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G L O S S A R Y Pronunciations for difficult terms appear online at www.mhhe.com/au/knox4e Glossary copyright McGraw-Hill mater 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. Glos s ary KNOX Endmatter.indd 1139 1139 7/8/09 8:10:28 AM 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. 1140 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. G lo s s a ry KNOX Endmatter.indd 1140 7/8/09 8:10:28 AM 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. Glos s ary KNOX Endmatter.indd 1141 1141 7/8/09 8:10:28 AM 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. 1142 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. G lo s s a ry KNOX Endmatter.indd 1142 7/8/09 8:10:28 AM 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. Glos s ary KNOX Endmatter.indd 1143 1143 7/8/09 8:10:29 AM 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. G lo s s a ry KNOX Endmatter.indd 1144 7/8/09 8:10:29 AM 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. Glos s ary KNOX Endmatter.indd 1145 1145 7/8/09 8:10:29 AM 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 G lo s s a ry KNOX Endmatter.indd 1146 7/8/09 8:10:29 AM 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. Glos s ary KNOX Endmatter.indd 1147 1147 7/8/09 8:10:29 AM 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. G lo s s a ry KNOX Endmatter.indd 1148 7/8/09 8:10:30 AM 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). Glos s ary KNOX Endmatter.indd 1149 1149 7/8/09 8:10:30 AM 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. G lo s s a ry KNOX Endmatter.indd 1150 7/8/09 8:10:30 AM 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 Glos s ary KNOX Endmatter.indd 1151 1151 7/8/09 8:10:30 AM 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 1152 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. G lo s s a ry KNOX Endmatter.indd 1152 7/8/09 8:10:30 AM 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. Glos s ary KNOX Endmatter.indd 1153 1153 7/8/09 8:10:30 AM 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. 1154 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. G lo s s a ry KNOX Endmatter.indd 1154 7/8/09 8:10:31 AM 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. Glos s ary KNOX Endmatter.indd 1155 1155 7/8/09 8:10:31 AM 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. 1156 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. G lo s s a ry KNOX Endmatter.indd 1156 7/8/09 8:10:31 AM 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. Glos s ary KNOX Endmatter.indd 1157 1157 7/8/09 8:10:31 AM 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. 1158 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. G lo s s a ry KNOX Endmatter.indd 1158 7/8/09 8:10:31 AM 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 Glos s ary KNOX Endmatter.indd 1159 1159 7/8/09 8:10:32 AM 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). G lo s s a ry KNOX Endmatter.indd 1160 7/8/09 8:10:32 AM 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 Glos s ary KNOX Endmatter.indd 1161 1161 7/8/09 8:10:32 AM 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. 1162 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. G lo s s a ry KNOX Endmatter.indd 1162 7/8/09 8:10:32 AM 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. Glos s ary KNOX Endmatter.indd 1163 1163 7/8/09 8:10:32 AM 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 G lo s s a ry KNOX Endmatter.indd 1164 7/8/09 8:10:33 AM 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. Glos s ary KNOX Endmatter.indd 1165 1165 7/8/09 8:10:33 AM 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 1166 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. G lo s s a ry KNOX Endmatter.indd 1166 7/8/09 8:10:33 AM 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. Glos s ary KNOX Endmatter.indd 1167 1167 7/8/09 8:10:33 AM 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. 1168 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. G lo s s a ry KNOX Endmatter.indd 1168 7/8/09 8:10:33 AM 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). Glos s ary KNOX Endmatter.indd 1169 1169 7/8/09 8:10:33 AM 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. 1170 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. G lo s s a ry KNOX Endmatter.indd 1170 7/8/09 8:10:34 AM 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. Glos s ary KNOX Endmatter.indd 1171 1171 7/8/09 8:10:34 AM 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. 1172 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). G lo s s a ry KNOX Endmatter.indd 1172 7/8/09 8:10:34 AM 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. Glos s ary KNOX Endmatter.indd 1173 1173 7/8/09 8:10:34 AM 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. G lo s s a ry KNOX Endmatter.indd 1174 7/8/09 8:10:34 AM 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). Glos s ary KNOX Endmatter.indd 1175 1175 7/8/09 8:10:35 AM 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 1176 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. G lo s s a ry KNOX Endmatter.indd 1176 7/8/09 8:10:35 AM 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. Glos s ary KNOX Endmatter.indd 1177 1177 7/8/09 8:10:35 AM 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. 1178 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. G lo s s a ry KNOX Endmatter.indd 1178 7/8/09 8:10:35 AM 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. Glos s ary KNOX Endmatter.indd 1179 1179 7/8/09 8:10:35 AM 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). 1180 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. G lo s s a ry KNOX Endmatter.indd 1180 7/8/09 8:10:36 AM 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 Glos s ary KNOX Endmatter.indd 1181 1181 7/8/09 8:10:36 AM 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. 1182 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. G lo s s a ry KNOX Endmatter.indd 1182 7/8/09 8:10:36 AM 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. Glos s ary KNOX Endmatter.indd 1183 1183 7/8/09 8:10:36 AM 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. 1184 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. G lo s s a ry KNOX Endmatter.indd 1184 7/8/09 8:10:36 AM 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. Glos s ary KNOX Endmatter.indd 1185 1185 7/8/09 8:10:37 AM 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. G lo s s a ry KNOX Endmatter.indd 1186 7/8/09 8:10:37 AM 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). Glos s ary KNOX Endmatter.indd 1187 1187 7/8/09 8:10:37 AM 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. 1188 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. G lo s s a ry KNOX Endmatter.indd 1188 7/8/09 8:10:37 AM 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. Glos s ary KNOX Endmatter.indd 1189 1189 7/8/09 8:10:37 AM 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. 1190 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. G lo s s a ry KNOX Endmatter.indd 1190 7/8/09 8:10:37 AM 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 KNOX Endmatter.indd 1191 1191 7/8/09 8:10:38 AM