
is captured by plant, algal or cyanobacterial
cells and used to synthesise organic molecules
from CO2 and H2O (or H2S).
photosystems I and II Two light-harvesting
systems consisting of protein molecules
associated with pigments, embedded in the
thylakoid membranes of chloroplasts.
phototroph An organism that uses radiant
energy (light).
phototropism A tropism in which light is the
stimulus; for example the bending or turning
of a stem towards a light source.
phycobilin A type of water-soluble pigment
characteristic of cyanobacteria, giving them
a bluish colour; also called ‘blue-green algae’.
phycocyanin A photosynthetic pigment;
see red alga.
phycoerythrin A photosynthetic pigment;
see red alga.
phycologist A scientist who studies algae.
phycoplast A system of microtubules orientated
in the plane of cell division following the
collapse of the spindle during cytokinesis;
a feature of many green algae.
phyllode Laterally compressed petiole and
rachis; foliage that replaces true leaves in
most Australian acacias.
phyllotaxy The geometric pattern of leaf
arrangement on a stem of plants (e.g. spiral).
phylogenetic species concept The members
of a single species are identified by having a
unique combination of characteristics.
phylogeny Evolutionary relationships of
organisms, usually depicted as a branching
tree diagram (phylogenetic tree).
phylum A higher level taxon (grouping) used in
classification of organisms, below the level of
kingdom.
physical digestion The breakdown of food into
small particles by grinding or chewing.
physical isolation When populations do not
interbreed because they are geographically
separated.
physiology The study of the functions of cells
and body parts of living organisms.
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phytochrome A plant pigment that absorbs
light; it exists in two interconvertible forms,
Pr (inactive form) and Pfr (active form);
it is involved in the timing of flowering,
dormancy and seed germination.
phytoplankton Microscopic algae and
cyanobacteria that float or actively move
through water.
pigment A molecule that can absorb light
energy.
pilus (pl. pilli) The structure (sex pilus) in some
bacteria that enables transfer of DNA from
one cell (‘male’) to another (‘female’) during
conjugation.
pinacocyte See pinacoderm.
pinacoderm The outer surface of a sponge,
consisting of a layer of flattened cells
(pinacocytes).
pineal gland In vertebrates, an outgrowth in
the midline of the roof of the third ventricle
that is used to measure photoperiod; in
fishes, amphibians and some reptiles it may
contain photoreceptor cells (the ‘third eye’);
in mammals and birds, it is a neurosecretory
organ releasing melatonin at night.
pinealocyte Modified photosensory cells with
a secretory function only; it is located in the
pineal gland of mammals, birds and snakes; a
major hormone released is melatonin.
pinna (pl. pinnae) A leaflet of a compound leaf.
pinocytosis The mode of capillary exchange
whereby large particles and lipid-insoluble
materials are exchanged slowly via numerous
tiny vesicles.
pistil A flower structure that may consist of a
single carpel or multiple, fused carpels, and is
differentiated into stigma, style and ovary.
pit A small cavity in a plant cell wall where
secondary wall materials such as lignin are
absent.
pit connection The connection between
adjacent cells in the thallus of red algae.
pith Parenchyma cells that lie centrally within
the vascular tissue of stems and some roots.
pituitary gland A multilobed endocrine gland
sitting directly below the hypothalamus.
placenta An organ that enables exchange
between an embryo and the maternal
circulation; it is present in marsupial and
placental mammals.
placoderm The extinct, large armoured, jawed
fish of the late Silurian and Devonian
periods.
plant hormone An organic substance produced
in one part of a plant and transported to
another part where it has its effect.
plant tissue culture A laboratory process to
produce thousands of identical plants having
the same desirable characteristics.
Plantae A kingdom that includes all green land
plants.
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planula A ciliated type of animal larva.
planula larva The ciliated free-swimming larva
of a hydrozoan coelenterate such as the
hydra.
plasma The transparent, slightly yellowish fluid
component of blood without the cellular
components; it is obtained by centrifugation.
plasma cell A non-dividing, antibody-secreting
B cell formed after binding of antigen to the
specific receptor of a B cell in the presence of
helper T cells.
plasma membrane The boundary of living cells
separating a cell from its environment; it is
formed from a phospholipid bilayer.
plasmid A small circular extrachromosomal
DNA molecule in bacterial cells, often
carrying genes conferring antibiotic
resistance.
plasmodesma (pl. plasmodesmata) The special
channel connecting plasma membranes
and cytosols of adjacent plant cells through
primary cell walls.
plasmodium A large multinucleate cell
resembling a slimey scum, often bright
yellow; a stage in the life cycle of acellular
slime moulds (myxomycetes).
plasmogamy Cytoplasmic fusion between
eukaryote cells.
plasmolysis The shrinkage of cytoplasm due to
loss of water by osmosis, drawing the plasma
membrane away from the wall.
plastid A general name given to organelles
found in plant and algal cells, which are
bound by two membranes and contain
DNA and large amounts of chlorophyll
(chloroplasts) carotenoids (chromoplasts), or
starch (amyloplasts).
plastron In turtles and tortoises; a ventral,
protective shield covered by horny plates.
plate tectonics A modern geological theory that
recognises that the earth’s crust and part of
the upper mantle (together the lithosphere)
are divided into a number of plates that
move relative to one another.
platelets Cell fragments in the blood of
mammals that play a crucial role in the
formation of blood clots.
Platyhelminthes A phylum of flatworms.
platypus An egg-laying mammal in the order
Monotremata, often described as duck-billed.
platyrrhine A New World monkey (South
America); flat-nosed; nostrils far apart and
nose–lip area haired; many have a prehensile
tail.
pleiotropy The effect of one gene on multiple
traits.
plesiomorphic A primitive character considered
to be ancestral in a group of organisms.
pleural cavity The space within the pleural sac,
which is a double layer of sheath-like moist
membranes that encases each lung.
plumule The epicotyl of a plant with its two
young leaves.
pluripotent The ability of cells to produce a
range of cell types, for example, blood-forming
stem cells.
pneumatophore Upright aerial root of
mangroves; it is exposed at low tide and
functions in gas exchange.
point mutation A mutation that affects only
a single base pair within the DNA, or that
involves the addition or deletion of a single
base pair to a DNA sequence.
polar covalent bond The bond that forms when
two atoms with different electronegativities
form a covalent bond and the shared
electrons are closer to the atom of higher
electronegativity than to the atom of
lower electronegativity. The distribution of
electrons around the atoms creates a polarity,
or difference in electric charge, across the
molecule.
polar molecule A molecule containing
significant numbers of polar bonds.
polar nuclei The two nuclei present in the central
cell of the embryo sac of flowering plants.
polarity 1. Refers to unequal charge distribution
in a molecule such as water, which has
a positive region and a negative region
although it is neutral overall. 2. Refers to axial
differences in a developing embryo that result
in anterior–posterior and dorsal–ventral axes
in a bilaterally symmetrical animal.
pole cell The cell that forms at the posterior
end of the Drosophila embryo and gives rise
to germ line cells.
pollen A collective term for pollen grains; the
microgametophytes of seed plants, which
develop sperm and can be transported by
wind or animals.
pollen grain The products of microsporogenesis
in a seed plant.
pollen tube A mature male gametophyte
consisting of a germinated pollen grain with
a long, thin pollen tube that carries haploid
sperm cells.
pollination The process in which pollen of
flowering plants is transferred by animals,
air or water currents to the stigma for
fertilisation.
pollination syndromes The pattern of coevolved
traits between particular types of flowers and
specific pollinators.
pollinator An animal that carries pollen
between angiosperm flowers (or cones of
gymnosperms).
poly A tail A string of adenine nucleotides
at the 3′ end of most mature mRNAs in
eukaryotes.
polyadenylation The process in which a series
of A residues are added to the 3′ end of an
RNA transcript.
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polyandry A form of mating in which one
female mates with several males.
polyarch xylem A pattern of xylem
development in roots of monocotyledons
among flowering plants in which the xylem
does not fill the centre, but is divided into
many ridge-like projections (archs).
polychaete A marine annelid worm of the class
Polychaeta, having a distinct head and paired
parapodia that bear bristles and are used in
swimming.
polycistronic mRNA A transcript that contains
more than one reading frame, which is
translated to produce more than one
protein.
polyclonal antibody An antibody response
in which an antigen elicits many different
antibodies, each fitting a different portion of
the antigen surface.
polygenic A phenotype influenced by multiple
genes.
polygyny A form of mating in which one male
mates with more than one female in a single
breeding season, but females mate only with
one male.
polyisoprenoid lipid A diverse group of lipids
formed by polymerisation of isoprenoid
building blocks; it plays a role in membranes
and cell–cell signalling.
polymer A large molecule formed by linking
together many smaller molecules called
monomers.
polymerase chain reaction (PCR) A technique
to make many copies of a gene in vitro;
primers are used that flank the region of
DNA to be amplified.
polymorph The shortened term for
‘polymorphonuclear neutrophil’; a phagocytic
white blood cell characterised by an irregular
nucleus and neutral staining granules in the
cytoplasm containing digestive enzymes.
polymorphic locus A locus in which two or
more alternative allelic genes occur in a
population, where at least two of the alleles
occur frequently.
polyp An attached tubular form of a cnidarian
with its mouth upwards.
polypeptide chain A molecule of many amino
acids joined together by peptide bonds; a
protein is a large polypeptide chain.
polyphyletic group A group or taxon of
organisms that are unrelated to one another
and based on superficial resemblance due to
convergent evolution.
polyplacophoran Chitons; exclusively marine,
bilaterally symmetrical molluscs, elongated
and dorsoventrally flattened with eight
overlapping calcareous shell plates.
polyploidy The existence of more than two
sets of homologous chromosomes in a
genome.
polyprotein A large molecule consisting of a
number of protein parts; for example some
viral proteins.
polyprotodont A marsupial with more than one
pair of incisor teeth in the lower jaw.
polysaccharide A carbohydrate composed of
many monosaccharides joined in long, linear
or branched chains (polymers).
polysome A number of ribosomes attached to
the same mRNA strand, each at different
stages of protein synthesis; it may be free
in the cytosol or attached to endoplasmic
reticulum.
pome A fleshy fruit from an ovary of an
epigynous flower.
Pongidae The family of great apes that includes
the orang-utan, gorilla and two species
of chimpanzee; it is traditionally separate
from the family Homindae, which includes
humans, although Pongidae is a paraphyletic
taxon.
population A group of organisms of the same
species living in a defined geographic area.
population bottleneck Occurs when a
population decreases dramatically in size for
a period of time; usually leads to genetic drift
and increased homozygosity.
population ecology The study of how
populations grow and what promotes and
limits growth.
population genetics The study of genes and
genotypes in a population.
