Download F215: Control, Genome and the Environment

F215: Control, Genome and the Environment
Module 1: Cellular Control and Variation
5.1.1: Cellular Control
(a) State that genes code for polypeptides, including enzymes
(b) Explain the meaning of the term ‘genetic code’
The genetic code is the sequence of base pairs on a length of DNA. It codes (contains the
genetic information) for the production of polypeptides (proteins), as well as containing
non-coding lengths of DNA. There are several main features of the genetic code:
- It is a ‘triplet code’ – a sequence of 3 bases codes for an amino acid (called a
‘codon’)
- It is ‘degenerate’ – every amino acid (except one) has more than one coding
triplet
- It is widespread but not universal
- It also contains ‘stop’ and ‘start’ codons – e.g. TAG is a stop codon
(c) Describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a
polypeptide
A polypeptide is made up of a chain of amino acids, each connected by a peptide bond.
Amino acids are each coded for by specific base triplets (a sequence of three bases). DNA
in the nucleus is transcribed into mRNA, which is then used as a template for the
construction of a polypeptide in the cytoplasm of the cell.
(d) Describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to construct a polypeptide,
including the roles of mRNA, tRNA and ribosomes
Transcription:
- RNA polymerase attaches to the length of DNA to be transcribed, and causes it
to unzip and unwind (by dipping into the nucleolus, causing the hydrogen
bonds between complementary base pairs to break by hydrolysis). This creates
two single-stranded lengths of DNA, one of which is able to act as the ‘template
strand.’
- Free, activated RNA nucleotides anneal to the exposed template strand by
complimentary base pairing, catalysed by the enzyme RNA polymerase.
- Uracil replaces thymine as a base in RNA.
- Free RNA nucleotides continue to be added to the growing mRNA strand, until
a ‘stop’ codon is reached.
- When a ‘stop’ codon is reached, the completed mRNA strand leaves the nucleus
via a nuclear pore, and attaches to the small subunit of a ribosome, in the
cytoplasm.
- The hydrogen bonds between base pairs on the template strand of DNA reform,
causing the DNA to resume its double-stranded, helical structure.
Translation
- The completed mRNA strand attaches to a small ribosomal subunit, and is
exposed to the larger ribosomal subunit.
- A molecule of tRNA, with an anticodon complementary to the first codon on the
mRNA strand, binds to its specific amino acid. The anticodon of the tRNA
molecule then anneals to the first codon on the mRNA strand by hydrogen
bonding.
-
A second molecule of tRNA, with an anticodon complementary to the second
codon on the mRNA strand, also binds to its specific amino acid, and also binds
to the mRNA strand.
Ribosomal enzymes cause a peptide bond to form between the two amino acids.
The first tRNA molecule leaves the ribosome, and is free to pick up another
amino acid
A third molecule of tRNA binds to the mRNA strand, and a peptide bond is
formed between the second and third amino acids.
This process continues, with the ribosome gradually moving along the mRNA
strand, until a stop codon is reached, at which point, the polypeptide is
complete.
(e) State that mutations cause changes to the sequence of nucleotides in DNA molecules
(f) Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions
Beneficial
The change in the base sequence of the DNA changes the amino acid sequence,
which conveys an advantageous phenotypic characteristic to the individual, causing
them to have a survival advantage. This could be as a result of increased protein
function.
Neutral
The change in the base sequence of the DNA is either a silent mutation (the new
base sequence still codes for the original/chemically similar amino acid), or causes a
change in amino acids away from the active site of the protein – so function is not
affected.
Harmful
The change in the base sequence of DNA causes a change in the amino acid
sequence, which conveys a disadvantageous phenotypic characteristic to the
individual. This could be the result of an alteration in the active site of a protein, or
due to a change in the structure of the protein – for example, sickle cell anaemia is
due to a change in the base sequence of red blood cells, which causes bonding
within the cell to change, producing a ‘sickle’ shape.
(g) State that cyclic AMP activates proteins by altering their 3D structure
(h) Explain genetic control of protein production in a prokaryote using the ‘lac operon’
E. coli bacteria normally respire glucose, however they are also able to respire lactose if no
other substrates are available. When first placed on the lactose culture medium, little
substrate is respired as the bacteria have a very small amount of the enzymes necessary to
respire lactose. However, after a relatively short period of time, the E. coli are able to
metabolise the lactose medium at a normal rate. The lactose acts as an inducer, causing the
molecular switch to be turned on, and the necessary enzymes – β-galactosidase and lactose
permease – to be synthesised. Here’s how:
Lactose NOT present
The regulator gene is expressed and synthesises the lac repressor, which has
two binding sites (one for binding to the operator region, and one for
binding to lactose). The lac repressor binds to the operator region (in front
of the structural genes), which prevents RNA polymerase from binding to
the promoter region. Transcription of the structural genes is prevented.
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Lactose present
Lactose is taken up by the bacterium, and then binds to the lac repressor,
causing it to change shape. This shape change causes the repressor protein to
dissociate from the operator region, which allows RNA polymerase to bind
to the promoter region, and hence transcribe the structural genes. These
structural genes code for the production of the enzymes necessary to
metabolise lactose.
(i) Explain that the genes that control development of body plans are similar in plants, animals and fungi, with reference to
homeobox sequences
Homeobox sequences control the development of body plans in plants, animals and fungi
in similar ways. Homeotic genes contain homeobox sequences (a length of DNA which
codes for approximately 60 amino acids), which code for the homeodomain, which acts as
a transcription factors in developmental genes, to either activate or repress transcription.
In animals, homeobox sequences are concerned with developmental events, such as
segmentation. Homeobox sequences are responsible for the development of the
anterior/posterior axis in the Drosophila fruit fly, and are also responsible for the correct
development of limbs in more complex organisms.
(j) Outline how apoptosis (programmed cell death) can act as a mechanism to change body plans
Apoptosis is a biochemical mechanism which allows the tidy and ordered break down of
unneeded cells, to ensure that the rate of cell destruction is equal to the rate of cell
production. This is opposed to cell necrosis, where the cell breaks down and releases
harmful hydrolytic enzymes. Apoptosis is involved in the refinement of body parts, such as
the separation of fingers and toes, and the removal of tadpole tails. When apoptosis is
triggered, the following reaction occurs in the cell:
- Enzymes break down the cell cytoskeleton
- Cytoplasm becomes dense with organelles tightly packed
- Cell surface membrane changes and blebs form
- Chromatin condenses and the nuclear envelope breaks down
- DNA breaks into fragments
- Cell breaks down into vesicles, which are then absorbed by phagocytosis
- Cellular debris is disposed of or recycled by other cells
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5.1.2: Meiosis and Variation
(a) Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated
behaviour of the nuclear envelope, cell membrane and centrioles (names of the main stages are expected, but not the
subdivisions of prophase)
Meiosis is the process by which gametes (sex cells) are produced in the sexual organs. In
order to combine to form a zygote with the correct, ‘haploid’, number of chromosomes
(46), each gamete must contain half the normal number of chromosomes – known as the
‘diploid’ number (23 chromosomes). In order to do this, meiosis allows each sex cell to
divide twice, forming four genetically dissimilar daughter cells. Here’s how it works:
Meiosis I
1. Interphase
During interphase, each chromosome replicates, forming two ‘sister
chromatids,’ which are joined together at the centromere. Each sister
chromatid is genetically identical as they contain exact copies of the same
length of DNA.
2. Prophase I
Chromatin condenses and supercoils.
Chromosomes come together in homologous pairs (one maternal
chromosome pairs with one paternal chromosome), to form a ‘bivalent.’
Each chromosome has the same genes at the same loci, however they have
different alleles, so are not genetically identical.
Non-sister chromatids form ‘chiasmata’ (crossing over points), where
sections of DNA containing different alleles are swapped between the nonsister chromatids. This is known as ‘crossing over,’ and allows the
reassortment of alleles.
The nuclear envelope breaks down.
Centrioles move to the poles of the cell and a spindle forms.
3. Metaphase I
Bivalents line up randomly along the equator of the cell, each attached to the
centrioles by spindle fibres at the centromere.
Each member of the homologous pair faces opposite poles.
4. Anaphase I
The spindle contracts, pulling the chromosomes to the poles of the cell.
Homologous chromosomes separate, with maternal and paternal
chromosomes pulled towards opposite centrioles. The centromeres do not
divide. The bivalent is initially held together at the chiasmata because of
strong forces of attraction between sister chromatids, however the chiasmata
eventually break, and the crossed over alleles remain with the chromatid to
which they are newly attached.
5. Telophase I
In most animals, two new nuclear membranes form, one around each set of
chromosomes at either pole of the original cell. Cytokinesis may occur.
In most plants, the cell goes straight from anaphase I to prophase II.
Meiosis II
1. Prophase II
If a nuclear envelope has formed, it breaks down again.
Chromosomes condense.
Centrioles move to the poles of the cell and a spindle forms.
2. Metaphase II
Sister chromatids line up randomly on the equator of the cell, attached to the
centrioles by spindle fibres attached at the centromere.
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3. Anaphase II
The spindle fibres contract.
Sister chromatids are separated at the centromere and pulled to opposite
poles of the cell.
The chromatids randomly segregate (any combination of chromatids is
possible in each of the daughter cells).
4. Telophase II
Nuclear envelopes form around the daughter cells.
Cytokinesis (cell membrane separation) occurs in animals, to form a total of
four genetically dissimilar haploid daughter cells.
In plants, a tetrad of four haploid cells is formed.
(b) Explain the terms:
Allele
An alternative version of a gene, at the same locus on the chromosome.
Locus
The fixed position of a specific gene on a specific chromosome.
Phenotype
The observable characteristics of an individual organism, produced by the genotype.
Genotype
The alleles present within the cells of an individual organism, which code for a
particular characteristic.
Dominant
An allele which is always expressed in the phenotype, even when only one copy of
the allele is present (in heterozygous individuals). Shown with a capital letter.
Recessive
An allele which is only expressed in the phenotype when two recessive copies of the
gene are present (homozygous recessive individuals). Shown with a lowercase
letter.
