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Populations and Ecosystems
Ecology is the study of the interaction between living organisms and their environment, including
biotic and abiotic factors
...
It is made up of all
the biotic and abiotic factors in a particular area
...
There are two key processes
that occur in an ecosystem: energy flow and the nutrient cycling
...
Energy doesn’t need to be
recycled in an ecosystem, so it flows through the ecosystem
...
This chemical energy is then consumed by primary
consumers, which are then consumed by secondary consumers etc
...
They must therefore be
recycled through an ecosystem, so that they are replenished and continuously made available to
organisms
...
A community is a group of
interacting populations of organisms in a habitat at a particular time
...
The distinction between
different populations of the same species is important as different populations of the same species
may occupy different habitats and therefore show variation
...
Two groups of the same species that lived at
different times are also different populations
...
Within an ecosystem, there are many different
habitats that can each support a unique community of organisms
...
Within a habitat, there may be many different
microhabitats, which can themselves support populations or even small communities of organisms
...

A niche describes how a particular species fits into its environment
...
No two species can occupy the same niche
...
This is the competitive exclusion principle
...


Investigating Populations
Ecosystems and populations are investigated in order to determine the abundance of different
species within the environment
...

In order to measure the abundance, the number of individuals in the environment must be found
...
Samples are taken which are representative of
the population, and can therefore be used to measure the abundance
...

When sampling with quadrats, three factors must be taken into account: the size of the quadrat, the
number of quadrats and the position of the quadrats
...
However, for species that are not evenly distributed throughout
an area, a large number of small quadrats is more accurate than a small number of large
quadrats
...
However, laying out many quadrats is time consuming and requires more
effort, so a balance must be struck between the time and resources available and the
representativeness of the sample
The position of the quadrats in the area must be random in order to avoid sampling bias that
would affect the results
...


Systematic sampling using transects involves placing a transect (either line or belt) along a
particular area and recording the species and number of individuals of the species that cross the
transect
...
For example, a sea shore is better sampled using a transect due to the changing
environmental conditions caused by the tide and the proximity to water
...
There are two
main ways of measuring this abundance: frequency and percentage cover
...
E
...
if a species
occurs in 15 out of 30 quadrats, then its frequency is 50%
...
It provides a quick view of the species
present in an area and their general distribution
...
This is
useful for species that are difficult to count individually and for species that are very
abundant, as the number of individuals doesn’t have to be counted
...


Using quadrats and transects are very effective for plant and some insect species, however for most
animals they are ineffective
...
To test the abundance of animal populations, mark release
capture techniques are used
...
A second group of
animals are then captured and the number of marked individuals is identified
...
Size = no
...
of individuals from sample 2 / number of marked
individuals recaptured
This is based on a number of assumptions:







That the proportion of marked to unmarked individuals in the sample is the same as in the
population
...

That there is no immigration or emigration in the population size that would change the
population size
...

That the marker used doesn’t increase the risk of predation of the individual
That the marker used doesn’t poison the individual
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A statistical difference (where the test is for 0
...


Variation in Population Size
The size of populations of different species in their respective habitats varies according to abiotic
and biotic factors within their environments
...
They exert environmental resistance on the population size
...
This is because there are only a small number of organisms
so they reproduce at a slow rate
...
The increasing numbers of organisms reproduce at a
correspondingly higher rate, so the size of the population increases exponentially
...

A period of decline until the population size is stable
...
A balanced equilibrium is reached where the population size remains constant, with
small fluctuations at each interval of time
...
These factors impose a limit on the stable population size in the given
area over a long period of time
...
Biotic factors include predation, competition and disease
...


Abiotic factors:








Temperature
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Temperature affects enzyme action in plants
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Therefore the growth rate of the population will decrease
...
Mammals are also affected by
environmental temperature even though they can regulate their body temperature
...
This means that they have less energy for growth and
reproduction, so the population size is limited
...
As light is often a limiting factor of photosynthesis, an increase in the light intensity
results in an increase in the growth rate and size of a plant population
...
It also means that the size of the animal
populations that feed on these plants will increase, as there is more food available for them
...
pH affects enzyme action, so if the pH is above or below the optimum, enzymes will
denature and enzyme action will be decreased
...

Water and humidity
...
Humidity affects transpiration in plants and the
evaporation of water from the bodies of animals
...