Porifera A phylum of sponges.
porphyrin ring In a chlorophyll molecule, a
central metal atom (magnesium) surrounded
by a ring of alternating single and double
bonds.
positive feedback The acceleration of a process,
leading to what is sometimes called an
explosive system.
positive pressure filling The method by which
amphibians ventilate their lungs—the
animals gulp air and force it under pressure
into the lungs, as if inflating a balloon.
positive regulation The regulation of a gene
such that expression of the gene is induced
by the presence of a particular molecule.
postabsorptive state One of two alternating
phases in the utilisation of nutrients; occurs
when the gastrointestinal tract is empty
of nutrients and the body’s own stores
must supply energy. The other phase is the
absorptive state.
posterior Refers to the rear (tail-end) of an
animal.
posterior pituitary gland (neurohypophysis)
In most vertebrates; it is a neurohaemal
organ containing axons whose cell bodies
are located in the walls of the third ventricle;
it secretes peptides that affect water balance
and reproduction.
posttranscriptional control A mechanism of
control over gene expression that operates
after the transcription of mRNA is
complete.
postzygotic isolating mechanism A mechanism
that prevents interbreeding by blocking the
development of a viable and fertile individual
after fertilisation has taken place.
potential distribution Of a species; the range
over which individuals could theoretically
survive and reproduce.
potential energy Stored energy, such as
chemical energy stored in the bonds of atoms
and molecules.
potyvirus The most common type of plant
virus; named after the potato virus Y; causes
diseases of crops but also attractive colour
changes in flowers.
power In mechanical terminology, it is the rate
at which work is done; it is measured in
joules per second (equals watts).
Precambrian The oldest era in the geologic time
scale; before 570 million years ago.
precapillary sphincter The band of smooth
muscle at the entrance of a true capillary;
it opens or closes the vessel in response to
local influences.
predator An organism that catches and kills
another organism for food.
predator–prey cycle Oscillations in population
size where predator numbers follow those of
the prey.
preferential channel The main capillary
route for blood flow through a tissue when
metabolic demands are minimal.
preprophase band A band of microtubules that
forms prior to mitosis at the site of the future
cell division plane.
pressure potential The energy level of water as a
result of hydrostatic pressure or suction.
prezygotic isolating mechanism A mechanism
that stops interbreeding by preventing the
formation of a zygote.
primary active transport A type of transport
that involves pumps that directly use energy
and generate a solute gradient.
primary cell wall The first wall of a plant cell,
composed of cellulose, pectins and noncellulosic polysaccharides.
primary consumer An organism that obtains its
food by eating primary producers; also called
a herbivore.
primary growth In vascular plants, growth
originating in the apical meristems of shoots
and roots; results in an increase in length.
primary lymphoid organ The site of
differentiation of lymphocytes, for example
thymus and bone marrow.
primary meristem A meristematic tissue that
increases plant length and produces new
organs.
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primary metabolism The synthesis and
breakdown of molecules and macromolecules
that are found in all forms of life and are
essential for cell structure and function.
primary pit-field An area of the primary cell
wall where plasmodesmata are concentrated
and the site where a pit will occur following
development of a secondary cell wall.
primary producer An autotroph, which
typically harvests light energy from the sun;
located at the base of the food chain.
primary response The initial immune
response to an antigen, usually resulting in
immunological memory, which causes a later
immune response to the same antigen to be
larger and more rapid.
primary structure Of a protein; determined by
the sequence of amino acids.
primary tissue Plant tissue generated as a result
of primary growth at apical meristems.
primary transcript The RNA molecule
produced by transcription prior to
processing.
primary vascular tissue Plant tissue composed
of primary xylem and phloem.
primase The RNA polymerase that synthesises
short RNA strands (primers) that are used
by DNA polymerase to initiate DNA
synthesis.
Primates An order of mammals including
lemurs, tarsiers, monkeys and great apes.
primer A short segment of RNA, typically 10
to 12 nucleotides in length, that is needed to
begin DNA replication.
primitive See plesiomorphic.
primordium In plants, a bulge on the young
shoot produced by the apical meristem;
primordia can differentiate into leaves, other
shoots or flowers.
principle of parsimony The preferred
hypothesis is the one that is the simplest.
prion A proteinaceous infectious particle; it
is an infectious agent that is virus-like but
appears to lack nucleic acid and consists only
of protein.
probe A term given to DNA that is labelled
with radioactivity or other markers and
hybridised to detect complementary DNA
sequences.
proboscis The coiled tongue of a butterfly or
moth, which can be uncoiled, enabling it to
drink nectar from flowers.
procambium A type of primary plant tissue
meristem that produces vascular tissue.
producer An organism that synthesises the
organic compounds used by other organisms
for food.
productivity The rate at which biomass
accumulates; primary productivity refers
to productivity of producer organisms
(e.g. plants).
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proglottid The segment-like body unit of a
tapeworm; new segments are added at the
anterior end.
programmed cell death (apoptosis)
The process of cell suicide that involves
a characteristic series of events leading to
death of the cell.
progymnosperms An extinct group of plants,
having wood but no seeds, that evolved
before the gymnosperms.
prokaryote A bacterium; a small single-celled
organism; a prokaryotic cell lacks membrane
organelles such as a nucleus or mitochondria
or chloroplasts; compare eukaryote.
prometaphase A phase of mitosis in which the
nuclear envelope breaks down, allowing the
mitotic spindle to interact with and move
chromosomes.
promoter The specific sequence of DNA that
binds RNA polymerase, promoting initiation
of transcription of the coding region.
proofreading The ability of DNA polymerase to
identify a mismatched nucleotide and remove
it from the daughter strand.
prophase The initial phase of mitosis in which
the mitotic spindle assembles, and dispersed
chromatin in the nucleus condenses into
paired chromatids.
proplastid The precursor organelle of all types
of plastids.
proprioceptor In vertebrates, a sensory
receptor that senses the body’s position and
movements.
prosimian A member of a class of primates that
includes the smaller species such as bush
babies, lemurs, pottos and tarsiers.
prosoma In arthropods, the anterior part of the
body, the head or cephalothorax.
prosthetic group Small molecules that are
permanently attached to the surface of an
enzyme and aid in catalysis.
prostomium The anterior, presegmental part of
an annelid (segmented worm), which houses
the brain.
protandry In animals: a situation in which
an individual starts life as a male, changing
to a female at some later stage. In plants:
a condition where a flower first opens in
the male phase (anthers dehisce) and later
becomes female (stigmas receptive).
Proteaceae A Southern Hemisphere plant
family that includes banksias, grevillias and
waratahs.
protease An enzyme that cuts proteins into
smaller polypeptides.
protein A functional unit composed of one
or more polypeptides. Each polypeptide is
composed of a linear sequence of amino
acids.
proteoid root A cluster of hairy rootlets in some
Proteaceae and Fabaceae that forms a dense
mat at the soil surface to enhance nutrient
uptake in nutrient-poor soil.
proteomics Techniques used to identify and
study groups of proteins.
prothoracicotropic hormone A protein secreted
by neuroendocrine cells in the insect brain; it
is the first in a suite of hormones that control
the moulting process.
Protista The kingdom of eukaryotes that excludes
plants, fungi and animals; it includes many
unicellular aquatic organisms, free-living or
parasitic, but also large multicellular seaweeds;
it is not a monophyletic taxon.
protoderm A type of primary plant tissue
meristem that generates the outermost
dermal tissue.
protogyny In animals, a situation in which an
individual starts life as a female, changing
to a male at some later stage; in plants, a
condition where a flower firsts opens in the
female phase (stigma receptive) and later
becomes male (anthers dehisce).
proton A positively charged particle of an
atomic nucleus; a hydrogen ion.
proton pump A protein channel in a membrane
of the cell that expends energy to transport
protons against a concentration gradient;
involved in the chemiosmotic generation of
ATP.
protonema A mass of branched filaments that is
the first gametophytic plant in the life cycle
of a moss resulting from the germination of a
haploid spore.
protonephridium (pl. protonephridia) The
tubular excretory organ of animals that has
a flame cell to filter coelomic fluid into the
tubule for excretion.
proto-oncogene A normal cellular gene that
when mutated leads to tumour formation.
protoplasm The cytoplasm and nucleus of
eukaryotic cells.
protostome Animal in which the blastopore
becomes the mouth (e.g. flatworms, annelids,
molluscs, arthropods).
Prototheria A subclass of mammals with one
order, the Monetramata (the echidna and the
platypus); the only egg-laying mammals.
protoxylem The first-formed and earliest
maturing xylem tissue that forms towards the
centre of the stem in plants.
protozoa A term commonly used to describe
diverse heterotrophic protists.
proximal convoluted tubule The first part of
the vertebrate nephron tubule, responsible
primarily for reabsorption of solutes and water.
proximate explanation An explanation that
considers the causation of a particular
behaviour.
pseudocoel The body cavity of an animal, such
as a nematode, that is not lined on all sides
by mesoderm.
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pseudocoelomate An animal with a
pseudocoelom.
pseudomurien A component of the cell wall
of archaea; it is similar to peptidoglycan in
structure and function but contains different
components.
pseudoplasmodium A mass of amoebae that
aggregate to form a ‘slug’ in the life cycle of
a cellular slime mould; it is similar to the
plasmodium of acellular slime moulds.
pseudopodium (pl. pseudopodia) The
protrusion of part of the protoplasm of an
amoeboid cell used for locomotion, ingestion
of food, etc.
Psilophyta The phylum of fork ferns, the most
primitive living vascular plants.
psychrophile An organism, such as a fungus,
that can live in cold conditions.
pteridophytes A phylum of vascular plants
having euphylls, but not seeds; Pteridophyta.
Pterygota A class of insects; all have wings.
pulmocutaneous circulation The routing of
blood from the heart through different vessels
to the gas exchange organs (lungs and skin)
of frogs and some other amphibians.
pulmonary circuit The pathway from the
right ventricle of the heart to the lungs and
back to the left atrium; it is responsible for
oxygenating the blood.
pulvinus A motility organ at the base of the leaf
petiole in certain plants, such as legumes,
that controls the position of the leaf.
pump A transporter that directly couples its
conformational changes to an energy source,
such as ATP hydrolysis.
pupa A developmental stage in some insect
life cycles between the larva and adult;
non-feeding, immobile and sometimes
encapsulated or in a cocoon.
pure-breeding Strains in which individuals and
their progeny have the same phenotype.
purine A type of nitrogenous organic base
(nucleotide), adenine and guanine, which
pairs with pyrimidine bases in DNA.
purinotely The pattern of nitrogenous waste
excretion where excess nitrogen (mainly from
digested protein) is excreted as purines (such
as uric acid, guanine and xanthine).