Codominant
Two alleles which are both expressed in the phenotype of the individual, neither
one is recessive to the other.
(c) Explain the terms:
Linkage
Two genes for different characteristics, present at different loci on the same
chromosome, so are likely to be expressed together (however crossing over can
change this).
Crossing over
The process by which the alleles at various loci on non-sister chromatids are
exchanged during prophase I of meiosis. Exchange occurs via the formation of
chiasma (plu. chiasmata). After crossing over, each chromatid contains the same
genes, but different combinations of alleles, producing genetic diversity.
(d) Explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles
Meiosis:
1. During prophase I, crossing over causes ‘reshuffling’ of alleles
2. During metaphase I, the independent assortment and distribution of
chromosomes means that any combination of maternal and paternal
chromosomes can enter the daughter cells
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3. During metaphase II, the independent assortment and distribution of sister
chromatids means that any combination of chromatids could enter the
daughter cells
4. Random mutations
Fertilisation:
1. Any female can mate with any male
2. Any sperm cell (male gamete) can fuse with any egg cell (female gamete)
(e) Use genetic diagrams to solve problems involving sex linkage and codominance
Sex linkage: (e.g. haemophilia)
First Generation
Sufferer female
x
Normal male
XhXh
XHY
First Cross (Second Generation)
Female Gametes
Xh
H
X
XHXh
Carrier female
XHXh
Carrier female
Xh
Second Cross (Third Generation)
Female Gametes
XH
H
XhXh
XhY
Y
XHY
Normal male
XhY
Sufferer male
XHY
XHXh
XhY
Y
XhY
Sufferer male
XhY
Sufferer male
Male Gametes
X
XHXH
Normal female
XHXh
Carrier female
Xh
XhY
Male Gametes
XHY
XhY
XHY
XHXh
XHXH
Haemophilia is a sex linked disease. The recessive allele ‘h’ is carried on the X
chromosome, of which males have only one copy. This means that males will always
express the recessive allele, making them more likely to suffer from haemophilia than
females. In order for females to suffer from the disease, they must have two copies of the
recessive allele ‘h.’ However, females are able to ‘carry’ the genetic information for
haemophilia, without expressing it themselves. This is possible because the allele ‘h’ is
recessive to the dominant allele ‘H,’ meaning that, although a carrier female is able to pass
the disease onto her offspring, she will not suffer from the disease herself.
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Codominance: (e.g. blood group)
First Generation
Heterozygous Group ‘A’ Female
BABo
Second Generation
Female Gametes
BA
x
Heterozygous Group ‘B’ Father
BB B o
Male Gametes
BB
Bo
BABB
BABo
Group AB
Group A
B o
BB
B oB o
Group B
Group O
x
Heterozygous Group ‘B’ Father
BB B o
Male Gametes
BB
Bo
A B
B B
BABo
Group AB
Group A
A B
B B
BABo
Group AB
Group A
x
Homozygous Group ‘B’ Father
BBBB
Male Gametes
BB
BB
A B
B B
BABB
Group AB
Group AB
B o
BB
BB Bo
Group B
Group B
x
Homozygous Group ‘B’ Father
BBBB
Male Gametes
BB
BB
A B
B B
BABB
Group AB
Group AB
A B
B B
BABB
Group AB
Group AB
Bo
OR
First Generation
Homozygous Group ‘A’ Female
BABA
Second Generation
Female Gametes
BA
BA
OR
First Generation
Heterozygous Group ‘A’ Female
BABo
Second Generation
Female Gametes
BA
Bo
OR
First Generation
Homozygous Group ‘A’ Female
BABA
Second Generation
Female Gametes
BA
BA
Blood type shows codominance, because neither of the alleles ‘A’ or ‘B’ are recessive.
Both A and B are dominant to the third allele, O. This means that when A and B are
both present in the genotype, both will be expressed in the phenotype. The blood
group ‘O’ only occurs in individuals homozygous for the recessive O allele.
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(f) Describe the interactions between loci (epistasis)
Many different alleles can have an effect upon the same characteristic. These alleles
interact to produce the phenotype. Epistasis is the interaction of different gene loci so that
the phenotypic expression of one gene locus masks or suppresses the effect of another gene
locus.
Recessive epistasis:
Recessive epistasis occurs only when two copies of the dominant epistatic
allele are present (when the individual is homozygous for the epistatic
allele).
Dominant epistasis:
Dominant epistasis occurs even when only one copy of the dominant
epistatic allele is present (when the individual is either heterozygous or
homozygous for the epistatic allele).
(g) Predict phenotypic ratios in problems involving epistasis
Recessive epistasis:
9:3:4
Dominant both: Dominant epistatic, recessive other: Recessive epistatic
Dominant epistasis:
12:3:1
Dominant epistatic: Recessive epistatic, dominant other: Recessive both
(h) Use the chi-squared (X2) test to test the significance of the difference between observed and expected results
Chi-squared is a statistical test which allows us to test whether the difference between
observed and expected results (e.g. in a genetic cross) is due to chance, or whether we have
made an incorrect assumption in our hypothesis.
o = Observed result
e = Expected result
The smaller the value of X2, the more likely it is that the difference observed is due to
chance, and therefore not a significant difference. In order to apply chi-squared, a ‘null
hypothesis’ must first be made: we assume that there is no significant difference between
observed and expected results.
When the value for X2 has been calculated, we compare it to the ‘critical value.’ The
critical value is obtained from a X2 table, using the degrees of freedom (number of classes –
1), and a probability of 0.05 (5%).
If the value of X2 is lower than the critical value, we accept the null hypothesis.
(i) Describe the differences between continuous and discontinuous variation
Continuous variation:
Continuous variation is used to measure quantitative differences between
phenotypes. There is a wide range of variation between individuals, with no
distinguishable categories: for example, height in humans.
Discontinuous variation:
Discontinuous variation is used to measure qualitative differences between
phenotypes. Individuals fall into easily distinguishable categories, with no
intermediates: for example, blood group.
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(j) Explain the basis of continuous and discontinuous variation by reference to the number of genes which influence
variation
Continuous variation:
Most continuous variation is controlled by multiple genes (known as ‘polygenic’).
Multiple gene loci interact together to form a large range of possible phenotypes.
Different alleles at the same locus have small effects.
Different gene loci have the same, often additive, effect on the trait.
Discontinuous variation:
Most discontinuous variation is controlled by one, or very few, genes (known as
‘monogenic’).
Different alleles at a single gene locus have a large effect upon the phenotype.
Different gene loci have different effects on the trait.
(k) Explain that both genotype and environment contribute to phenotypic variation
Although an individual may possess the genetic potential to achieve a certain
characteristic, the environment may influence the organism is such a way as to prevent it
from achieving the full scope of that potential. In this way, the environment around an
individual possesses the ability to influence its phenotype. For example, a ‘tall pea plant’
may possess the genetic potential to grow to 1.2m tall, however, the environment may cause
it to lack essential nutrients, water, or sunlight, and in this way may limit the growth of the
plant. As a result, the plant may only grow to be 0.8m tall.
Often, characteristics that are influenced by the environment may change over an
organism’s lifetime: for example, accent in humans may change, depending on the accent of
those around you, and where you live.
(l) Explain why variation is essential in selection
Both natural and artificial selection are based on the principle that phenotypic variation
exists between individuals of a species, with the result (in natural selection) that some
organisms are better adapted to survive than others. Differences in phenotype mean that –
were the environment to change – some individuals would possess certain characteristics
which conferred to them a survival advantage, meaning that these individuals would be
more likely to reproduce and pass on their advantageous alleles, allowing the species to
adapt and continue.
(m) Use the Hardy-Weinberg principle to calculate allele frequencies in populations
Hardy-Weinberg principle states that:
p+q=1
p = frequency of dominant allele
q = frequency of recessive allele
All of the individuals within a population must have either the dominant or the recessive
allele, therefore the total allele frequency = 1 (100%).
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q2 = frequency of homozygous recessive genotype
All of the individuals within a population must be either homozygous for one genotype,
homozygous for the other genotype, or heterozygous. Therefore, the total genotype
frequency must = 1 (100%).
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(n) Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection
Depending on the environment, selection can be either stabilising or directional
(evolutionary).
Stabilising:
When the environment is fairly constant, for example in a temperate climate with
relatively little change between the seasons, selection has a stabilising effect upon the
alleles within the population. This means that the individuals with phenotypes closest
to the mean are most likely to survive and reproduce, and so alleles close to the mean
become more common.
For example, fur length in rabbits shows continuous variation: some rabbits have very
long fur, some have medium fur, some have very short fur, and most are somewhere
inbetween. In a temperate climate, rabbits with very long fur are likely to overheat in
the warmer summer, and so are less likely to survive and reproduce. Rabbits with very
short fur are likely to get too cold in the winter, and so are less likely to survive and
reproduce. Rabbits with average fur length are more likely to survive, reproduce, and
pass on the alleles for average fur length. Gradually, alleles for extreme fur length will
‘die out.’
Directional:
When the environment is changing, it is likely that some alleles in the population will
give a survival advantage, and other alleles will be disadvantageous. Directional
selection causes an increase in the frequency of extreme types of alleles, because these
alleles convey a survival advantage.
For example, if the temperate climate suddenly became extremely cold, rabbits with
the alleles for long fur length would be more likely to survive and reproduce to pass on
the advantageous alleles, whereas rabbits with the alleles for average or short fur length
would freeze, and so these alleles would gradually ‘die out.’
(o) Explain how genetic drift can cause large changes in small populations
Genetic drift is the effect of chance upon the frequency of alleles – with no survival
advantage/disadvantage – within a population. Genetic drift occurs when, by chance, the
alleles for one characteristic, ‘A,’ are passed on to the next generation more than the alleles
for another characteristic, ‘B.’ By chance, the next generation also passes on the allele ‘A’
more than the allele ‘B.’ Gradually, allele A become common in the population, and allele B
becomes rare. Neither characteristic has any effect upon the survival of the individual, so
natural selection has no effect upon allele frequency. Genetic drift has a much larger
influence over small populations, when the effects of chance are greater. In larger
populations, chance factors normally even out across the whole population.