Competition
Competition is where individuals compete with each other for resources such as food, water and
light
...
There are two main types of competition:
intraspecific competition and interspecific competition
...
The
availability of resources therefore determines the size of the population and are limiting factors to
the growth of the population
...
The population
growth will slow or decrease
...

Intraspecific competition is seen in all species as, in a given habitat, there is a limited number of
resources and the size of the population is usually larger than what the ecosystem can support
...
In high light conditions, the larger oak trees
grow at a high rate, blocking light from reaching the smaller oak trees
...
Birds also compete for resources
...
When food is scarce, the breeding territory must become larger in order to support the
family, meaning less space is available
...

Interspecific competition is competition for resources between different species
...
This means that two or more species will compete
for those resources
...
The species with the competitive advantage will out-compete the other
species, leading to its elimination from the ecosystem and the niche
...

In order to look at how availability of resources and predation affect population size, it is necessary
to look at the birth and death rate of the population
...


Predation
Predation is an interspecific relationship between predators and prey
...
Predators are adapted to find and consume prey,
and prey are adapted to evade and disguise themselves from predators
...
They
therefore impose selection pressures on each other and stimulate natural selection
...
In a laboratory, the outcome of putting a predator and prey
population together is almost always the extermination of the prey by the predator
...
This
is because the prey have many adaptations that allow them to evade predators that cannot be used
in an artificial environment, such as camouflage
...
Data that is obtained from the wild can only be done so through
sampling, which is variable in accuracy
...
All this means that data on predation is difficult to collect and must be
represented with caution, as it may not be representative or valid
...

As the prey population decreases, there is less available food for the predators population,
so intraspecific competition increases due the limited resources
...

The fall in the predator population means that there is less predation of prey, so the prey
population increases
...
This cycle then repeats
...
Although predation
does affect the population size, other factors also play a role, such as disease and water availability
...


Human Populations
Human populations have a different growth rate and growth pattern to other organisms
...
The result is a continued
increase in the size of human populations
...
These two factors have effectively removed limiting factors to
population growth
...
Disease also doesn’t affect the population in the same way, as the
development of medical care and drugs limits the effect of disease
...

In order to determine the change in the size of a human population, four factors must be taken into
account: birth rate, death rate, immigration and emigration
...
growth during period/pop
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of births per year/total pop
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of deaths per year/total pop
...

The structure of populations of different countries varies over time due to economic, environmental
and social factors
...
This population type is mostly seen in developed countries, while the population type of the
low life expectance and high death rate is mostly seen in developing countries
...

The growth curve of the population returns to the normal sigmoid shape
...
Different types of
pyramid show the different states a population is in
...
There will be a birth rate that is proportional to the number of elderly in the
population
...
This means it has a very
high birth rate that is greater than the death rate
...


Energy and ATP
Energy is the ability to do work
...

There are many different forms of energy, but energy cannot be created or stored, so the body
cannot use raw energy
...
It is a molecule that is continually
produced that is used to release energy that fuels the body
...
Light
energy is captured by plants and used to produce chemical energy (in the form of organic molecules)
in photosynthesis
...

These reactions make up the respiratory process and produce ATP
...
The phosphate bonds are high energy bonds, which
means that they release a large amount of energy when broken
...
This means that a relatively large amount of energy
can be produced from an ATP molecule readily, to be used in metabolic processes
...
The reaction itself is reversible, so a
molecule of water and energy is added to ADP and Pi (condensation reaction) to convert it back to
ATP
...
The process of synthesising ATP from ADP is
phosphorylation – the addition of a phosphate group onto the ADP molecule
...







Photophosphorylation occurs in the light dependent reaction of photosynthesis, in the
electron transport chain of the thylakoid membrane
...

Oxidative phosphorylation occurs in the electron transport chain of mitochondria, in aerobic
respiration
...

Substrate level phosphorylation occurs in plant and animal cells in many different metabolic
reactions
...


ATP is an immediate energy source
...
However, the ease at which it releases energy means that ATP is
unstable and cannot therefore be stored
...
This isn’t a problem as ATP is so easily reformed from ADP
...
ATP releases much less energy
than an entire glucose molecule, so the energy release is smaller and can be controlled
...
It is also
much easier to release energy from ATP (hydrolysis) and much easier to reform ADP from ATP
(condensation reaction)
...


Photosynthesis
Photosynthesis is the process by which plants convert light energy into chemical energy
...