Purkinje fibres A group of fast-conducting
cells that stimulate the ventricles of the
mammalian heart to contract; these cells
originate from the atrioventricular bundle
and radiate throughout the ventricle muscle.
purple bacteria A type of photoheterotrophic
bacteria that uses sunlight for energy but
ready-made organic compounds as building
blocks; bacteria have a characteristic set of
photosynthetic pigments different from
plants.
pygidium The posterior, postsegmental part of
an annelid.
pygostyle In modern birds, a plate-like structure
formed from caudal (tail) vertebrae to which
specialised tail feathers are connected.
pyramid of biomass A measure of efficiency in
which the organisms at each trophic level are
weighed.
pyramid of numbers An expression of trophiclevel transfer efficiency, in which the number
of individuals decreases at each trophic level,
with a huge number of individuals at the
base and fewer individuals at the top.
pyrimidine A single ring molecule; it forms
three of the bases of nucleic acids—cytosine,
thymine and uracil.
pyruvate (C3H3O3) The end product of
glycolysis.
Q10 The measure of the sensitivity of
biochemical processes to temperature,
measured as the increase in the rate of a
physiological process or reaction for a 10°C
rise in temperature.
quantitative trait A trait that is determined
by the effects of more than one gene; such
a trait usually exhibits continuous variation
rather than discrete either/or values.
Quaternary period The geological period from
1.8 million years ago to the present.
quaternary structure The specific association
of several globular protein units to form a
functional protein.
quorum sensing A mechanism by which
prokaryotic cells are able communicate when
they reach a critical population size.
R plasmid A resistance plasmid; a conjugative
plasmid that picks up antibiotic resistance
genes and can therefore transfer resistance
from one bacterium to another.
r strategist Species that are opportunists, with
high population growth rates (r), large
numbers and short life cycles (such as plant
weeds or insect pests).
race A group of organisms having common
characteristics that distinguish them from
other members of the same species, usually
forming a geographically isolated group;
subspecies.
radial canal Extensions from the stomach of a
medusa (jellyfish) radiating out to the margin
of the bell; see also ring canal.
radial symmetry The symmetry of an organism
such that any plane passing through the
central axis bisects the organism into equal
halves (e.g. jellyfish).
radicle The root axis of a germinating seed.
radiometric dating The quantitative
determination of the level of radioactivity
used as a method to date the geological age
of rocks and fossils.
radula A tongue-like structure with rows of
rasping teeth; it is present in the floor of the
foregut of molluscs, except bivalves.
ram ventilation The flow of water or air used
for ventilation, which occurs by forward
locomotion.
random genetic drift A random change in the
frequency of alleles in a population over time.
random mating It occurs in a population if the
polymorphic character being studied has no
influence on the choice of mate.
random-mating population A population in
which the polymorphic character being
studied has no influence on the choice of
mate.
random sampling error The deviation between
the observed and expected outcomes.
raphe A longitudinal slit in the valve of a
pennate diatom; it is associated with the
gliding motion of diatoms.
rate Of a chemical reaction: velocity at which it
proceeds towards equilibrium.
rate-limiting step The slowest step in a
pathway.
ratite An ancient group of flightless birds,
including the living Southern Hemisphere
emu, cassowary, kiwi, ostrich, tinamou, rhea
and recently extinct moa; they lack a keel on
the sternum.
ray initial A type of meristematic cell produced
by the vascular cambium differentiating into
wood rays.
ray-finned fishes The Actinopterygii, which
includes all bony fishes except the coelacanths
and lungfish.
reabsorption A physiological process in animals
whereby solutes are actively transported from
the urine or the gut contents back into body
fluids.
reaction centre A specialised chlorophyll
complex functioning as a photosynthetic unit
capable of channelling energised electrons
to an acceptor molecule; it is located on the
thylakoid membranes of chloroplasts.
reading frame Refers to the idea that codons are
read from the start codon in groups of three
bases each.
realised distribution Of a species; the range
over which individuals live and reproduce.
realised niche A species’ actual range of
environmental factors in which it survives.
receptacle 1. The fluid-filled beads of brown
algae such as Hormosira that are resistant to
drying. 2. The stem of a flower to which all
the parts are attached.
receptor A molecule, usually a protein, in the
plasma membrane or within a cell, that
undergoes a change as a result of a specific
interaction with a signal, leading to a
particular response.
renal cortex The outer layer of the mammalian
and avian kidney, containing the glomeruli
and proximal and distal convoluted tubules;
it is responsible for solute and water
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reabsorption (but not osmotic concentration)
and is equivalent to the kidney of lower
vertebrates.
recessive A phenotype, such as green colour in
seeds, which is apparent in the homozygous
(yy) individuals, but absent in the
heterozygous (Yy) individuals.
recombinant An offspring that has a different
combination of traits from the parental
generation.
recombinant DNA Any DNA molecule that
has been manipulated so that it contains
DNA from two or more sources.
recombination The mixing of alleles of
different genes on homologous chromosomes
(homologues), caused by crossing-over
between homologues during meiosis.
red alga Common multicellular seaweed
containing the pigment phycoerythrin (in
addition to chlorophyll a and phycocyanin)
which gives them a red colour.
red blood cell A cell that serves the critical
function of transporting oxygen throughout
the body; also known as an erythrocyte.
red tide Discolouration of the sea caused by
a population explosion of dinoflagellates,
which may be toxic.
redia (pl. rediae) A larval stage in the life cycle
of parasitic flukes that develops within
sporocysts and forms cercariae.
redox reaction A type of reaction in which the
electron that is removed during the oxidation
of an atom or molecule must be transferred
to another atom or molecule, which becomes
reduced; short for a reduction-oxidation
reaction.
reduction A process that involves the addition
of electrons to an atom or molecule.
reduction division The first cell division during
meiosis (meiosis I), during which the genetic
information is reduced (from 2n to n) when
the homologous chromosomes align with
each other and then separate.
refractory period The short period after a
neuron has fired an action potential when it
cannot be excited to fire another one.
regeneration A form of asexual reproduction in
which a complete organism forms from small
fragments of its body.
regulate Of a gene: control of its expression,
occurring at a variety of different levels,
including transcription, mRNA transcript
stability, translation and protein stability or
modification.
regulated secretion Secretion that only occurs
in response to a specific signal.
regulative development Development in which
the fate of blastomeres is determined by
interactions with surrounding cells.
regulatory gene A gene whose function is to
regulate the expression of other genes.
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regulatory protein Any of a group of
proteins that modulates the ability of RNA
polymerase to bind to a promoter and begin
DNA transcription.
regulatory transcription factor A protein that
binds to DNA in the vicinity of a promoter
and affects the rate of transcription of one or
more nearby genes.
reinforcement In speciation, the process by
which partial reproductive isolation between
populations is increased by selection against
mating between members of the two
populations, eventually resulting in complete
reproductive isolation.
relative fitness A measure of fitness in which
the fitness of the fittest genotype is assigned
a value of unity (one), and the fitness
values for the other genotypes are adjusted
proportionally.
release factor A protein that recognises the
three stop codons in the termination stage of
translation and promotes the termination of
translation.
renal cortex The outer layer of the mammalian
and avian kidney, containing the glomeruli
and proximal and distal convoluted tubules;
it is responsible for solute and water
reabsorption (but not osmotic concentration)
and is equivalent to the kidney of lower
vertebrates.
renal medulla The inner layer of the
mammalian and avian kidney, containing
the loops of Henle and collecting ducts; it
is responsible for osmotic concentration of
urine by counter-current multiplication.
renin An enzyme secreted by the macula densa
cells of the juxtaglomerular apparatus of the
mammalian kidney; it converts circulating
angiotensinogen into angiotensin I (which is
converted to angiotensin II in the lungs).
repetitive DNA DNA that contains the same
nucleotide sequence multiple times in the
genome.
replication 1. The performing of experiments
several or many times. 2. The copying of
DNA strands.
replicon A unit region of replication in
chromosomes of eukaryotes; from an origin,
bidirectionally, to termination of DNA
replication in both directions.
replisome The macromolecular assembly of
enzymes involved in DNA replication;
analogous to the ribosome in protein
synthesis.
repressible operon In this type of operon, a
small effector molecule inhibits transcription.
repressor protein A protein that binds to the
operator sequence and prevents transcription
initiation.
reproduction The process by which organisms
produce offspring.
reproductive effort The investment of parents
in sexual reproduction, which is dependent
on their own survival and future reproductive
success.
reproductive isolating mechanism A
geographical, physical, physiological
or behavioural barrier preventing the
interbreeding of individuals from different
species or populations.
reproductive success The number of surviving
offspring produced by an individual.
Reptilia A traditional class of vertebrates
including snakes, lizards, crocodiles, etc.,
but a paraphyletic taxon.
resistance The ability to withstand an
environmental impact, for example to ward
off infection and disease; bacterial resistance
to antibiotics.
resistance gene (R gene) A plant gene that
has evolved as part of a defence system in
response to pathogen attack.
resistance vessel An arteriole; these small
vessels are surrounded with muscle that
contracts to narrow the vessel diameter and
restrict flow; they produce the major control
over the distribution of blood in various
tissues.
respiratory system All components of the
body that contribute to the exchange of
gas between the external environment and
the blood; in mammals, includes the nose,
mouth, airways, lungs and muscles and
connective tissues that encase these structures
within the thoracic (chest) cavity.
resting membrane potential The difference in
electrical potential across a plasma membrane
at electrochemical equilibrium.
restoration A process to produce (restore)
a healthy, natural, self-regulating ecosystem
to a state similar to that before human
disturbance.
restriction endonuclease (restriction
enzyme) An enzyme that recognises specific
sequences within a double-stranded DNA
molecule and cleaves the DNA.
restriction fragment length polymorphism
(RFLP) A restriction fragment that differs
in length in different individuals (i.e. it is
polymorphic) because of differences in the
DNA sequence.
restriction fragment length polymorphism
(RFLP) Restriction enzymes recognise very
specific DNA sequences. Alleles of the same
gene or surrounding sequences may have
base-pair differences, so that DNA near one
allele is cut into a different-length fragment
than DNA near the other allele. These
different fragments separate based on size on
electrophoresis gels.
restriction sites The base sequences recognised
by restriction enzymes.
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rete A network of interdigitated arteries
and veins that function for efficient
countercurrent exchange.
reticulopodium A network of cytoplasmic
strands that connect individual amoeboid
cells of chlorarachniophytes (k. Protista).
retina A sheet-like layer of photoreceptors at the
back of the eye.
retrotransposon A class of transposon related to
retroviruses; see transposon and retrovirus.
retrovirus An RNA virus that utilises reverse
transcription to produce viral DNA that can
be integrated into the host cell genome.
reverse transcriptase A viral enzyme that
catalyses the synthesis of viral DNA, starting
with viral RNA as a template.
reversible binding Respiratory pigments must
be able to bind and release oxygen.
rhabdite A rod-like structure secreted by
epidermal gland cells in free-living flatworms.