Genetic drift also has a greater influence when there has been a ‘genetic bottleneck’ (when
a large population has suddenly been reduced to a small population, e.g. after a natural
disaster). This is because the surviving population is unlikely to be representative of the
allele frequencies of the previous population.
(p) Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographical),
seasonal (temporal) and reproductive mechanisms
If two sub-populations (‘demes’) are separated from each other, they are likely to evolve
separately as a result of the differing selection pressures acting on each of the subpopulations, and because mutations will occur independently in each deme. If the two
demes are unable to interbreed, they will eventually become reproductively isolated, at
which point, they have evolved into two new species – ‘speciation’ has occurred.
Allopatric isolating mechanisms may include:
- Geographical isolation (a physical barrier divides the demes)
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- Seasonal isolation (evolving different flowering/breeding seasons)
- Reproductive isolation (change to genitalia/mating behaviour)
Sympatric speciation may also occur, whereby a spontaneous change results in instant
reproductive isolation. Sympatric isolating mechanisms include:
- ‘Polyploidy’ in plants
- Mutations which prevent interbreeding
(q) Explain the significance of the various concepts of the species, with reference to the biological species concept and the
phylogenetic (‘cladistic’/‘evolutionary’) species concept
The traditional definition of a species, under the biological species concept, is ‘a group of
similar organisms which are able to interbreed to successfully to produce viable (fertile)
offspring.’ However, there are several problems with this definition:
- Not all ‘species’ interbreed in the wild, so we do not know whether they would
produce viable offspring.
- Not all possible crosses can be tested in the lab (e.g. crossing a human and a chimp)
- Some species are extinct so we cannot investigate their mating behaviour
- Some unrelated organisms can look very similar, without being members of the same
species
- Not all organisms reproduce sexually (e.g. bacteria)
As a result of these problems, we sometimes use the phylogenetic species concept, which
defines a species as: ‘a group of organisms with similar morphology, physiology,
embryology and behaviour, and occupy the same ecological niche.’ This concept involved
studying the evolutionary history of a group of organisms, to see how genetically similar to
another group of organisms they may be. All organisms evolved from a common ancestor,
and the more recently the groups diverged away from each other, the more closely related
they are. The phylogenetic linkage is called a ‘clade.’ However, this theory also has issues:
- There is no definitive ‘cut off point’ at which point two groups can be
established as the same species (e.g. humans and chimps share 94% of the same
DNA).
(r) Compare and contrast natural selection and artificial selection
Similarities:
In both natural and artificial selection, individuals within a population will
demonstrate variation.
Both change the allele frequencies within a population.
Both make use of beneficial mutations when they occur.
Differences:
Natural Selection
The selection pressure comes from the
environment: the individuals that survive and
reproduce are those best adapted to the
environment they live in.
Evolution is in best interests of the species, as
having the fittest survive ensures that the species
will continue.
The result of natural selection is unpredictable, as
the species will change depending upon the
environment they are exposed to. The end result is
unknown.
Artificial Selection
Selection pressure comes from humans: the
individuals that reproduce are chosen by humans
because they have a useful characteristic which
humans want to pass to their offspring.
Evolution is in the best interests of humans, as
‘useful’ characteristics will become more common
in the population, as artificial selection continues.
Artificial selection aims for a pre-determined
result: humans choose individuals with desirable
characteristics (e.g. high milk yield), and breed
together the ‘best’ individuals for this trait, to
increase its frequency within the population.
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(s) Describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat (Triticum
aestivum)
Modern Dairy Cow:
1. Cows with the highest milk yield were selected.
2. Bulls with proven progeny (daughters with high milk yields) were selected.
Few bulls are required as artificial insemination (‘AI’) means that one bull can
fertilise many cows.
3. Selected cows bred with the selected bulls.
4. Some high value cows may be given fertility hormones so they produce many
eggs, which are then fertilised in vitro, and the embryos implanted into
surrogate cows.
5. The embryos could also be separated, so that many clones of the offspring are
produced.
6. This process continues over several generations, until a cow with an extremely
high milk yield is produced.
Bread Wheat (Triticum aestivum):
1. Plants with a high wheat yield (large cob length) are selected.
2. High yielding plants are crossed, using methods to prevent cross pollination
with another, lower quality, plant.
- Pollen is transferred by hand, using small paintbrushes
- Stamen is protected from other pollen sources using a muslin bag
placed over the plant
3. The offspring with the highest wheat yields are selected and crossed, until a very
high yielding variety is produced.
4. Wheat is able to undergo polyploidy, where the chromosome number increases
in subsequent generations. Modern bread wheat is hexaploid – it has 42
chromosomes in the nucleus of each cell, which means that each cell is larger, so
the plant produces more wheat. Modern bread wheat has three times the
number of chromosomes in ‘wild einkorn’ – one of its ancestors.
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Module 2: Biotechnology and Gene Technologies
5.2.1: Cloning in Plants and Animals
(a) Outline the differences between reproductive and non-reproductive cloning
Reproductive Cloning
An entire organism is produced, which is
genetically identical to the cloned organism. This
may be produced by nuclear transfer (clone of
parent), or embryo splitting (clone of sibling)
Can be used by farmers to clone high-value animals
Non-Reproductive Cloning
Embryonic stem cells are used to produce
replacement cells, tissues or organs
Stem cells have the ability to differentiate into any
kind of cell
Always artificial
Can be done naturally or artificially
(b) Describe the production of natural clones in plants, using the example of vegetative propagation in elm trees
Some plants (e.g. the elm tree) are able to produce clones of themselves asexually, by a
process known as ‘vegetative propagation.’ Clones are produced from normally dormant
basal sprouts, or ‘suckers,’ which are grown on shallow roots. In times of stress, for
example if the plant is dying, the suckers are activated, and begin to grow from
meristematic tissue. Normally, the root suckers are located far enough away from the
parent plant to avoid the source of stress that triggered their production. Eventually, the
sucker will form a completely separate tree.
(c) Describe the production of artificial clones of plants from tissue culture
Plants can be reproductively cloned by a process known as ‘tissue culture.’ This is done in
several stages:
1. Cells are removed from the plant to be cloned. Cells are removed from either the apical
or the lateral meristems, as these cells still have the ability to differentiate. The tissue
removed is called the ‘explant.’ This is sterilised to kill any micro-organisms which
may inhibit the growth of the plant cells.
2. The explant is placed on a culture medium containing growth hormones. The cells
divide but do not differentiate. The resulting undifferentiated mass of cells is known as
a ‘callus.’
3. After a few weeks, individual cells can be removed from the callus and grown on a
culture medium containing plant growth substances, which encourage shoot growth.
4. After another few weeks, the young shoots can be placed on a third culture medium,
containing plant growth substances which encourage root growth.
5. The growing plants can then be transferred to a greenhouse, and finally grown outside.
(d) Discuss the advantages and disadvantages of plant cloning in agriculture
Advantages
Desirable genetic characteristics are always
reproduced, which cannot be guaranteed in sexual
reproduction.
Plants can be produced quickly, at any time of the
year.
Sterile plants, and plants that take a long time to
reproduce naturally, can be reproduced.
Disadvantages
Undesirable genetic characteristics are always
reproduced.
Production costs are high as trained staff are
required, and high energy costs are involved.
No genetic variation means that all of the clones
will be susceptible to the same disease – a lot of
plants could be wiped out at once.
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(e) Describe how artificial clones of animals can be produced
Parent animals can be artificially cloned using a process known as ‘nuclear transfer.’ This is
carried out in several stages:
1. A somatic (body) cell is taken from the animal to be cloned (‘animal A’). The
nucleus is removed from the cell and kept.
2. An egg cell is removed from another animal of the same species (‘animal B’). The
nucleus is removed from the cell and discarded, forming an ‘enucleated’ egg cell.
3. The nucleus from animal A is implanted into the enucleated egg cell from animal
B, and the cell is stimulated to divide. The egg cell now contains the genetic
information from animal A.
4. The resulting embryo is implanted into a surrogate mother, and continues to
develop.
Genetically identical siblings can also be artificially produced, in a process known as
‘embryo splitting,’ where cells from a developing embryo are removed and grown
separately. Each separate cell will go on to produce a separate, genetically identical,
organism. This process also occurs naturally, and forms identical twins.
(f) Discuss the advantages and disadvantages of cloning animals
Advantages
Desirable genetic characteristics are always
reproduced.
High value animals may be reproduced without
danger of injury to the animal – e.g. prize cows can
be reproduced without the danger of giving birth
to a calf.
Disadvantages
Undesirable genetic characteristics are always
reproduced.
Genetic uniformity means that all clones will be
susceptible to the same diseases. No variation
means that selective breeding is hard to achieve,
and the organisms would struggle to adapt to a
change in the environment.
There is some evidence that cloned animals do not
live as long as ‘naturally produced’ organisms.
Clones of high value animals are not always
produced with animal welfare in mind.
Reproductive cloning is very difficult, timeconsuming and expensive. It took 277 attempts to
clone Dolly the sheep.
Rare or endangered animals can be cloned to
preserve the species.
Infertile animals can be reproduced.
Genetically engineered characteristics can be
‘inserted’ into animals and then reproduced
quickly – e.g. sheep which produce pharmaceutical
chemicals in their milk.
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5.2.2: Biotechnology
(a) State that biotechnology is the industrial use of living organisms (or parts of living organisms),
to produce food, drugs or other products
(b) Explain why microorganisms are often used in biotechnological processes
- Microorganisms grow very quickly in favourable conditions
- They can be grown anywhere in the world
- They can be grown on a range of inexpensive media, which would otherwise be either
useless or toxic to humans
- Their ideal growth conditions are easily replicated
- They produce a range of chemicals during various growth stages, which may be harvested
for human use.
- Microorganisms normally produce product in a more pure form than those generated via
chemical processes. This reduces the cost of downstream processing.
- They can be genetically modified to produce certain products, e.g. human insulin
- They can be grown at any time of year
(c) With the aid of diagrams, describe and explain the standard growth curve of a microorganism in a closed culture
Lag Phase: Bacteria are acclimatising to the environment,
activating specific genes, and synthesising the necessary
enzymes for respiring the given substrate. Reproduction
rate is very slow.