Rhesus antigen A human blood group antigen
that may cause haemolytic reactions,
especially during pregnancy; blood
containing this antigen is called Rh positive,
while that lacking it is Rh negative.
rhinarium The nose pad surrounding nostrils in
groups of mammals.
rhizobia The collective term for proteobacteria
involved in the nitrogen-fixation symbioses
with plants that are important in nature and
to agriculture.
rhizoid A short root-like structure that
anchors bryophytes and some fungi to
their substrate.
rhizome An underground stem.
rhizomorph Fungal hyphae growing together
like rope with a tough, darkly pigmented
outer sheath that makes them resistant to
drying.
rhizopod An amoeba that is able to transiently
produce extensions of the cell surface.
rhizosphere The soil zone immediately
surrounding the root hairs in plant roots
where interactions occur between plant, soil
and microorganisms.
Rhodophyta The phylum of red algae.
rhodopsin The visual pigment in rods.
rhopalia The sense organ of a jellyfish that
contains a statolith or sometimes an
eyespot.
rhynchocoel A coelomic body cavity that
houses the proboscis of a proboscis worm
(nemertine).
Rhyniophyta An extinct phylum of the earliest
land plants; fossils date from the Silurian and
Devonian periods
ribonucleic acid (RNA) (Also known as ribose
nucleic acid) One of two classes of nucleic
acids; the other is deoxyribonucleic acid
(DNA). RNA consists of a single strand of
nucleotides.
ribose A five-carbon sugar; a sugar component
of RNA.
ribosomal RNA (rRNA) An RNA that forms
part of ribosomes, which provide the site
where translation occurs.
ribosome A cytoplasmic organelle where protein
synthesis occurs; it is formed from two rRNA
subunits in association with an mRNA
molecule.
ribozyme An RNA molecule that acts as an
enzyme.
ribulose bisphosphate (RuBP) The five-carbon
sugar that binds to CO2 in the first step of
carbon fixation in photosynthesis.
ribulose bisphosphate carboxylase-oxygenase
(Rubisco) An enzyme that catalyses the
first step of carbon fixation (see RuBP); it
constitutes 50% of protein in chloroplasts.
right–left axis In bilateral animals, one of the
three axes along which the adult body pattern
is organised; the others are the dorsoventral
axis and the anteroposterior axis.
ring canal A structure in the water vascular
system of echinoderms.
RNA interference (RNAi) Refers to a type of
mRNA silencing; miRNA interferes with the
proper expression of an mRNA.
RNA polymerase The enzyme that is
responsible for transcribing rRNA genes
RNA splicing A nuclear process by which
intron sequences of a primary mRNA
transcript are cut out and the exon sequences
spliced together to give the correct linkages
of genetic information that will be used in
protein construction.
Rodinia The oldest known supercontinent; it
formed about 1.8 billion years ago and broke
up into eight continents 750–800 million
years ago.
root apical meristem (RAM) The region where
new root tissues of plants are produced.
root cap A protective covering on the root tip
that is produced by the root apical meristem
of a plant.
root effect An increase in renal osmotic
concentration in the mammalian kidney by
solute transport and gas secretion into the
teleost swimbladder.
root meristem The collection of cells at the root
tip that generate all of the tissues of a plant
root.
root nodule The outgrowth of roots in a wide
range of plants, especially legumes, in which
symbiotic nitrogen-fixing bacteria (Rhizobium
or Frankia) occur.
root pressure The positive pressure generated by
roots that is responsible for the exudation of
sap from tapped or cut stems in spring.
root system The collection of roots and root
branches produced by root apical
meristems.
root The usually descending axis of a plant,
normally below ground, which anchors the
plant and serves as the major point of entry
for water and minerals.
Ross River alphavirus An arbovirus
(arthropod-borne) widespread in Australia
that infects mammals and is spread by
mosquitoes; in humans, it causes fever and
joint swelling.
rostellum An apical protrusion of the
tapeworm scolex that is armed at its base
with hooks.
rough endoplasmic reticulum (RER) An
endoplasmic reticulum with attached
ribosomes; it is involved in synthesis of
proteins usually destined for export from
a cell.
rowing The action of propelling an object
across or through a fluid using oars or
oar-like appendages.
rubisco The enzyme that that catalyzes the first
step in the Calvin cycle in which CO2 is
incorporated into an organic molecule.
rumen An ‘extra stomach’ in cows and related
mammals wherein digestion of cellulose occurs
and from which partially digested material can
be ejected back into the mouth.
ruminate The regurgitation and re-chewing of
partially digested food in mammals of the
order Artiodactyla Suborder Ruminantia—
sheep, cattle and deer.
rural dieback The premature and usually rapid
decline and death of native trees in rural
Australia caused by a variety of factors.
S phase A phase of the cell cycle in which DNA
replication occurs.
sac-like gut A part of the gut that is expanded
into a voluminous sac that allows digesta
to be subject to extensive mixing and
fermentation.
salicylic acid In plants, an organic molecule that
is a long-distance signal in systemic-acquired
resistance.
salt gland A gland that can secrete a salt
solution that is more concentrated than
body fluids (e.g. in reptiles, birds and
mangroves).
saltatory conduction The conduction of action
potentials along myelinated axons involving
action potentials skipping from node to
node.
saprophyte An organism that lives on dead
organic matter.
sapwood The outer region of wood (secondary
xylem) in a tree trunk containing living rays;
of lesser strength than heartwood.
sarcolemma The specialised cell membrane in a
muscle cell.
sarcomere One compete unit of the repeating
pattern of thick and thin filaments within a
myofibril.
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sarcoplasmic reticulum A cellular organelle that
provides a muscle fiber’s source of the cytosolic
calcium involved in muscle contraction.
Sarcopterygii See coelacanth and lungfish.
satellite DNA DNA that contains large
numbers of small nucleotide sequences
repeated in tandem.
satellite nucleic acid A simple virus-like
infectious agent; a genome of ssRNA circles
of about 300–400 nucleotides in length; they
rely on a helper virus for their proteins and
transmission.
satellite virus A type of virus only able to
replicate in cells infected with a specific
helper virus.
Sauropsida One of the two major lineages of
amniotes; it includes turtles, tuatara, lizards,
snakes, crocodiles and birds; the other lineage
(sister group) is the Mammalia.
scavenger An animal feeding on carrion (dead
organisms).
schistosomulum A juvenile stage of the blood
fluke, Schistosoma.
scientific method A series of steps to test the
validity of a hypothesis. The experimentation
often involves a comparison between control
and experimental samples.
scientific publication A critical part of the
scientific method. Publication of findings in
scientific journals (including aims, experiments,
results and conclusions) after a process of peer
review (consultation with experts in the field)
and checking for compliance with agreed codes
of practice. Published work is widely available
and can influence the thinking and work of
other scientists.
sclereid A plant cell with a thick, lignified wall;
the stone cells of fruit, etc.
sclerenchyma Plant tissue that has a support
role; cells with thickened lignified secondary
walls that impart rigidity as well as strength;
it includes sclereids, branched or more-or-less
even-shaped stone cells that form the hard
tissue of fruits and seed coats and fibres;
elongate cells.
sclerophyll A plant characterised by rigid, often
small leaves, and short internodes; able to
survive low soil nutrients, water stress and fire.
sclerotium (pl. sclerotia) A hard, resistant
resting body, composed of masses of tightly
compacted mycelium, formed by some
soil-inhabiting fungi.
scolex An anterior attachment organ, with
suckers and often hooks of a tapeworm.
scutellum The single cotyledon of
monocotyledonous flowering plants; it forms
an interface tissue between the embryo and
endosperm.
scyphistoma The stage in the life cycle of a
jellyfish (class Scyphozoa) that produces small
medusae called ephyrae.
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Scyphozoa A class of cnidarians; jellyfish.
second law of thermodynamics The entropy
of the universe is increasing; thus an input
of energy is needed to maintain the ordered
state of the universe.
second messengers Small molecules or ions that
relay signals inside the cell.
secondary cell wall An additional strengthening
layer added to plant cell walls during
development to make them more rigid; its
main component is lignin.
secondary consumer An organism that eats
primary consumers; also called a carnivore.
secondary growth In vascular plants, an
increase in stem and root diameter made
possible by cell division of the lateral
meristems.
secondary lymphoid organ An organ where the
immune response is induced, for example the
spleen and lymph nodes.
secondary metabolism Involves the synthesis
of chemicals that are not essential for
cell structure and growth and are usually
not required for cell survival, but are
advantageous to the organism.
secondary metabolite Molecules that are
produced by secondary metabolism.
secondary phloem The phloem tissue formed
from the vascular cambium during secondary
growth.
secondary production The measure of
production of heterotrophs and
decomposers.
secondary response The larger, more rapid
immune response to a particular antigen as a
result of immunological memory due to an
earlier primary immune response.
secondary structure The structure of a protein
that results from fold-backs (b-sheets) or coils
(a-helices).
secondary xylem The xylem tissue formed from
the vascular cambium during secondary
growth.
secretin A hormone released by gastrointestinal
mucosa in response to acid into local blood
vessels; the target organ is the pancreas where
it acts to optimise the release of digestive
secretions.
secretion The process by which cells release ions
and/or biochemicals in solution, for example
secretion of digestive enzymes, hormones,
saliva and bile.
seed A structure from which a new plant
develops; it is produced from a fertilised
ovule, containing an embryo, a food source
(cotyledons or endosperm) and usually a hard
outer seed coat (testa).
seed bank Ungerminated seeds in the soil of an
area. Regeneration of plants after events such
as fire often depends on the presence of a
seed bank.
seed coat (testa) A protective layer that
surrounds a seed; the seed coat develops from
the integuments.
seed dormancy A condition of arrested growth;
commencement of growth requiring special
environmental cues.
segmentation The division of the developing
animal body into repeated units;
segmentation allows for redundant systems
and more efficient locomotion.
segment-polarity gene A gene expressed in a
portion of each segment and required for
formation of proper segmental structures.
selectable marker A gene whose presence
can allow organisms (such as bacteria) to
grow under a certain set of conditions. For
example, an antibiotic resistance gene is a
selectable marker that allows bacteria to grow
in the presence of the antibiotic.
selection The process by which some
organisms leave more offspring than
competing ones, and their genetic traits
tend to appear in greater proportions among
members of succeeding generations than the
traits of those individuals that leave fewer
offspring.
selective breeding Programs and procedures
designed to modify traits in domesticated
species.
selective permeability The ability of membranes
to allow passage of some molecules and not
others.
self-fertilisation Fertilisation resulting from
self-pollination.
self-incompatibility (SI) Rejection of pollen
that is genetically too similar to the pistil of
a plant.
self-pollination Pollination by pollen from the
same flower or by pollen from another flower
on the same plant.
self-thinning rule When growing plants in a
plot (e.g. a crop), the average weight of an
individual becomes related to the density of
survivors.
semiconservative replication The mode
of DNA replication employed by living
organisms, in which each strand of a DNA
molecule is used as a template to produce
two new DNA molecules, each of which
contains one strand derived from the parent
and one that is newly synthesised.
semilunar valve A bicuspid valve between the
right ventricle and pulmonary artery in
mammals.
senescent Cells that have doubled many times
and have reached a point where they have
lost the capacity to divide any further.
sensory neuron A neuron that detects a sensory
stimulus.
sepal A flower organ that often functions to
protect the unopened flower bud.
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septum (pl. septa) A wall that divides, or
partially divides, a structure or cavity; a crosswall in fungal hypha; a septum dividing the
ventricle of the heart.
sequencing (nucleotides and amino
acids) Determining the order of amino acids
in a polypeptide chain or of nucleotides in a
DNA or RNA molecule.
sere The orderly sequence of ecological
communities that replace one another
over time.
serum The yellowish fluid isolated from
clotted blood; plasma without some of the
constituents bound to the clot.
seta (pl. setae) Chitinous bristle.
sex Often used as a synonym for sexual
reproduction.
sex chromosomes Chromosomes that differ in
morphology yet can pair and segregate during
meiosis; they are present in different numbers
in males and females and are involved in sex
determination.
sexual reproduction Formation of offspring
by the fusion of haploid gametes from two
different organisms.
sexual selection The differential ability of
individuals to acquire mates; it involves
contests between males or choice by females;
it leads to the selection of morphological
or behavioural traits relating to attracting
mates.
shared derived character A trait that is shared
by a group of organisms but not by a distant
common ancestor.
shared primitive character A trait shared with
a distant ancestor.
shivering thermogenesis Rapid muscle
contractions in an animal, without
any locomotion, in order to raise body
temperature.
shoot apical meristem The meristem at the
apex of the shoot.
shoot system The collection of plant organs
produced by shoot apical meristems.
short-day plant A plant that flowers only when
the night length is longer than a defined
period. Such night lengths occur in late
summer, autumn or winter, when days
are short.
sieve tube element A series of sieve-tube cells
arranged end to end and interconnected
via sieve plates; it is responsible for the
translocation of nutrients, especially sugars.
signal A stimulus, chemical (e.g. food or
hormones) or physical (e.g. light or heat),
that can be detected by cells, leading to a
particular response.
signal sequence A sequence within a protein
that is recognised by a receptor within
the cell so that the protein is targeted to a
particular site within the cell.
signal transduction mechanism The mechanism
by which a group of proteins converts an
initial signal to a different signal inside
the cell.
silencer A sequence in a eukaryotic gene that
binds transcription factors that repress
transcription.
silent mutation A gene mutation that does
not alter the amino acid sequence of the
polypeptide, even though the nucleotide
sequence has changed.
silicoflagellate A chrysophyte (golden flagellate)
with a star-shaped silica skeleton.
silk An extracellular b-sheet protein with high
strength and flexibility; it is secreted by
spiders and moths.
simple fruits Fruits that develop from one
carpel.
simple leaf A leaf with a single lamina.
simple pit A pore in the secondary wall of some
plant cells in which the pit diameter remains
more-or-less constant.
single-strand binding protein A protein that
binds to both of the single strands of parental
DNA and prevents them from re-forming a
double helix.
sinoatrial node The heart’s ‘pacemaker’; a small
group of non-contractible muscle cells in
the right atrium of higher vertebrates that
initiates the cardiac cycle; it is evolutionarily
derived from the sinus venosus of fishes.
sinus A cavity or space in tissues or in bone.
sinus venosus The first chamber of the fish
heart that collects blood from the major veins
and leads to the atrium.
siphon A specialised funnel in cephalopods that
can produce a jet of water for propulsion.
siphuncle A structure that regulates the amount
of gas and buoyancy in some cephalopods.
slash-and-burn agriculture A traditional
agricultural practice involving the clearing of
a small area of forest and burning the fallen
timber, releasing nutrients to support a crop.
sliding filament model A model of mechanism
of contraction of skeletal muscle, which
proposes that the relative motion between
actin and myosin myofilaments within
muscle cells produces shortening of
individual cells, resulting in muscle
contraction.
slime mould Amoeboid protists that produce
fruiting bodies and absorb nutrients from
their environment in a way similar, but
unrelated, to fungi.
small effector molecule With regard to
transcription, refers to a molecule that
exerts its effects by binding to a regulatory
transcription factor and causing a
conformational change in the protein.
smallpox A disease caused by variola virus (a
poxvirus); a cause of death of many humans
in the past with the virus spread mainly
by rodents.
smooth endoplasmic reticulum
(SER) Endoplasmic reticulum that lacks
attached ribosomes.
smooth muscle Spindle-shaped contractile
cells with a central nucleus and less regular
arrangement of myofilaments than striated
muscle cells; it lines the walls of internal
organs, arteries and veins and is under
involuntary control.
soaring An extremely energy-efficient form of
gliding, in which an animal uses air currents
to remain airborne, rather than slowly losing
height, as in gliding.
sodium–potassium pump Transmembrane
channels engaged in the active (ATP-driven)
transport of Na+, exchanging them for K+,
where both ions are being moved against
their respective concentration gradients;
maintains the resting membrane potential of
neurons and other cells.
soil acidification An induced increased soil
acidity, for example by overuse of fertilisers
in agriculture.
solute A molecule dissolved in a liquid.
solute potential (S) The osmotic potential;
an element in the water potential equation.
solvent The liquid medium in which a solute is
dissolved.
soma The middle, segmented part of an
annelid; the body of a neuron as distinct
from the dendrites and the axons.
somatic cells All cells in the body of a
multicellular organism other than germ
cells.
somatic embryogenesis The production of plant
embryos from body (somatic) cells.
somatostatin The peptide released by gamma
cells of the islets of Langerhans; it appears
to inhibit neighbouring a and b cells in a
paracrine fashion.
somite A block of mesoderm that forms
adjacent to the notochord in the vertebrate
embryo.
soredium (pl. soredia) A structure of a lichen
analagous to a spore; it consists of an algal
cell embedded in fungal hyphae.
sorocarp A fruiting body produced during the
life cycle of slime moulds.
sorus (pl. sori) A cluster of sporangia on the
margins or undersurface of a fern frond.
space constant The distance along a neuronal
membrane that it takes for an applied voltage
to fall to 1/e (about 37%) of its original
value.
speciation The formation of new species;
see allopatric, sympatric and parapatric
speciation.
species A group of related organisms that share
a distinctive form in nature.
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species concepts Different approaches for
distinguishing species, including the
phylogenetic, biological, evolutionary and
ecological species concepts.
species richness The number of species within
a community.
specific dynamic effect Increase in metabolic
rate during digestion.
specific heat The amount of heat that must be
absorbed or lost by 1 g of a substance to raise
or lower its temperature 1°C.
specificity Particularity (as opposed to
generality).
spectrin A scaffold of proteins that links plasma
membrane proteins to actin filaments in the
cytoplasm of red blood cells, producing their
characteristic biconcave shape.
sperm The male gamete.
sperm competition The competition for
fertilisation success between the ejaculates
of two or more males that are found
simultaneously within a female’s
reproductive tract.
spermatheca (pl. spermathecae) A small sac
containing sperm.
spermatocyte The male germ cell in the process
of meiosis within the testis.
spermatogenesis The process by which sperm
form from primordial germ cells.
spermatogonium A diploid male germ cell in
the process of mitosis in the testis.
spermatophore A structure enclosing many
sperm; see also spermatheca.
spermatozoa The male gamete, usually smaller
than the female gamete, and usually motile.
S-phase-promoting factor A specific cyclin and
cyclin-dependent protein complex whose
acitivity drives a cell from the G1 phase into
the S phase.
Sphenophyta The phylum of horsetails, early
vascular plants known mostly as fossils; one
living genus Equisetum.
sphincter In vertebrate animals, a ring-shaped
muscle capable of closing a tubular opening
by constriction (e.g., between stomach and
small intestine or between anus and
exterior).
spicule The skeletal component in sponges,
composed of calcium carbonate or silica.
spindle The structure composed of
microtubules radiating from the poles of the
dividing cell that will ultimately guide the
sister chromatids to the two poles.
spinneret A spider’s abdominal silk gland; also
found in the mouths of caterpillars.
spiracle (pl. spiracles) The small external
opening of the air-filled gas-exchange system
(tracheae) of spiders and insects.
spirilli Rigid, spiral-shaped prokaryotic cells.
spirochaetes Flexible, spiral-shaped prokaryotic
cells.
1186
splicing The process whereby regions of RNA
(introns) from the primary transcript are
removed and flanking exons are joined.
sponge The simple marine animals (p. Porifera)
that are ordered aggregations of cells but lack
tissues or organs; they are characterised by
collar cells; see choanocytes.
spongin The coarse collagenous proteinaceous
material forming skeletal fibres in sponges.
spongocoel The internal cavity of a sponge; it is
also called the atrium.
spongy mesophyll Irregularly arranged leaf
mesophyll cells with conspicuous intercellular
spaces.
spongy parenchyma Photosynthetic tissue of
the plant leaf mesophyll that contains cells
separated by abundant air spaces.
sporangiophore The stalk of a fungus that bears
a sporangiospore and spores.
sporangiospore A haploid asexual spore that
develops in a sporangium.
sporangium A sac-like cell or multicellular
structure in which asexual spores form.
spore A cell capable of producing a new
individual; it is often a dormant resistant
structure or functioning in dispersal.
spore capsule See sporangium.
sporocyst A sac-like structure in the life cycle of
parasitic flukes that produces rediae.
sporophyte The diploid stage of a plant life
cycle that produces spores.
sporopollenin A polymer, tougher than lignin
but with similar properties, composed chiefly
of carotenoids; it makes spores and pollen
grains of plants resistant to biodegradation.
sporozoite The stage in the life cycle of the
malarial parasite that has the apical complex
(see apical complex) and passes from the
salivary gland of the mosquito host to the
vertebrate host blood stream.
sporulation The production of spores.
stabilising selection A pattern of natural
selection that favours the survival of
individuals with intermediate phenotypes.
stamen A flower structure that makes the male
gametophyte, pollen.
standard free energy change The free energy
change in a biological reaction under
standard conditions (25°C, 1 atm pressure,
pH 7.0 and initial solute concentrations of
1 M).
Starling principle In capillary exchange, the
net fluid movement between the capillary
and the interstitial fluid is determined by
the balance between the hydrostatic pressure
and the colloid osmotic pressure across the
capillary wall.
start A stage of the cell cycle during the G1
phase in which a cell commits to progressing
through the remainder of the cell cycle and
dividing.
statocyst The balance organ.
statolith A small calcareous body found in
sensory organs for balance.
stele The central vascular cylinder of stems and
roots.