Log Phase: Growing conditions are favourable, bacteria
have plenty of space, nutrients and oxygen, and so are
able to reproduce at their maximum rate. The population
size doubles with every generation. Primary metabolites
are produced.
Stationary Phase: Population size remains constant as the
death rate is equal to the reproduction rate. Toxic waste
products start to build up, and bacteria have to compete
for nutrients and space. Secondary metabolites are
produced. In an open system, this would be the carrying capacity.
Death Phase: Population size decreases as nutrient exhaustion and toxic waste build up
causes the death rate to rise above the reproduction rate. In a closed system, all organisms
will eventually die.
(d) Describe how enzymes can be immobilised
- Encapsulated in an alginate bead
- Trapped in a silica gel matrix
- Covalently bonded to collagen/cellulose fibres
- Physical separation from the substrate mixture by use of a partially-permeable membrane
(e) Explain why immobilised enzymes are used in large-scale production
- Do not need to separate enzymes from the reaction mixture when the reaction is
complete
- Immobilised enzymes are more stable (harder to denature) than free enzymes
- Columns of immobilised enzymes can be washed and re-used (reduces cost of raw
materials)
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(f) Compare and contrast the processes of continuous and batch culture
Continuous Culture
Microorganisms are fermented in an open system
(nutrients are continuously added, and product is
regularly removed).
Batch Culture
Microorganisms are fermented in a closed system
(a fixed quantity of nutrients are added at the start
and no more are added. Product is only removed at
the end of the process).
Microorganisms undergo the lag phase but are then
Microorganisms undergo all of the phases of the
maintained at the log (exponential) phase.
standard growth curve, and eventually all of the
microorganisms will die.
If contamination occurs, the entire culture would If contamination occurs, only one batch is lost. It is
have to be discarded, which is extremely expensive.
much less expensive to replace a single
contaminated batch.
Continuous culture is more productive (high
Batch culture is less productive (low product yield)
product yield) because products are being
because the fermenter must be thoroughly cleaned
constantly produced.
and sterilised between each batch.
Set up is difficult, maintenance of necessary
Easy to set up and maintain the culture.
growing conditions can be hard to achieve and
requires constant monitoring by trained staff.
Useful when desired products are primary
Useful when the desired products are secondary
metabolites, or the microorganisms themselves.
metabolites.
(g) Describe the differences between primary and secondary metabolites
Primary metabolites are substances produced by a microorganism as part of its normal
growth pattern. Often, they are essential for the growth of the organism. The production
of primary metabolites matches the population increase of the microorganism culture,
therefore the highest primary metabolite production rate is during the log phase of
microorganism growth.
Secondary metabolites are substances produced by some microorganisms in times of stress.
These are produced in order to offer the organism a survival advantage, so that it is able to
out-compete other microorganisms which threaten its survival. The production rate of
secondary metabolites does not match the growth rate of the microorganism, as secondary
metabolites are produced at their highest rate during the stationary phase of
microorganism growth. For example, penicillin is an antibiotic produced by the fungus
Penicillium, and helps the fungus to kill bacteria that inhibit its growth.
(h) Explain the importance of manipulating the growing conditions in a fermentation vessel in order to maximise the yield
of product required
Temperature: if conditions are too hot, enzymes within the microorganisms may start to
denature. If conditions are too cold, the growth of the microorganisms will
be slowed.
pH: changes in pH can reduce the productivity of the enzymes involved in microorganism
growth.
Nutrients: bacteria are kept circulating around the fermentation vessel to ensure that all
the organisms are always in contact with fresh growth medium. This means
that microorganisms can always access the necessary nutrients for growth.
Oxygen concentration: most microorganisms are grown under aerobic conditions, so
oxygen concentration is maintained at the optimum level for aerobic
respiration. Lower oxygen concentration could result in a decreased growth
rate.
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(i) Explain the importance of asepsis in the manipulation of microorganisms
Asepsis is the practise of preventing culture contamination by unwanted microorganisms,
which could:
- Cause spoilage of the desired product
- Contaminate the microorganism culture (resulting in the culture having to be
discarded)
- Threaten the growth of the microorganism culture by competing for nutrients
and space (reducing the yield of product)
- Produce toxic chemicals
- Destroy the culture microorganisms
There are several aseptic techniques which are employed to prevent contamination by
unwanted microorganisms:
- Work surfaces are regularly disinfected to minimise contamination
- Gloves are worn and long-hair tied back
- Instruments used to transfer microorganisms are sterilised before and after each
use
- Lids are held over open containers to prevent unwanted microorganisms falling
into the containers
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5.2.3: Genomes and Gene Technologies
(a) Outline the steps involved in sequencing the genome of an organism
- Genes are mapped to identify which part of the genome they come from. Information that
is already known – such as the location of microsatellites is used.
- Samples of the genome are cut to produce fragments of around 100,000 base pairs (‘bp’) in
length, using restriction enzymes.
- Each fragment is inserted into a separate BAC (‘bacterial artificial plasmid’), each of which
is then taken up by a separate E. coli bacterium. Bacteria are encouraged to take up the
plasmid by ‘electroporation,’ which involves adding calcium salts to the bacteria/plasmid
mixture, dropping the temperature to 0oc, then quickly increasing the temperature to 40oc.
- The bacteria divide and each produce an individual clone library of their specific fragment.
Together, all of the clones are known as the ‘genomic DNA library.’
- The fragments are isolated and extracted from the plasmids in each colony. Each fragment
is then cut up using different restriction enzymes, which produces many fragments of
varying length. Some of these fragments overlap each other.
- Each piece of DNA is now sequenced using the ‘chain-termination method.’
Chain Termination Method:
- Four test tubes are set up, each containing: a copy of the DNA fragment to be
sequenced; DNA polymerase enzyme; free nucleotides; primers; and fluorescentlylabelled modified nucleotides.
- In each test tube, one of the four possible fluorescently-labelled modified
nucleotides is added: either A*, T*, C* or G*. When added to a growing nucleotide
chain, these nucleotides cause termination of the chain. They also fluoresce under
UV light.
- PCR is carried out in each test tube, producing many nucleotide chains of different
lengths, depending on where the fluorescently labelled modified nucleotide was
added to the chain.
- The DNA fragments in each test tube are then separated by size using
electrophoresis, and visualised under UV light.
- Each modified nucleotide will fluoresce in a different colour, so the sequence of
bases can be read from the bottom, upwards.
- Finally, powerful computers are able to put all of the fragments back in order to sequence
the genome as a whole.
(b) Outline how gene sequencing allows for genome-wide comparisons between individuals and between species
Between species
- Evolutionary relationships can be explored by considering the similarities in the
genomes of two species
- The identification of genes common to most/all living things can give clues as to the
relative importance of these genes to life
- Gene interaction can be researched: each the control of developmental genes by the
homeobox sequence
- Medical research can be carried out, by comparing the genome of pathogenic and nonpathogenic bacteria, to identify the genes responsible for causing disease
Between individuals
- Early human migration can be mapped by comparing the genomes of humans from
around the world
- Medical advances can be made by possibly producing drugs specific to an individuals
genome, to maximise its effect
- The genetics of human diseases can be studied, so that we may predict whether
someone has inherited a specific genetic disorder
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(c) Define the term recombinant DNA
Recombinant DNA is a section of DNA produced by combining DNA from two different
sources, often two different species. This is often in the form of a plasmid.
(d) Explain that genetic engineering involves the extraction of enzymes from one organism, or the manufacture of genes, in
order to place them in another organism (often of a different species) such that the receiving organism expresses the
desired product
The required gene is obtained from the ‘donor’ organism, either by extraction of DNA from
the nucleus of a cell, or from using reverse transcriptase enzyme on a section of mRNA
from the cytoplasm of a cell.
A copy of the gene is inserted into a vector (plasmid), by use of the same restriction
enzymes used to isolate the gene.
The vector is inserted into a host, using a range of possible methods.
The host expresses the inserted gene through protein synthesis.
(e) Describe how sections of DNA containing a desired product can be extracted from a donor organism using restriction
enzymes
Restriction enzymes are enzymes which recognise specific sequences of bases
(‘palindromic’ sequences) on a gene. The restriction site is complementary to the active
site of the enzyme, so they can only cut at this specific base sequence. When a restriction
enzyme comes across this specific base sequence, they cut through the sugar-phosphate
backbone using a hydrolysis reaction. This isolates the required gene if it is found between
two of these restriction sites. Some restriction enzymes produce ‘sticky-ends’ when they
cut the DNA. Sticky-ends are short, single-stranded lengths of DNA at either end of the
desired gene. These are able to bind to complementary sticky ends, for example in a
plasmid. This allows the DNA fragment to anneal to a plasmid.
(f) Outline how DNA fragments can be separated by size using electrophoresis
A fluorescent tag is added to each DNA fragment so they can be identified under UV light.
Each fragment is placed into a separate ‘well’ in one end (the negative end) of a slab of
electrophoresis gel.
The gel is immersed in buffer solution, and an electric current is passed through it for a
fixed period of time (normally two hours).
DNA is negatively charged because of its phosphoryl groups, so it migrates towards the
positive end of the gel.
Smaller fragments (few base pairs) meet less resistance in the gel and so are able to move
further through it in the fixed time period. Larger fragments (more base pairs) meet more
resistance in the gel and so cannot move as far in the fixed time period.
After the fixed time period has passed, the gel is placed under UV light, and the sequence
of DNA fragments is read.
(g) Outline how DNA probes can be used to identify fragments containing specific sequences
A DNA probe is a short section of fluorescently-labelled single-stranded DNA,
complementary to the DNA sequence being investigated.
Copies of the probe are added to a sample of the DNA under analysis, and incubated.
If the DNA contains the specific sequence being investigated, the probe will anneal to it
using hydrogen bonds between complementary base pairs.
The sample undergoes electrophoresis, and is exposed to UV light.
If the probe has annealed to the DNA sequence, it will fluoresce.
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(h) Outline how the polymerase chain reaction (PCR) can be used to make multiple copies of DNA fragments
1. A reaction mixture is produced, containing:
- The DNA sample
- Primers
- Free, activated nucleotides
- DNA polymerase enzyme
2. The reaction mixture is heated to 95oc, which breaks the hydrogen bonds holding
complementary base pairs together. This forms two single-stranded template DNA
chains.
3. The reaction mixture is cooled to 55oc, which allows the primers to anneal to each of the
template strands.
4. The reaction mixture is heated to 72oc, at which temperature DNA polymerase is best
able to work. DNA polymerase adds free, activated nucleotides along the template
strand by complementary base pairing. When the enzyme reaches the end of the
template strand, the cycle of PCR is complete, and two new DNA strands are produced.
5. With each cycle of PCR, the amount of DNA increases exponentially.
(i) Explain how isolated DNA fragments can be placed in plasmids, with reference to the role of ligase
The gene and the plasmid are both cut with the same restriction enzyme.
If the gene does not have sticky ends, sticky ends are added.
The gene is incubated with the plasmids. Some plasmids will take up the gene, which will
anneal by complementary base pairing between sticky ends. DNA ligase is then used to
seal the nicks in the sugar-phosphate backbone, to form recombinant DNA.
Some plasmids will not take up a copy of the gene, and will instead re-seal in the presence
of DNA ligase.
(j) State other vectors into which DNA may be incorporated
Yeast artificial chromosomes;
Bacteriophages (viral DNA);
Liposomes
(k) Explain how plasmids may be taken up by bacterial cells in order to produce a transgenic microorganism that can
express a desired gene product
Electroporation:
Large quantities of plasmids are mixed with bacterial cells.
Calcium salts are added to the mixture.
The mixture temperature is decreased to 0oc.
The temperature is then quickly raised to 40oc.
This increases the membrane permeability of the bacteria, so they are more likely to
take up a plasmid.
Micro-injection
Bacteriophages:
Infect the bacterium by injecting its DNA into the bacteria.
(l) Describe the advantage to microorganisms of the capacity to take up plasmid DNA from the environment
Plasmids may contain:
- Genes for antibiotic resistance. This will increase the chances of survival of the
bacteria.
- Genes which help the bacteria to invade hosts.
- Genes which help the bacteria to break down different nutrients or sugars.
- Genetic variation.
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(m) Outline how genetic markers in plasmids can be used to identify the bacteria that have taken up a recombinant
plasmid
Not all bacteria will take up a plasmid, and some bacteria will take up plasmids without
the recombinant DNA. Because of this, marker genes are used as a means of identifying the
transformed bacteria (those with the recombinant plasmid).
Genetic markers can either code for antibiotic resistance, or for fluorescence. If the marker
gene is for antibiotic resistance, it is inserted into the plasmid along with the desired gene,
on the plasmid. The bacteria are then grown on a culture containing the antibiotic, so that
only the bacteria with the resistance gene are able to grow successfully.
If the marker gene is for fluorescence, bacteria are grown until each has produced a colony.
Then, the colonies are exposed to UV light, and those that fluoresce are the bacteria with
the recombinant plasmid.
(n) Outline the process involved in the genetic engineering of bacteria to produce human insulin
1. The gene for human insulin is isolated and extracted. This is done by identifying
mRNA in the cytoplasm of ß-cells in the Islets of Langerhans (pancreas), which codes
for the production of insulin. Reverse transcriptase is then used, which reverse
manufactures single-stranded cDNA from mRNA. DNA polymerase is used to produce
double-stranded cDNA.
2. A single sequence of nucleotides is added to each end of the DNA to make sticky ends.
3. A plasmid is cut using a restriction enzyme, and complementary sticky ends are added.
4. The plasmid and gene are mixed in the presence of DNA ligase, which seals the nicks in
the sugar-phosphate backbone.
5. The plasmid is inserted into a host (bacterial) cell, which then reproduces.
6. The gene for human insulin is expressed by each of the bacteria, and can then be
extracted and purified for human use.
(o) Outline the process involved in the genetic engineering of ‘Golden Rice TM’
Golden Rice TM is genetically modified to include beta-carotene, which the human body is
able to convert to vitamin A. Vitamin A deficiency can cause blindness, and so Golden Rice
TM was engineered to provide beta-carotene for areas which rely on rice for the staple diet.
1. The ‘psy’ gene (from a daffodil), and the ‘crtl’ gene (from a soil bacterium) are
isolated and inserted into TI plasmids (using restriction enzymes and
complementary sticky ends), along with a marker gene.
2. The plasmids are inserted in Agrobacterium tumefaciens (a bacterium).
3. Rice plant cells are incubated with the transformed A. tumefaciens bacteria,
which infect the rice plant cells, inserting its genes into the rice plant cell DNA.
This produces transformed rice plant cells.
4. The rice plant cells are then grown on a selective medium, so only the
transformed cells are able to grow.
5. The rice plant cells then express the genes for beta-carotene, which is converted
to vitamin A when eaten.
(p) Outline how animals can be genetically engineered for xenotransplantation
- Pigs have been genetically engineered without the sugar Gal-alpha(1,3)-Gal attached to
their cell-surface proteins. This protein does not occur in humans, and is a key trigger for
transplant rejection. This is done by removing the gene for the unwanted sugar from the
nucleus of a somatic pig cell. Nuclear transfer is then used to produce a pig without the
genes for the unwanted sugar.
- Pigs have also been genetically engineered with human cell-surface proteins. These pigs
are produced by injecting the gene into a newly fertilised pig embryo.
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(q) Explain the term ‘gene therapy’
Gene therapy encompasses any therapeutic technique which aims to improve the function
of a particular organism. This usually involves the insertion of a functioning dominant
allele, to either replace or silence a faulty or recessive allele, which cause malfunctioning of
that particular gene. Recessive conditions are easier to treat, as a dominant allele can be
inserted, however dominant conditions can also be treated by ‘silencing’ the faulty allele
(done by inserting a length of non-coding DNA into the middle of the faulty allele, so that
it is not expressed).
(r) Explain the differences between somatic and germ-line gene therapy
Somatic Gene Therapy
Germ-line Gene Therapy
The functioning allele of the gene is introduced to
The functioning allele is introduced to the germthe target cells of the patient only. Cells are not
line (embryonic) cells of the patient, so that every
reproduced with the functioning allele.
cell will reproduce to contain the functioning allele.
Because cells do not reproduce with the
The cells reproduce the functioning allele, so no
functioning allele, any treatment is short-lived and further treatment will be necessary. Any offspring
must be regularly repeated. The functioning allele
may also contain the inserted, functioning allele.
will not be passed to offspring.
There can be difficulties in introducing the allele to
It is considered unethical to genetically engineer
the target cells. Liposomes have been tried but may
human embryos. It is not possible to know
be inefficient.
whether any unintentional changes have been made
which may damage the embryo.
(s) Discuss the ethical concerns raised by the genetic manipulation of animals (including humans), plants and
microorganisms
Animals (humans):
- It is feared that humans may be genetically engineered to produce desirable
traits, such as intelligence. This could produce a genetic underclass.
- Some people think that genetically engineering animals for xenotransplantation
could cause them suffering.
- There are fears that we do not understand the long-term implications of genetic
engineering.
- Religious objections (e.g. in some cultures, cows are considered sacred and pigs
are unclean).
Plants:
- Super-weeds (cross between pesticide-resistant crops and weeds).
- Monoculture (bad for biodiversity).
- Poor farmers sold expensive crops they could not afford.
- Eating GM plants may be detrimental to long-term health.
Microorganisms:
- Superbugs (produced from the use of antibiotic resistant marker genes).
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Module 3: Ecosystems and Sustainability
5.3.1: Ecosystems
(a) Define the term ‘ecosystem’
An ecosystem is a dynamic (constantly changing) system made up of all the biotic (living)
and abiotic (non-living) features of a particular area, and their interactions.
(b) State that ecosystems are dynamic systems
(c) Define the following terms, using named examples
Biotic factor
Any living features of an ecosystem, e.g. availability of food, predation, disease
Abiotic factor
Any non-living features of an ecosystem, e.g. climate, water availability, temperature
(d) Define the terms
Producer
An organism (plant) which produces complex organic molecules from simple
organic molecules (sunlight energy)
Consumer
An organism which gains energy from complex matter (by eating other organisms)
Decomposer
An organism which gains energy by breaking down organic molecules found in dead
organisms or other undigested waste products
Trophic level
The stage in a food chain at which a particular species feeds
(e) Describe how energy is transferred through ecosystems
Energy is transferred when one organism consumes another organism.
Energy transfer is shown in a food web, with the arrows representing the flow of energy.
(f) Outline how energy transfers between trophic levels can be measured
Bomb Calorimeter
- A sample of prey and predator are collected (e.g. a known mass of grass, and one
mouse which feeds on the grass).
- Each sample is dried in an oven and then weighed.
- The samples are burned in a bomb calorimeter, and the energy given off is used
to heat a known mass of water.
- The energy content of both prey and predator are calculated using the
temperature rise and the latent heat capacity of water.
- The difference between energy content of each sample is calculated.
- The energy content of the ecosystem can be calculated by multiplying the energy
content of each sample by the size of their respective populations.
Measuring Dry Mass
- A sample of prey and predator are collected.
- The samples are dried in an oven.
- The dry mass of each sample is taken (by weighing the samples)
- The difference in energy content is taken by the difference in mass between each
trophic level.
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(g) Discuss the efficiency of energy transfer between trophic levels
The amount of energy transferred decreases at each trophic level, due to the fact that a lot
of the gross energy intake of the prey organism will be lost through processes such as
respiration and egestion, and also due to the fact that a lot of the mass of the prey will
consist of indigestible organic matter (cellulose fibres in plants, bones in animals).
Efficiency of energy transfer is shown by both gross and net productivity:
- Gross productivity is the total energy intake of an organism. This is never as high
as it could be, due to the fact that organisms never eat all of the available food; and
plants never absorb all of the available sunlight.