stem The main part of the aerial shoot of
plants, usually bearing leaves, lateral branches
and reproductive organs.
stem cell A cell that divides and supplies the
cells that construct the bodies of all animals
and plants.
stenohaline An animal that has little tolerance
to change in its external environment due to
a reliance on passive homeostasis.
stenothermal A thermoregulating ectotherm
whose body temperature is kept within
narrow limits (while active); a constant body
temperature optimises thermal conditions
for physiological processes, but with minimal
energetic costs.
sternum The breast bone; in birds, it is
enlarged, with a bony keel for the attachment
of pectoral muscles giving power for flight.
steroid The family of molecules that are
derivative of polyisoprenoid lipids, with a
multiple ring structure.
sticky ends Single-stranded ends of DNA
fragments that will hydrogen-bond to each
other due to their complementary sequences.
stigma The terminal cells of the pistil of a
flower that receive and recognise pollen
grains during interactions that may lead to
fertilisation.
stigma, of Euglena A small red organ in the
Euglena cell involved in the detection of
light.
stipe A stalk connecting the blades of a brown
alga to the holdfast.
stipules Leaf-like appendages that occur at the
base of some flowering plant leaves or stems.
stochastic model A model (e.g. in population
ecology) including chance, random variables.
stolon A horizontally growing stem or runner.
stoma (pl. stomata) A specialised pore in the
epidermis of leaves and stems that allows
uptake of CO2 from the atmosphere for
photosynthesis; the pore through which
transpiration occurs.
stomium The zone of thin-walled cells where
sporangia or anthers rupture to release spores
or pollen grains.
stonewort A type of green alga, a charophyte,
encrusted with calcite (CaCO3); it is related
to land plants.
stop codon One of three three-base sequences—
UAA, UAG, and UGA—that signals the end
of translation.
storage parenchyma The tissue of parenchyma
cells containing storage reserves such as starch
granules, lipid droplets or protein storage
organelles.
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stratification The process of exposing seeds
to low temperatures for an extended period
before attempting to germinate them at
warmer temperatures.
Strepsirhini Lemurs and lorises, primates that
have a naked rhinarium (nose pad) and
nostrils that are slit-like (hence the name
Strepsirhini).
stress A term used to describe a force acting
on, or within, a body, tending to deform it,
that incorporates a measure of the area over
which the force acts; it is usually measured in
newtons (N) per square metre (m–2) (equals
Pascals, Pa; where 1 N m–2 = 1 Pa).
striated muscle A muscle with a highly
organised array of actin and myosin filaments
giving the appearance of cross-striations when
viewed under the light microscope; includes
skeletal and cardiac muscle.
strobila The tape or body of a tapeworm.
strobilisation Asexual reproduction in some
animals; the process of splitting the body
to produce small medusae in jellyfish; the
process of adding new proglottids in the
growth of a tapeworm.
strobilus (pl. strobili) In plants: a cone, which
is a collection of sporangia. The body of a
tapeworm.
stroma (fungal) A mat of fungal hyphae bearing
spores.
stroma (of chloroplast) A matrix enclosed
within an inner membrane.
stromatolite A concentrically layered rock, the
layers being formed by the successive growth
of thin mats of cyanobacteria; fossil and
present day.
structural diversity The diversity of a plant
community measured in terms of the size and
shape of plants irrespective of the species.
structural gene Refers to most genes, which
produce an mRNA molecule that contains
the information to specify a polypeptide with
a particular amino acid sequence.
style The pathway for pollen tubes between
stigma and ovary in the pistil of flowering
plants; it may comprise solid transmitting
tissue or a canal.
stylet A sharp, piercing organ in the mouth of
nematodes and some insects.
subduction The descent of sea floor back into
the earth’s mantle (at deep-sea trenches).
submetacentric A chromosome in which the
centromere is off centre.
subspecies A geographically defined
population or group of populations
within a single species that has distinctive
characteristics.
substitution A mutation in which a nucleotide
in a particular position is changed to a
different nucleotide.
substrate The reactants of a chemical reaction.
substrate-binding amino acid An amino acid
whose R-groups lining the active site of
an enzyme are concerned with the specific
binding and orientation of substrate
molecules.
succession The process of replacement over
time of one ecological community by
another.
succulent A xerophytic plant with fleshy
leaves or stems and highly mucilaginous
cell sap.
sucker An underground shoot that arises
from the roots or lower stem of a plant and
emerges from the soil as a new plant.
sucrose A disaccharide; the transport and
storage form of carbohydrate (sugar) in
plants.
super kingdom A domain; the highest grouping
of living organisms (an informal taxonomic
rank).
supercool To cool a fluid so that it remains
liquid below its nominal freezing point.
supercooling The cooling of a fluid so that it
remains liquid below its nominal freezing
point.
surface tension The surface tension at an
air-liquid interface is the result of
intramolecular attractive forces in the liquid,
providing the potential energy that draws
molecules from the surface and therefore
shrinks the interfacial area.
suspensor A filament of cells below an embryo
of flowering plants that connects it to the
ovule.
swimmeret An abdominal appendage in a
crustacean that provides movement.
symbiosis Interactions in which two organisms
(symbionts) live together in a close
relationship that is beneficial to at least one
of them.
symbiotic microorganism As symbiosis refers
to a relationship between two organisms in
which both partners derive some benefit
from the relationship, microorganisms that
live naturally in the gut and that provide
some benefit to the host animal are therefore
symbiotic microorganisms.
sympathetic nervous system The division of the
autonomic nervous system that innervates the
enteric nervous system and controls vascular
changes in organs.
sympatric speciation Populations specialising
on different resources diverge and form new
species without geographic isolation.
symplast The portion of a plant tissue
constituted by the protoplasm and vacuoles
of the tissue cells collectively.
symplastic pathway The pathway in plants for
the uptake of water, solutes and ions from
cell to cell via the cytosol; the only pathway
for crossing the endodermis of roots.
symporter A type of transporter that binds two
or more ions or molecules and transports
them in the same direction.
synapomorphy In systematics, a derived
character that is shared by clade members.
synapse A small area of close contact between
an axon terminal and a post-synaptic cell
across which information is transmitted,
usually by chemical neurotransmitters; it may
be excitatory or inhibitory, and also electrical.
synapsis The pairing of homologous
chromosomes during prophase of meiosis I.
synaptic vesicle A vesical in the presynaptic
terminal that contains transmitter chemicals.
synaptonemal complex The molecular scaffold
on which crossing over occurs between paired
chromosomes during prophase of meiosis I.
synergid In flowering plants, one of a pair of
cells adjacent to the egg at the micropylar
end of an embryo sac, one of which acts to
receive the pollen tube.
synonymous codons Codons of different
sequence that encode the same amino acid.
synonymous mutation A change from one
codon for an amino acid to another codon
for the same amino acid; a neutral variation
that has little effect on the survival or
reproduction of the carrier.
systematics The field of biology that studies the
phylogenetic relationships and classification
of organisms.
systemic acquired resistance (SAR) A wholeplant defensive response to pathogenic
microorganisms.
systemic circuit The pathway from the left
ventricle of the heart to the body and back
to the right atrium; it is responsible for
delivering nutrients, etc., to the body.
systole The phase of the cardiac cycle involving
a muscle contraction and ejection of blood
from a heart chamber.
T lymphocyte (T cell) A lymphocyte that
matures in the thymus and recognises antigen
by means of the T-cell receptor; it functions
independently to kill microorganisms and
controls B-cell responses.
tagma (pl. tagmata) A structure formed by the
fusion of segments in arthropods.
tagmatisation The organisation of segments
into groups with differing structures and
functions.
tapetum The inner layer of the anther wall
of a flower, comprising cells dedicated to
nutrition of the developing microspores.
taproot system The root system of eudicots,
which has one main root with many branch
roots.
Taq polymerase A heat-stable form of DNA
polymerase; one of several reagents required
for synthesis of DNA via polymerase chain
reaction (PCR).
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target cell A cell that responds specifically to a
particular hormone by means of specialised
receptor molecules located on the surface or
inside the cell.
TATA box One of three features found in most
promoters; one of several reagents required
for synthesis of DNA via polymerase chain
reaction (PCR).
taxon (pl. taxa) Any formal name (rank) in
the classification of living organisms (e.g.
phylum, class, order, family, genus and
species).
taxonomy The methods and principles of
classification of organisms.
T cell A type of lymphocyte that directly kills
infected, mutated or transplanted cells.
T-cell receptor (TCR) A dimeric receptor on
a T cell that recognises antigen epitopes
presented by MHC.
tegument An outer resistant body coat
of parasitic animals such as flukes and
tapeworms.
teleomorph The sexual form of a fungus.
teleost A ray-finned fish; a diverse group of
20 000 species, including catfishes, trout,
cod, etc.
telocentric A chromosome in which the
centromere is at the end.
telomerase An RNA-dependent DNA
polymerase that extends the 3′ end of DNA
molecules in chromosomes, which would
otherwise shorten because of the strictly 5′
to 3′ activity of DNA polymerase and the
requirement for an RNA primer.
telomere The DNA sequence at the ends of
chromosomes of eukaryotes.
telophase The final phase of mitosis, in which
new nuclear envelopes form, surrounding
each of the two newly separated groups of
chromosomes.
telson The posterior tagma (tail end) of
arthropods, which bears the anus.
template A DNA strand on which a
complementary DNA strand is synthesised.
temporal isolation When populations or species
do not interbreed as a result of differences
in the timing of reproduction; for example,
related, co-occurring plant species that have
different flowering times.
terminally differentiated A cell that has ceased
dividing and has irreversibly differentiated
into a particular cell type.
terminator A sequence that specifies the end of
transcription.
terminus The sequence of DNA that terminates
replication.
territorial behaviour Where an individual
defends an area that contains a resource
against other individuals.
tertiary consumer An organism that feeds on
secondary consumers.
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tertiary structure The final folded threedimenional shape of a protein, either
globular or extended rods (fibrous proteins).
test See foram.
testcross A cross to a homozygous recessive
individual.
testis (pl. testes) Specialised organ in a male
where the germ cells undergo differentiation
into sperm, and hormones are produced to
regulate reproductive function.
tetanus A condition of skeletal muscle in which
it produces a continuous maximal contractile
force due to stimuli that cause repeated
contraction without pause.
tetraploid An organism or cell that has four sets
of chromosomes.
tetrapod A land vertebrate with four limbs
(‘four footed’) with separate digits.
thallus Body of an alga, fungus or plant that
lacks special tissue systems or organs.
thecodonts Of mammals and certain reptiles:
having teeth that grow in sockets. Also
any extinct Triassic reptile of the order
Thecodontia, which had teeth set in sockets
and gave rise to the dinosaurs, crocodiles,
pterodactyls and birds.
theory A general principle based on hypotheses
that have survived falsification tests and
explain all the observations.
therapsids Any extinct Permian to Triassic
reptile of the order Therapsida, considered
to be the ancestors of mammals.
Theria A subclass of mammals that includes
Metatheria (marsupials) and Eutheria
(so-called placentals).
thermal conductance An endotherm’s capacity
for heat exchange with the environment.
thermals The rising bodies of relatively warm
air that are used by glider aircraft and some
birds to gain height.
thermoconform Of animals, to possess the same
body temperature as their environment.
thermodynamics The study of energy
interconversions.
thermoneutral zone The range of ambient
temperatures where metabolic rate is minimal
and constant (basal metabolic rate) and
the animal does not have to use energy to
regulate body temperature.
thermophile An organism that grows best in
hot conditions between 30°C and 50°C.
thermoreceptor A type of receptor that detects
heat or cold.
thermoregulate Of animals: to maintain a
constant body temperature.
thigmotherm An ectotherm that uses
conductive heat gain as its major mechanism
to increase body temperature above ambient
air temperature (for example pressing against
warm rocks).
thigmotropism Touch responses in plants.
threatened species Those species that are likely
to become endangered in the future.
threshold potential The potential difference
across a membrane at which certain voltagedependent channels (usually sodium
and occasionally calcium) increase their
permeability.
thrombin An enzyme that converts soluble
fibrinogen into insoluble filaments of fibrin
during the formation of a blood clot.
thrombocyte A cell fragment in mammals
(platelet) or a whole cell in other vertebrates,
involved in blood clot formation by adhering
to fibrin strands in a wound.
thrombus A blood clot.
thylacine A tasmanian tiger or marsupial wolf,
Thylacinus cynocephalus; it is presumed
extinct.
thylakoid The flattened disc-like sac that
forms part of the internal membrane system
of chloroplasts; the site of location of
photosynthetic pigments.
thylakoid lumen The fluid-filled compartment
within the thylakoid.
thylakoid membrane The pigmented membrane
of a thylakoid.
thymine (T) A pyrimidine base found in DNA.
thymus A fleshy lymphoid organ sitting
atop the heart in humans where T cells
differentiate.
thyrotoxicosis A condition of elevated
metabolic rate caused by excessive thyroid
hormone activity.
thyroxine (T4) An inactive form of thyroid
hormone; the major product of the thyroid
gland; it is converted into active T3 in target
cells.
tidal ventilation A type of breathing in which
the lungs are inflated with air, and then the
chest muscles and diaphragm relax and recoil
back to their original positions as an animal
exhales. During exhalation, air leaves via the
same route that it entered during inhalation,
and no new oxygen is delivered to the
airways at that time.
tidal volume The volume of air that is normally
breathed in and out at rest.
tight (occluding) junction A connection that
provides an impenetrable seal between
adjacent cells and restricts the lateral
movement of membrane components.
tissue culture The growth of individual cells or
tissues of an organism in sterile culture.
tissue system In plants: any of the three types
of tissue; called a system because the tissue
extends throughout the roots and shoots.
tissue The association of many cells of the same
type, for example, muscle tissue.
tissue trophism The phenomenon whereby
certain bacteria only cause disease in certain
body locations.
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tobacco mosaic disease A plant disease caused
by the tobacco mosaic virus; it is spread via
the sap of plants; it causes the mottling of
tobacco leaves.
toll-like receptor A primitive pathogenrecognition molecule; stimulation leads to
production of antimicrobial defence peptides.
Contributes to the self-recognition system. In
vertebrates, also initiates and guides the specific
immune response.
tone The continual nervous stimulation of an
organ; in the circulation, for example, the
vagus nerve can continually reduce the heart
rate, or nervous stimulation of the arterioles
can continually constrict them and reduce
the flow rate.
tonicity The potential of a solution to move
water by osmosis across a semipermeable
membrane; it depends on the solute
concentrations of the solution and the
relative permeability of the membrane.
tonoplast The semipermeable lipoprotein
membrane that encloses the vacuole of a
plant cell and separates the cytoplasm from
the vacuole.
topoisomerase Any of a class of enzymes that
can change the topological state of DNA to
relieve torsion caused by unwinding.
torpor The strategy in the smallest endotherms
of lowering internal body temperature to just
a few degrees above that of the environment
in order to conserve energy.
totipotency The ability of plant cells to
regenerate a new plantlet from a single cell.
toxin A poisonous compound; a product of
certain microorganisms, plants (ricin), snakes
(venom) and poisonous fungi.
trace element A micronutrient required by
plants in small amounts, such as iron, zinc
and copper, to carry out various metabolic
functions.
trace fossil The preserved tracks, such as
footprints, of extinct organisms.
trachea 1. A sturdy tube arising from the
spiracles of an insect’s body and involved in
respiration. 2. The name of the tube leading
to the lungs of air-breathing vertebrates.
tracheal system In insects, a series of finely
branched air tubes called tracheae that lead
into the body from pores called spiracles.
tracheid A type of conducting cell in the
xylem of all vascular plants; see also vessel
element.
tracheole The finest tube arising from an
arthropod trachea, where gas exchange occurs
directly with body tissues.
trait A characteristic or phenotype.
trans-acting A DNA segment that does not
need to be adjacent to the gene(s) that it
regulates. The lac inducibility site is an
example of a trans-acting element.
transcription The synthesis of RNA from a
DNA template.
transcription factor A protein that interacts
with the regulatory sequences of genes to
control their transcription.
transcription unit A region of DNA from
which an RNA molecule is transcribed.
transduction The conversion of external energy,
such as sound or light, into electrical signals
in a sensory neuron; the transfer of DNA
from one bacterium to another by infection
with a virus (bacteriophage), which results in
gene transfer and genetic variation.
transfer cell A type of parenchyma cell in
plants; it is characterised by primary wall
ingrowths, resulting in the massive increase
in the surface area of the plasma membrane,
and allowing rapid transfer of molecules to
adjacent cells, especially the vascular system.
transfer RNA (tRNA) An RNA that carries
amino acids and is used to translate mRNA
into polypeptides.
transferase An enzyme that catalyses a reaction
where a chemical group or molecular unit is
moved from a donor substrate to an
acceptor.
transformation The process of introducing
DNA into a cell so that the DNA is stably
maintained within that cell.
transgenic The term, used to describe an
organism that carries genes that were
introduced using molecular techniques such
as gene cloning.
transition A base substitution (mutation) in
DNA in which a pyrimidine or purine is
replaced by another pyrimidine or purine
respectively.
transition state In a chemical reaction, a state
in which the original bonds have stretched
to their limit; once this state is reached, the
reaction can proceed to the formation of
products.
transition state activation See transition state.
translation The synthesis of a protein from a
mRNA template.
translocation The transport of assimilates
(sugar) in the phloem of vascular plants from
the site of production in leaves (source) to
other parts of the plant (sink).
transmembrane domain Hydrophobic region of
a transmembrane protein that anchors it in
the membrane. Often composed of a-helices,
but sometimes utilising b-pleated sheets to
form a barrel-shaped pore.
transpiration The loss of water from a plant
by evaporation through stomata in leaves;
requiring energy from incoming solar
radiation to vaporise water.
transport ATPase A transmembrane protein
that couples the hydrolysis of ATP to the
transport of a molecule across the membrane.
transporter A membrane protein that binds
a solute and undergoes a conformational
change to allow the movement of the solute
across a membrane.
transposable element A segment of DNA that
has the capacity to transpose to new sites
within the genome.
transposon Regions of DNA that exist in
multiple copies and can move around from
chromosome to chromosome.
transverse flagellum See dinoflagellate.
transverse septum The tissue (septum) that
divides the annelid coelom internally into
segments.
transversion A base substitution (mutation) in
DNA in which a pyrimidine is replaced by a
purine or vice versa.
Trematoda A class of platyhelminths
(flatworms); endoparasitic flukes.
triacylglycerol A simple lipid (neutral fat or
oil); an ester of glycerol and three long-chain
fatty acids.
trichome An outgrowth of the plant epidermis,
such as simple hairs, stinging hairs, glandular
hairs, scales and vesicles.
tricuspid valve The valve between the right
atrium and right ventricle in the heart of
mammals.
triglyceride (triacylglycerol) An individual fat
molecule, composed of a glycerol and three
fatty acids.
triiodothyronine (T3) An active form of
thyroid hormone; it stimulates growth and
development in immature vertebrates and
metabolic rate in mature vertebrates.
trilobite An extinct group of arthropods with
hard skeletal parts; the most common marine
mutlicellular animal of the early Cambrian.
triple fusion nucleus During double
fertilisation, the product of fusion between
the central cell from the embryo sac and one
of the two sperm cells from the pollen tube;
the product of this fusion develops into the
endosperm.
triplet A group of three bases that function as
a codon.
triploid An organism or cell that has three sets
of chromosomes.
trochophore The free-swimming ciliated
larva typical of protostome animals such as
polychaetes.
trophic level The position of an organism
in a food chain (e.g. primary producer or
first-order consumer).
trophic relationships Feeding relationships; the
most important relationships determining
ecosystem structure and functioning.
trophic-level transfer efficiency The amount of
energy at one trophic level that is acquired by
the trophic level above and incorporated into
biomass.
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trophoblast In vertebrate embryos, the outer
ectodermal layer of the blastodermic vesicle;
in mammals, it is part of the chorion and
attaches to the uterine wall.
trophozoite The stage in the life cycle of the
malarial parasite in which it eats the contents
of the red blood cells of the vertebrate host;
trophozoites divide regularly and induce lysis
of the red blood cells, causing the release of
toxins and the periodic fever and chills of
malaria.
tropism Response to an external stimulus.
Trp repressor A repressor protein in E. coli
that interacts allosterically with tryptophan
to change its conformation, and thus bind
to the operator and repress transcription
(the Trp repressor alone does not bind),
preventing the synthesis of tryptophan.
trypanosome A kinetoplast, unicellular
flagellate parasite; it causes African sleeping
sickness and Chaga’s disease; it is transmitted
by blood-sucking insects.
tube feet A short, tubular, external projection
of the body wall of echinoderms containing
an extension of the radial canals of the water
vascular system; it functions in gas exchange,
attachment, locomotion and catching prey.
tube nucleus The nucleus of the pollen tube.
tuber A thickened, fleshy, underground root
(e.g. Dahlia) or underground stem (e.g.
potato), which functions as a storage organ.
tubular floret A type of flower in a daisy head
(usually towards the centre) that has fused.
tubulin A protein that forms the major
cytoskeletal scaffolding elements,
microtubules; it is composed of equal
amounts of two forms: a- and b-tubulin.
tumour suppressor gene A normal cellular
gene that, when made non-functional by
mutation, leads to tumour formation.
tunic A supportive and protective ‘coat’ of
tunicates secreted by the ectoderm.
tunica In the shoot apex of flowering plants, the
outer one to three layers of the apical dome
of cells that contributes to leaf and flower
formation.