- Net productivity is the amount of energy taken in, which is eventually converted
into complex organic molecules (stored as body-weight), and therefore available
to the next trophic level. This is never as high as gross productivity, because most
(75%) of the energy taken in by an organism is used in respiration and keeping
warm, or lost through urination and egestion.
(h) Explain how human activities can manipulate the flow of energy through ecosystems
Sometimes, it is beneficial to humans to alter the flow of energy through an ecosystem, so
that more energy can be made available to a particular organism or species. For example,
the more energy and nutrients available to a field of crops, the higher the net productivity
of the crops, so more food is available to humans (as well as more profit for the farmer).
There are several methods for altering the flow of energy:
- Use of pesticides to remove pests, which feed off the crops and reduce
productivity;
- Use of fungicides to kill infections, which damage crops and reduce
productivity;
- Use of herbicides to kill weeds, which compete with the crops for sunlight,
nutrients, space and water, and reduce productivity;
- Addition of fertilisers, which replace lost nutrients in the soil and maximise
productivity;
- Use of soil irrigation to replace lost water in the soil;
- Sheltering organisms from damaging environmental factors;
- Intensively rearing livestock, by restricting their movement and keeping them
warm, so that little energy is lost through respiration or heat loss.
Humans also manipulate the flow of energy through an ecosystem by deflecting succession,
for example, in maintaining a grassy field or lawn.
(i) Describe one example of primary succession resulting in a climax community
Bare Rock
- Algae and lichens are the pioneer species (first ‘seral stage’) They attach to the
bare rock surface, and begin to break it down. Natural erosion and weathering
of the rock aid this process.
- As the rock is broken down and algae and lichens start to die, a small amount of
organic matter begins to build up on the rock surface, forming a very thin layer
of basic soil.
- Eventually, enough soil builds up to support slightly larger plants, such as
mosses and small ferns (second seral stage). The root structures of these plants
offer more stability to the soil, and an increasing amount of dead organic matter
produces a slightly thicker layer of soil. The soil is now able to hold more water.
The mosses and ferns outcompete the pioneer species to become dominant.
- Larger plants, such as grasses, small flowing plants (third seral stage), and
eventually shrubs and trees (climax community), are now able to be supported.
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-
With each plant comes an increase in the volume and complexity of the soil, so
that even larger plants can be supported. Each successive seral stage
outcompetes the organisms from the previous stage, to become the dominant
species.
The climax community is the largest and most complex community of plants
and animals that the ecosystem can support.
(j) Describe how the distribution and abundance of organisms can be measured, using line transects, belt transects,
quadrats and point quadrats
Line Transect
A line (tape measure) is laid across an ecosystem, and every organism which touches
the line is recorded, along with its position on the line (distribution). The transect
must encompass every change in the ecosystem (e.g. from the shade under a large hedge,
right out to the middle of an open field). Abundance is calculated by counting the
number of times each species touches the line.
Belt Transect
A line (tape measure) is laid across an ecosystem as with the line transect. Instead of
measuring every species which touches the line, quadrats are laid continuously along
the line, and the number of each species in each quadrat are recorded. The quadrats
must be numbered so that the distribution of species is also recorded.
Quadrats
A quadrat is a square frame, usually 1m2, divided into 100 smaller squares by strings
running across the frame. They are placed randomly within the area under
investigation (random placement achieved by dividing the area into a grid and using a
random number generator to produce co-ordinates), and the number of each species
present within the quadrat is recorded. Abundance can be estimated by calculating
percentage cover: counting the number of squares covered by a particular species (e.g.
grass). This is useful when it is impractical to count every individual member of a
species.
Point Quadrats
Point quadrats consist of a frame, with holes along its length for long pins to be lowered
vertically through. They are placed randomly within the area under investigation, and
pins are dropped through each hole. The species touched by the pins on the way down
are recorded. Percentage cover can be calculated by working out the number of times a
particular species was hit, as a percentage of total pins dropped.
(k) Describe the role of decomposers in the decomposition of organic material
Decomposers break down dead or undigested organic material, releasing the energy which
would otherwise remain locked within these organic molecules. Decomposers are
important for the recycling of nutrients, and elements such as carbon and nitrogen, within
an ecosystem.
(l) Describe how microorganisms recycle nitrogen within ecosystems (only Nitrosomonas, Nitrobacter and Rhizobium
need to be identified by name)
Nitrogen Fixation
Nitrogen is fixed (converted to ammonia) from atmospheric nitrogen by nitrifying
bacteria such as Rhizobium, which live in the root nodules of leguminous plants, as
well as free-living in the soil. Rhizobium form a mutualistic relationship with
leguminous plants, receiving carbohydrates in return for the production of
ammonia.
Nitrogen fixation also occurs when lightening hits the earth.
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Ammonification (Decomposition)
When animals and plants die, nitrogen remains locked inside the dead material in
the form of DNA and amino acids. Ammonium ions are released by decomposers
when they break down and digest this organic matter, which recycles the nitrogen
to ammonium compounds in the soil.
Nitrification
Ammonium compounds in the soil cannot be taken up by plants, so they must first
be converted to a useable form. Nitrosomonas bacteria obtain energy by oxidising
ammonium compounds to nitrites in the soil. Nitrobacter bacteria obtain their
energy by oxidising nitrites to nitrates under anaerobic conditions, which can then
be taken up and used by plants.
Denitrification
In anaerobic conditions, some denitrifying bacteria in the soil will reduce nitrates
back to nitrogen gas, using the nitrates as a source of oxygen. This usually happens
in waterlogged soil.
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5.3.2: Populations and Sustainability
(a) Explain the significance of limiting factors in determining the final size of a population
A habitat cannot support a population greater than its carrying capacity. This is because
several factors quickly become limiting, and prevent further increase in population size.
Examples of limiting factors include:
Biotic
- Predation
- Intra- and inter-specific competition
- Food availability
- Spread of disease (lots of individuals in a small space)
Abiotic
- Space (nesting sites)
- Water availability
- Light availability
- Oxygen availability
- Shelter
(b) Explain the meaning of the term ‘carrying capacity’
The maximum number of individuals of a species that an ecosystem or habitat can
sustainably support.
(c) Describe predator-prey relationships and their possible effects on the population size of both the predator and the prey
The larger the population of the prey, the more food is available to predators.
The more food available to predators, the more the predators will breed.
The more the predators breed, the more individual predators need to be sustained by the
prey population.
More prey is eaten by the predators, so the prey population decreases.
As less prey is available to be eaten by predators, more predators will die from lack of food.
The predator population decreases.
With less predators to eat the prey, the prey population gradually increases again.
The cycle continues.
(d) Explain, with examples, the terms interspecific and intraspecific competition
Interspecific Competition
This is competition between different species. If two species occupy a similar niche,
they will compete for resources such as food, space and nesting sites. Interspecific
competition reduces the population size of both species, and one species may even
outcompete the other species entirely.
An example is the competition between red and grey squirrels in Britain. The native
red squirrel is outcompeted by the larger grey squirrel in woodland of less than 75%
conifer trees. However, in woodland of 75% or more conifer trees, the smaller red
squirrel outcompetes the grey squirrel.
Intraspecific Competition
This is competition between individuals of the same species. If the food supply
becomes a limiting factor, individuals often need to compete for food, in order to
survive. Intraspecific competition allows natural selection to work, as those
individuals with the alleles for a specific survival advantage are more likely to
survive and reproduce, passing on their beneficial alleles.
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(e) Distinguish between the term ‘conservation’ and ‘preservation’
‘Conservation’ involves the dynamic management of an ecosystem to allow it to be
sustainably exploited for human benefit, without permanent damage to the ecosystem or
habitats, while maintaining biodiversity. ‘Preservation’ involves preventing any change to
an ecosystem, so nothing is added and nothing is removed.
(f) Explain how the management of an ecosystem can provide resources in a sustainable way, with reference to timber
production in a temperate country
Strip Felling
Woodland is divided into strips, or small areas. Each year, one strip or area is
cleared, leaving the rest untouched. This produces a supply of timber without
damaging the ecosystem, or removing irreplaceable habitats. This also maximises
biodiversity, as the woodland supplies a range of habitats with different levels of
shelter, because different areas of the woodland have grown back to a different
extent. This method also prevents soil erosion.
Selective Felling
The largest trees (those that will produce the greatest yield), and diseased or
damaged trees are felled. This allows more room (less competition) for the other
trees, and provides a supply of timber.
Coppicing
Trees are felled close to the ground, leaving a small stump. Shoots are able to regrow quickly from the stump (due to its well developed root system), producing
several thin poles. Once they have matured, these poles can then be felled regularly.
(g) Explain that conservation is a dynamic process involving management and reclamation
In order to maintain biodiversity, conservationists must dynamically manage an ecosystem,
so that they are able to adapt their strategy to encompass any naturally occurring changes.
This is demonstrated by the fact that often, the climax community of a particular
ecosystem is not as biodiverse as it could be. Conservationists therefore may deflect
succession slightly, in order to allow an area to support as many different species as
possible, which may not be a natural seral stage.
Sometimes, it may be necessary to ‘reclaim’ and area which has lost its biodiversity. This
could be due to soil erosion from clear felling of trees, or bad management of an ecosystem.
In this case, conservationists must work to re-establish biodiversity, and then slowly adapt
their approach, so that it best fits the particular ecosystem, climate, and species they are
working with.
(h) Discuss the economic, social and ethical reasons for conservation of biological resources
Economic
- Many valuable resources are located in biodiverse areas, e.g. existing and
potential medicines in the rainforest.
- Many resources are traded on both a local and international scale.
- Many species provide a valuable food source.
- Genetic diversity of many species may be needed in the future for certain
characteristics.
- Ecotourism makes money from tourists visiting area with high biodiversity.
Social
- Aesthetic value of biodiverse areas.
- Ecotourism.
- People enjoy and use the areas for certain activities, e.g. bird-watching.
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Ethical
-
Human stewardship (conservation for future generations).
All organisms have the right to exist and value in their own right.
(i) Outline, with examples, the effects of human activities on the animal and plant populations in the Galapagos Islands
- In the 19th century, explorers and sailors visited the islands. They hunted the giant
tortoise to extinction for food.
- An increase in tourism on the islands has lead to increased development:
- Airports have increased in size to accommodate more visitors
- Sanitation services have been improved
- The number of building have been increased
- Pollution has increased
- Population of the islands has increased
- Non-native animals have been introduced to the islands. These compete with native
species:
- Dogs, cats and black rats predate on young giant tortoises and Galapagos land
iguanas
- Pigs destroy iguana nests and eat their eggs
- Goats eat much of the plant life on the islands, competing with native species for
food
- Non-native plants have been introduced to the islands. These also compete with native
species:
- The quinine tree outcompetes native trees for light and space, causing reduced
numbers of native trees
- Overfishing causes a depletion in the number of native sea species:
- Hammerhead sharks have been overfished
- Sea cucumbers have been overfished
- Galapagos green turtles have been overfished and are accidentally killed in fishing
nets
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Module 4: Responding to the Environment
5.4.1: Plant Responses
(a) Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress
In order to survive and reproduce, plants must be able to respond to their environment.
Some plants respond to the threat of predators by producing toxic substances, which
discourage predators from eating them. Plants also respond to abiotic stress, for example,
some carrots are able to produce their own form of antifreeze in extremely cold weather.
Under normal circumstances however, plants also need to respond to abiotic conditions
such as lack of water, lack of sunlight, and extreme weather conditions (depending on
their particular habitat). Many plant responses are co-ordinated by plant growth
substances.
(b) Define the term ‘tropism’
A tropism is a directional growth response of a plant, in which the direction of the
response is determined by the direction of the particular stimulus.
(c) Explain how plant responses to environmental changes are co-ordinated by hormones, with reference to responding to
changes in light direction
A change in the external environment can often act as a stimulus for a plant, producing an
alteration in its growth, or direction of growth. There are several plant growth hormones
which can produce a change in plant growth, however ‘auxins’ are the most common
growth substances involved in changing direction of growth.
- Auxins are produced in the apical buds of plants, and cause an increase in growth of
shoots, but a decrease in growth of roots.
- Auxins are responsible for cell elongation, and work by promoting the active
transport of hydrogen ions through the ATPase enzyme, into the cell wall. This
decreases the pH and allows optimum conditions for the wall loosening enzymes to
work. These enzymes break the bonds within cellulose fibres, so cell walls become
less rigid and can expand as the cell takes in water.
- Auxins are distributed throughout the plant using diffusion and active transport (and
via the phloem over long distances). This often results in uneven distribution of
auxins.
- In sunlight, the auxins are distributed more towards the shaded side of shoots.
- This causes the part of the shoot in the shade to grow by cell elongation, causing the
shoot to grow towards the light (‘positive phototropism’). This mechanism allows
for maximum light absorption for photosynthesis.
- In the roots, auxins are also transported to the more shaded side. However, auxins
inhibit growth in the roots, and so the roots grow away from the light (‘negative
phototropism’).
(d) Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the
control of stem elongation
Auxins:
Auxins control apical dominance by stimulating cell elongation of the apical bud, and
preventing side shoot formation further down the plant. This can be shown through
experimental evidence:
- Take 30 plants of the same species and similar age, height and weight
- Plant 10 untouched (as the control)
- Remove the apical bud from 10 and apply a paste containing auxins
- Remove the apical bud from the last 10 and apply a paste without auxins
- Count and record the number of side shoots on each plant
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-
Leave the plants to grow for six days, then re-count and record the number of
side shoots per plant
This experiment should show that the plants without auxins grew more side-shoots than
both the control group and the group with the paste containing auxins. This is evidence to
show that auxins prevent side-shoot formation.
Gibberellins:
Gibberellins stimulate stem elongation, side-shoot formation, seed germination, and
flowering. We can test the effects of gibberellins on stem length experimentally:
- Take 40 dwarf plants of the same species and similar age, height and weight
- Plant all 40 in the same conditions, measure and record the height of each
- Water 20 plants with normal water
- Water 20 plants with a dilute solution of gibberellin
- Leave the plants to grow for 28 days, water them all every day
- Measure and record the height of the plants each week
This experiment should show that the dwarf plants watered with a dilute solution of
gibberellin grew considerably taller than the dwarf plants watered with plain water. This
is evidence to show that gibberellins control stem elongation.
(e) Outline the role of hormones in leaf loss in deciduous plants
- Cytokinins prevent leaf loss (‘senescence’) by making the leaves a sink for phloem
transport. This guarantees the leaves a good nutrient supply.
- If cytokinin production drops, the leaves no longer receive this supply of nutrients, and
senescence is triggered.
- Auxins prevent leaf loss and are produced by young leaves. As leaves age, less auxins are
produced.
- Senescence stimulates a drop in auxin production and concentration.
- Ethene stimulates leaf loss and is produced by older leaves, when auxin concentration
drops. Ethene works by stimulating cells in the ‘abscission layer’ to expand. This breaks
the cell walls of the abscission layer cells at the base of the leaf stalk, causing the leaf to
fall.
- The combined (‘antagonistic’) effect of auxins and ethene is leaf loss.
(f) Describe how plant hormones are used commercially
Plant growth hormones are often used commercially to maximise profit and minimise
labour for those involved in the growth and transport of various plants for consumption:
- Ethene can be used to stimulate the ripening of fruit. This means that fruit can be
picked and transported before it is ripe, then sprayed with ethene so that it ripens on
the shelves of shops and in people’s homes. Ethene works by stimulating enzymes
which break down cell walls and chlorophyll, and convert starch in sugar.
- Auxins and gibberellins make unpollinated fruit develop. This can be used to produce
seedless fruit, e.g. seedless grapes.
- Being sprayed with a low concentration of auxins during early development prevents
fruit drop. Being sprayed with a high concentration of auxins at a later stage of
development encourages fruit drop. Growers can use this to ensure that all fruit drop
at the same time, to minimise labour.
- Auxins can be used in selective weed-killers. They make the apical bud of the weeds
grow too fast, so that the weed cannot take up enough nutrients. This makes the
weed fall over and die.
- Auxins are also used as rooting hormones for plant cuttings (e.g. in cloning by
micropropagation).
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5.4.2: Animal Responses
(a) Discuss why animals need to respond to their environment
Animals need to respond to their environment to maximise their chances of surviving to
reproduce and pass on their genes. This is done using a combination of nervous and
hormonal communication to produce a desired response. It is also important that animals
have the ability to maintain an optimal internal environment (homeostasis).
(b) Outline the organisation of the nervous system in terms of central and peripheral systems in humans
Central Nervous System
(CNS)
Peripheral Nervous System
(PNS)
Somatic Nervous System
(Under conscious control)
Autonomic Nervous System
(ANS)
Sympathetic Nervous System
(Fight or Flight)
Parasympathetic Nervous System
(Rest and Digest)
(c) Outline the organisation and roles of the autonomic nervous system
The autonomic nervous system is divided into the sympathetic and parasympathetic
nervous systems. These two systems are antagonistic in that they have opposite effects:
Sympathetic
- Active in times of stress
- Prepares the body for movement
- Increases heart and breathing rate
- Increases tidal volume of the lungs
- Diverts blood from the skin and gut to the skeletal muscles
- Stimulates the conversion of glycogen to glucose in the muscles
- Neurotransmitter between neurone and effector is noradrenaline
Parasympathetic
- Active during sleep and relaxation
- Decreases heart and breathing rate
- Promotes digestion
- Neurotransmitter between neurone and effector is acetylcholine
(d) Describe, with the aid of diagrams, the gross structure of the human brain, and outline the functions of the cerebrum,
cerebellum, medulla oblongata and hypothalamus
Cerebrum
- Divided into the cerebral hemispheres, connected by the corpus callosum
- Has a highly folded cortex
- Responsible for higher thought: thinking, seeing, hearing and learning
Cerebellum
- Also has a highly folded cortex
- Located underneath the cerebrum
- Responsible for co-ordinated fine movement, posture and balance
Medulla Oblongata
- Located at the base of the brain, at the top of the spinal cord
- Automatically controls heart and breathing rate
Hypothalamus
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- Located just beneath the middle part of the brain
- Controls homeostasis and the pituitary gland
(e) Describe the role of the brain and nervous system in the co-ordination of muscular movement
The conscious decision to move voluntarily is initiated in the cerebellum. Neurones from
the cerebellum carry impulses to motor areas of the brain, so that motor output to the
effectors can be adjusted appropriately.
(f) Describe how co-ordinated movement requires the action of skeletal muscles about joints, with reference to the
movement of the elbow joint
Skeletal muscles are under conscious control. All muscles work in antagonistic pairs, as
muscles do not have any power to ‘push,’ they are only able to ‘pull’ and relax. Muscles are
attached to bones by tendons. Ligaments attach bone to bone. In order to move a bone, for
example the lower arm bones, skeletal muscles must use their ‘pull’ force. In this example,
the biceps of the upper arm must contract whilst the triceps relax. This causes the bone in
the lower arm to be pulled upwards from the elbow joint. In order to move the bone back
down, the biceps must relax while the triceps contract. In this way, antagonistic muscles
and joints work together to move the skeleton.
(g) Explain, with the aid of diagrams and photographs, the sliding filament model of muscular contraction
The Neuromuscular Junction:
- An impulse travels down the neurone and reaches the
presynaptic knob.
- Calcium ions diffuse into the presynaptic knob, causing vesicles
containing acetylcholine to move towards and fuse with the
presynaptic membrane.
- Acetylcholine diffuses across the synaptic cleft, and binds to
nicotinic cholinergic receptor sites on the motor end plate.
- This causes ligand-gated sodium ion channels to open. Sodium
ions diffuse through the membrane, which depolarises the
sarcolemma.
- Depolarisation spreads down the T-tubules to the sarcoplasmic
recticulum.
- Acetylcholine-esterase is stored in clefts on the motor end plate, and breaks down
acetylcholine in the synaptic cleft to prevent continual depolarisation of the
sarcolemma.