Turbellaria A class of platyhelminths; free-living
flatworms.
turgid The term used to describe a plant cell
whose cytosol is so full of water that the
plasma membrane presses right up against
the cell wall; as a result, turgid cells are firm
or swollen.
turgor The pressure developed within a walled
cell due to the uptake of water by osmosis.
turgor pressure The hydrostatic pressure within
a cell that has a cell wall.
tusk shells Any of various burrowing sea-shore
molluscs of the genus Dentalium and related
genera that have a long, narrow tubular shell
open at both ends; class Scaphopoda.
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twitch A single, usually relatively rapid,
transient force produced by muscle activation
following a short duration stimulus.
tympanum In some groups of insects, a thin
membrane associated with the tracheal air
sacs that functions as a sound receptor;
paired on each side of the abdomen.
ultimate explanation An explanation that is
concerned with the evolution and function
of a particular behaviour.
undulatory swimming The propulsion of
an animal through water as a result of a
sinusoidal wave motion propagated along
the length of the body or parts of the body
(e.g. the fins of some fishes).
uniporter A type of transporter that binds
a single molecule or ion and transports it
across the membrane.
unipotent Stem cells that produce only one
type of cell, for example, skin stem cells.
uniramous Unbranched limbs, such as occur in
insects.
upstream activator sequence (UAS) A
regulatory element that binds upstream of
the target gene.
uracil (U) A pyrimidine base found in RNA.
urediniospore An asexual spore of a rust fungus.
ureo-osmoconform The pattern of iono- and
osmo-regulation in marine vertebrate animals
where body fluids are maintained at a lower
ion concentration than sea water but at
the same osmotic concentration, with the
‘osmotic gap’ filled mainly by urea.
ureotely The pattern of nitrogenous waste
excretion where excess nitrogen (mainly from
digested protein) is excreted as urea.
ureter The drainage duct of the kidney that
leads to a storage bladder or directly to an
excretory pore.
uricotely The pattern of nitrogenous waste
excretion where excess nitrogen (mainly from
digested protein) is excreted as the purine,
uric acid.
urinary system The structures that collectively
act to filter blood or hemolymph and excrete
wastes while recapturing useful compounds.
In humans, it includes the two kidneys,
two ureters, the urinary bladder and the
urethra.
urine The liquid waste filtered from the blood
by the kidney and stored in the bladder
pending elimination through the urethra.
Urochordata The subphylum of chordates that
includes tunicates or sea squirts.
vaccination A deliberate infection (e.g. a strain
of smallpox virus) to give stable immunity to
a disease; immunisation.
vaccine A preparation of bacteria or virus
rendered harmless and administered in
order to induce immunity to subsequent
infection.
vacuole Specialised compartments found in
eukaryotic cells that function in storage, the
regulation of cell volume, and degradation.
valve, of diatoms See frustule.
van der Waals force A weak attractive force
between two polar atoms or molecules; it
arises due to the non-uniform distribution
of electric charge at any instant on an
atom.
variable A factor that influences a process,
outcome, or observation. In experiments,
scientists attempt to isolate variables to test
hypotheses.
variable numbers of tandem repeats
(VNTRs) Segments of DNA that differ
between individuals because they have
different numbers of direct repeats at a
particular locus.
variable region A domain within an
immunoglobulin that serves as the
antigen-binding site.
variation Differences in the phenotype or
genotype of individuals in a population;
phenotypic variation that has an underlying
genetic basis is the type of variation that can
lead to evolution.
varicella zoster herpesvirus The cause of the
diseases chickenpox and shingles; a type of
herpesvirus.
vascular bundle A prominent structural feature
of primary growth in shoots, roots and leaves,
comprising the transport system; it consists
of xylem and phloem.
vascular cambium A meristem responsible
for producing wood (secondary xylem and
secondary phloem).
vascular plant A plant that can transport water,
sugar and salts throughout the plant body via
xylem and phloem tissues.
vasomotor centre The centre in the brain that
controls the pressure and distribution of
blood by affecting the contraction of smooth
muscle in arterioles.
vasopressin A posterior pituitary (antidiuretic)
hormone that influences blood pressure and
water balance.
vector An agent, such as an insect, able to
transfer a pathogen from one organism to
another.
vector DNA DNA that carries an origin of
replication and a selectable marker gene into
which segments of donor DNA are ligated
to create recombinant DNA that can be
propagated in a host cell.
vegetal plate The first sign of gastrulation; a
flattening of the vegetal pole region caused
by a lengthening of epithelial cells.
vegetative cell The largest cell of a pollen grain
that contains the generative cell and, in some
cases, the sperm cells, and produces the
pollen tube at pollen germination.
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vegetative growth The production of new
tissues by the shoot apical meristem and root
apical meristem during seedling development
and growth of mature plants.
vein In animals: a large blood vessel channelling
blood towards the heart. In vascular plants:
a network of vascular bundles in the leaf
lamina.
veliger The second stage in the life cycle of
molluscs; it develops from the larva and
forms a foot, mantle and shell.
ventilation Convection of the external medium
at the site of gas exchange, which is generated
by the animal itself.
ventral Refers to the lower side of an animal.
ventricle A heart chamber with strong
muscular walls that develops most of the
force necessary to pump blood through the
circulatory system.
venule Small blood vessels that collect blood
from the capillaries.
vernalisation The induction of flowering
in certain plants by exposure to low
temperatures.
vertebra (pl. vertebrae) A segment of
backbone.
vertebrate A chordate animal (superclass
Gnathostomata) that has a backbone
composed of vertebrae, which develop
around and replace the notochord during
development.
vesicle A small membrane-enclosed sac within
a cell.
vesicle-mediated transport The bulk movement
of substances across a membrane following
the formation of membrane sacs that enclose
the substances.
vessel A series of vessel elements arranged
end to end in which the end walls become
partially or totally perforated to form a
tube-like structure.
vestigial organ A reduced and simpler structure
than a corresponding part in another
organism but with no apparent function; it
indicates relatedness of organisms.
vibrios Comma-shaped prokaryotic cells.
virion A complete mature virus particle, which
is metabolically inert and is the transmission
(infective) phase.
viroid An infectious agent that is virus-like but
lacks a protein coat.
virotype A classification based on the array of
virulence factors of a virus strain.
virulence The capacity of a microorganism to
cause disease.
virus A subcellular genetic parasite that
reproduces only in the cells of a susceptible
host and may cause disease.
vitelline membrane A tough, clear elastic
envelope found in many eggs and embryos;
it lies adjacent to the cell membrane.
viviparity A mode of sexual reproduction in
animals in which offspring develop inside
the maternal body and are released as live
young or eggs; in plants, seeds germinate
while still attached to the parent plant
(e.g. in mangroves).
voltage-gated channels Membrane ion channels
that open or close in response to changes in
voltage across the membrane.
Wallace’s Line The line dividing the IndoMalayasian and Austro-Malaysian faunas; it
runs between Bali and Lombok in the south
and Borneo and Sulawesi in the north.
water The one molecule that exists in
abundance as a liquid at the temperature
of the surface of the earth; the medium
in which other molecules interact and in
which life first evolved; a polar covalent
compound consisting of two hydrogen and
one oxygen atoms, with hydrogen bonding
capacity.
water cycle The circulation of water through
ecosystems on earth—the atmosphere,
precipitation, soil, ground water, waterways
and oceans.
water potential The total energy level of water,
which is the sum of the osmotic potential
and pressure potential of water.
water vascular system A system of coelomic
canals lined with ciliated cells and filled with
fluid; includes a circular water canal and
radial canals leading to tube feet; it is unique
to echinoderms; see also tube feet.
water-use efficiency The quantity of carbon
assimilated by a plant compared to
the quantity of water lost by it during
transpiration over the same period.
wattle Plant in the genus Acacia from the family
Mimosaceae. Legumes; characterised by
compound leaves that are often replaced by
phyllodes, and flowers with showy anthers.
wax Esters of fatty acids with long-chain
monohydric alcohols; the water-repellent
coating on plant leaves, animal fur, etc.
weed A plant growing where it is not wanted;
often an introduced species.
West Nile flavivirus (WNV) An arbovirus
(arthropod-borne) related to yellow fever;
it is widespread in Africa but also occurs in
other countries such as the US; it kills birds
and some humans.
white blood cell A cell that develops from the
inner parts (the marrow) of certain bones
in vertebrates; all white blood cells (known
as leukocytes) perform vital functions that
defend the body against infection and
disease.
whorls In a flower, four concentric rings of
sepals, petals, stamens and carpels.
wild-type The phenotype found in most
individuals in a population.
windkessel vessel A large artery near the heart; its
walls contain elastin and muscles that stretch
during ejection of blood into them when the
ventricle contracts (systole); during ventricular
relaxation (diastole), the energy in the stored
wall is transferred back into blood pressure;
this damps the pressure oscillations from
the heart and keeps arterial blood pressure
somewhat steady.
wound response In plants, a signalling
pathway initiated by leaf damage, such as
being chewed by a herbivore; leads to the
production of proteinase inhibitors that give
herbivores indigestion.
xerophyte A plant tolerant of dry conditions.
xerotolerance The tolerance of dry conditions.
xylem A plant vascular tissue that conducts
water, minerals and organic compounds.
yeast A fungus that can occur as a single cell
and that reproduces by budding.
yeast artificial chromosome (YAC) A vector used
to generate recombinant DNA molecules that
carry very large genomic fragments.
yolk sac An extraembryonic membrane in
the amniotic egg that encloses a stockpile
of nutrients, in the form of yolk, for the
developing embryo.
zeatin A natural cytokinin isolated from maize.
zone of elongation The area above the root
apical meristem of a plant where cells extend
by water uptake, thereby dramatically
increasing root length.
zone of maturation The area above the zone
of elongation in a plant where root cell
differentiation and tissue specialisation
occur.
zoochlorellae See zooxanthella.
zooplankton Aquatic organisms, including
minute animals consisting of some worms,
copepods, tiny jellyfish and the small larvae
of invertebrates and fish that graze on the
phytoplankton.
zoospore A motile spore.
zooxanthella (pl. zooxanthellae) Dinoflagellate
endosymbiont found in the tissues of corals,
sea anemones and molluscs.
zygomorphic A flower with parts arranged
asymmetrically.
Zygomycota A phylum of fungi; zygomycetes
have coenocytic hyphae and produce large,
distinctive zygospores as a result of sexual
reproduction; for example the bread mould
Rhizopus.
zygospore A dormant spore with a thick cell
wall enclosing a zygote.
zygote A diploid cell resulting from the fusion
of the male and female gametes.
zygotic gene A gene that is transcribed and acts
in the developing zygote.
zymogen An inactive precursor of a protease
(an enzyme that breaks down proteins).
Glos s ary
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