Influx of Calcium Ions:
- The depolarisation of the sarcolemma spreads down the T-tubules until it reaches the
sarcoplasmic recticulum, which is then stimulated to release stored calcium ions
(Ca2+) into the sarcoplasm.
- Calcium ions bind to a protein called troponin, which is bound to another protein
called tropomyosin (which, in an unstimulated muscle, is located in such a place as to
block the actin-myosin binding site and prevent contraction).
- This causes the troponin to change shape, pulling the tropomyosin out of the actinmyosin binding site on the actin filament.
- This exposure of the actin-myosin binding sites means that the globular myosin head
is now able to bind to the actin filament (forming actin-myosin cross bridges).
The Power Stroke and the Role of ATP:
- The globular myosin head now moves in a rowing motion, pulling the actin filament
along. This is the power stroke, and causes muscle contraction. When the actin
filament has been moved as far as possible, ATP binds to the myosin head, and the
cross-bridge is broken.
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- The ATP is then broken down (hydrolysed) into ADP and Pi. This provides the
energy to move the myosin head back into the ‘cocked’ position.
- The cycle is now able to begin again: Pi is released as the myosin head springs forward
and attaches to the actin filament (forms an actin-myosin cross bridge), further along
from where the previous cross bridge was formed.
- ADP is then also released as the myosin head pulls the actin filament forwards.
- Finally, ATP again binds to the myosin head, causing the cross bridge to break.
Muscle contraction will continue for as long as the calcium ions are present in the
sarcoplasm. When the contraction impulse stops, and the sarcolemma is no longer
polarised, Ca2+ is actively transported back into the sarcoplasmic recticulum. This causes
troponin to return to its original shape, pulling tropomyosin back into its resting position,
blocking the actin-myosin binding sites. With no cross bridges to hold it in place, the
actin filament slides back to its resting position, relaxing the muscle.
(h) Outline the role of ATP in muscular contraction, and how the supply of ATP is maintained in muscles
Role of ATP:
- ATP is essential as it provides the energy needed to break the actin-myosin cross
bridge, which allows the myosin head to re-attach, further down the actin filament.
- When ATP has been hydrolysed to ADP and Pi, it also provides the necessary
energy for the power stroke, which moves the actin filament along, causing muscle
contraction.
Maintenance of ATP Supply:
ATP is produced by aerobic respiration in the mitochondria of muscle fibres.
However, this only works in the presence of oxygen, meaning that it is good for long
periods of low-intensity exercise.
ATP is produced by anaerobic respiration via glycolysis. However, the end product
of this process is pyruvate, which is converted to lactate by lactate fermentation.
This causes a build up of lactic acid in muscle cells, causing fatigue.
ATP is produced using the ATP-PCr (phosphocreatine) system. ADP is
phosphorylated using a phosphate group taken from PCr. This produces ATP and
Cr. This process is alactic and anaerobic, however the supply of PCr runs out very
quickly, so is good for short bursts of vigorous activity.
(i) Compare and contrast the actin of synapses and neuromuscular junctions
Synapse
Neuromuscular Junction
Neurotransmitter
Various
Acetylcholine
Post-Synaptic Cell
Post-Synaptic Receptors
Another neurone
Various
Muscle cell (sarcolemma)
Nicotinic-cholinergic receptor sites
Number of Post-Synaptic Receptors
Post-Synaptic Membrane
Effect of Depolarisation of
Membrane
Fewer
Smooth
May cause another action
potential to fire (must reach
threshold value)
Various ways
Many
Has clefts containing AChE
Always causes muscle contraction
Removal of Neurotransmitter
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Broken down by AChE
(j) Outline the structural and functional differences between voluntary, involuntary and cardiac muscle
Voluntary
Striated
Multi-nucleate
Long muscle fibres
Involuntary
Smooth
Single nucleus
Small, spindle-shaped muscle
cells (approx 0.2mm long)
Made of many muscle cells joined
Controls blood pressure and
together
peristalsis
Under voluntary control
Under involuntary control
Allows movement of the skeleton Surrounds hollow internal organs
Can be either twitch fibres
Contract slowly, don’t fatigue
(contract quickly) or tonic fibres
(contract slowly). Both fatigue.
Cardiac
Partially striated
Single nucleus
Small, cylindrical-shaped muscle
cells (approx 0.2mm long)
Branched with intercalated disks
Myogenic
Found only in walls of the heart
Contract rhythmically, don’t
fatigue
(k) State that responses to environmental stimuli in mammals are co-ordinated by nervous and
endocrine systems
(l) Explain how, in mammals, the ‘fight or flight’ response to environmental stimuli is co-ordinated by the nervous and
endocrine systems
Nervous
- Sensory neurones in the somatic nervous system carry impulses from receptors
to the sensory area of the cerebrum, giving information about the danger in the
environment.
- The sympathetic nervous system of the ANS is activated.
- Impulses are sent down the accelerator nerve to the SAN of the heart,
stimulating it to increase the heart rate and stroke volume.
- The adrenal glands are stimulated to secrete adrenaline.
Endocrine
The release of adrenaline causes several responses:
- Smooth muscle surrounding the bronchioles relaxes, to increase air flow to the
lungs;
- Blood flow to the skin and gut is reduced and redirected to the involuntary
muscles;
- Glycogen is broken down into glucose in muscle cells;
- Mental awareness is increased.
These responses prepare the body for the sudden use of muscles in order to either fight, or
escape from, the source of danger.
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5.4.3: Animal Behaviour
(a) Explain the advantages to organisms of innate behaviour
Innate behaviour is inbuilt from birth, and therefore does not need to be learned. This is
advantageous as innate behaviour often confers an immediate survival advantage from
birth (e.g. suckling provides food for the infant). It is also important for organisms with a
short life-span, as they do not have the time required to learn appropriate behaviour.
Innate behaviour is stereotyped, in that all members of the same species are born with the
same innate responses. This allows the responses to become appropriate for a particular
habitat, as natural selection allows fine-tuning of the alleles controlling it.
(b) Describe escape reflexes, taxes and kineses as examples of genetically determined innate behaviours
Escape Reflexes
This is an innate response to potential danger. The organism will automatically
move to avoid the danger which has caused the response.
Taxes
This is a tactic response to move the organism away from unpleasant external
stimuli. The movement is directional, and depends upon the direction of the
stimulus.
Kineses
This is a kinetic response to unpleasant external stimuli. It produces an increase in
the rate of movement of the organism, however it is not directional: the increased
movement of the organism allows it to escape the unpleasant stimuli by chance.
Movement is decreased when the stimuli is no longer detected.
(c) Explain the meaning of the term ‘learned behaviour’
Learned behaviour is behaviour which is modified as a result of experience.
(d) Describe habituation, imprinting, classical and operant conditioning, latent and insight learning as examples of
learned behaviours
Habituation
The organism learns not to respond to unimportant stimuli. After repeated
exposure to a stimulus which yields neither reward nor punishment, the organism
learns to ignore the stimulus so as not to waste energy reacting to it.
E.g. a baker will no longer salivate over the smell of freshly baked bread.
Imprinting
An organism is susceptible to imprinting for only a very short period (‘critical
period’) after birth. During this critical period, the new born organism learns to
recognise its parents: usually this is the first moving object that it sees. This allows
the young organism to learn from its parents, and to recognise a mate in later life.
E.g. ducklings will imprint on either their parent duck, or, if hand-reared from birth,
possibly a human.
Classical Conditioning
The organism learns to associate a natural response with an unnatural stimulus. If
the unnatural stimulus regularly coincides with a natural stimulus (which produces
the natural response), the organism will eventually learn to respond to the
unnatural stimulus alone.
E.g. Pavlov’s dogs
Operant Conditioning
The organism learns to associate a particular response with either a reward or a
punishment.
E.g. rats regularly placed in a Skinner Box
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Latent Learning
The organism does not show that it has learned anything until it is stimulated by
either reward or punishment. This involves learning through repeatedly doing the
same task or exploring the surroundings. Organisms may use this learning to escape
from danger.
E.g. rabbits will learn the layout of their home warren, and can escape quickly if
threatened by a predator.
Insight Learning
The organism uses previous experiences, and reason, to work out how best to
achieve a desired outcome. This is much quicker than trial-and-error, because
actions are planned and worked out.
E.g. chimps placed in a playpen with boxes, clubs, sticks, and bananas hanging from
the ceiling out of reach. The chimps are able to pile up the boxes and beat the
bananas down with the clubs and sticks.
(e) Describe, using one example, the advantages of social behaviour in primates
Baboons
- Baboons live in groups of 50 or so individuals.
- They hunt together to cover a wider area, and communicate good food sources
to the rest of the group.
- When moving through their territory, males stay on the outside of the group,
while female and infant baboons travel in the middle. This allows the males to
protect the young from danger, and to ensure that there are enough females to
mate with.
- There is a clear-cut male hierarchy within the group, so that no time or energy is
wasted by fighting.
- Baboons groom each other. This is hygienic, and so helps to prevent spread of
disease, and also strengthens social bonds within the group.
(f) Discuss how the links between a range of human behaviours and the dopamine receptor DRD4 may contribute to the
understanding of human behaviour
Much understanding of how the human brain works has come from studying those with
‘mental disorders.’ This comparison between a ‘normal’ brain, and one whose physiology
causes certain behavioural changes, allows experts to begin to pinpoint which areas of the
brain are responsible for which mental alterations.
In humans, dopamine is a neurotransmitter released when a person is ‘happy.’ Because of
this, it is considered a reward hormone. Some people have more DRD4 receptors in their
brain than others, and this can cause certain behavioural abnormalities. For example,
people with schizophrenia have more D4 receptors in their brain than people without this
disorder.
This link has been tested, and the supporting evidence includes:
- People with schizophrenia have a higher density of D4 receptors in their brain.
- If drugs which stimulate the dopamine receptors are given to healthy people,
they begin to display schizophrenic traits.
- Drugs that block the action of dopamine reduce the symptoms of schizophrenia
in those with the disorder.
- One of the drugs that is used to treat schizophrenia binds better to the D4
receptors than it does to other dopamine receptors.
ADH and Parkinson’s disease are also thought to be linked to differing levels of dopamine
in the brain.
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