The leaf is the main site of photosynthesis in plants
...
The leaf has many adaptations to allow
photosynthesis to occur at a high rate:










The leaf has a large surface area for the maximal absorption of light and diffusion of CO2 into
the leaf
...
These stomata also open and close depending on the light intensity, so the rate
of photosynthesis can be optimised according to the environmental conditions
...

Palisade cells are arranged at the top of the leaf so absorb the maximal amount of light
...

The leaves are thin, which decreases the diffusion pathway of CO2 and allows light to
penetrate the leaf easily
...

Xylem and phloem transport water and mineral ions to the leaf for use in photosynthesis
and transport the sugars produced in photosynthesis away to other tissues in the plant,
respectively
...
The first stage is the capture of light energy by
chlorophyll pigments in the chloroplast
...
The light excites the electrons in these ions
...
This is where the excited electrons travel down the
electron transport chain, producing ATP and NADPH
...
The last stage is the light independent reaction (the Calvin cycle)
...

The main sites in the chloroplast where these two reactions take place are the thylakoid membrane
and the stroma
...
The light independent reaction takes place in the stroma of the chloroplast
...
It is
referred to as the light dependent reaction as it actively uses light in order to form ATP and NADPH,
as well as for the photolysis of water
...
Photosystem 2 is a complex of different proteins that is
embedded into the thylakoid membrane
...
Photosystem 2 contains a chlorophyll molecule
...
This
excitation is so great that they leave the chlorophyll molecule and are taken up by an electron
carrier
...
The electron pair passes along a sequence of electron carriers in a series of redox reactions
...
Each electron carrier is at a slightly lower energy level
than the last one, so with each jump between electron carriers, the electrons lose energy
...
This is photophosphorylation:
light energy being used to phosphorylate ADP to ATP
...
Photosystem 1 is an adjacent photosystem with a very
similar structure to photosystem 2 – a central chlorophyll molecule and numerous electron carriers
...
This raises their excitation energy once more, and they pass
down a sequence of electron carriers in a series of redox reactions, with ATP being synthesised in
the same way as in photosystem 2
...
This is done through the photolysis of water
...
The electrons produced
replace those lost by the chlorophyll molecule from photosystem 2
...
The protons are combined with the electrons from the
electron transport chain and NADP+
...
It is
reduced when it is combined with the H+ ions and electrons
...
This H- ion combines with the NADP+
to form NADPH (reduced NADP+)
...
The products of
the light dependent reaction (ATP and NADPH) are used in the light independent reaction
...






It has a large surface area for the absorption of photons and the attachment of enzymes
A network of proteins in the grana holds the chlorophyll in a precise position in order to
maximise light absorption
...


The Light Independent Reaction
The light independent reaction is the second stage of photosynthesis where carbon dioxide is
reduced to form sugars such as glucose
...
However, as the light independent reaction uses the products of the light
dependent reaction, it therefore indirectly relies on light and would stop if no light were present
...

The cycle is called the Calvin cycle, after Melvin Calvin who discovered it
...
CO2 diffuses into the leaf through the stomata
...
It then diffuses into the chloroplasts and into the stroma
...
The CO2 is combined with ribulose biphosphate (RuBP) by the RUBISCO (ribulose
biphosphate carboxylase) enzyme to form an unstable six carbon compound
...
The ATP from the light dependent reaction is used to activate the G3P
...
As the phosphate bonds are unstable, the
reactivity of the G3P is increased and therefore its activation energy is lowered
...
The NADPH formed from the light dependent reaction is used to reduce the G3P to triose
phosphate (TP)
...

5
...
5/6 TP molecules are
reformed into RuBP, whereas only 1/6 are converted to sugars
...
The stroma has a
number of adaptations that allow it to carry out these reactions at a high rate
...

The stroma is very close to the thylakoid membrane, so the ATP and NADPH needed do not
have to diffuse far, speeding up the reaction
...


The Calvin cycle was discovered in the lollipop experiment
...

1
...

2
...
In this way, any intermediate can be tracked as they will contain radioactive carbon
atoms that must have come from the radioactive carbon dioxide
...
At 5 second intervals, algal cells were dropped into hot methanol to completely stop any
chemical reactions from taking place
...


Limiting Factors of Photosynthesis
The rate of photosynthesis is determined by many interacting factors: light, temperature, carbon
dioxide concentration, nitrate concentration etc
...
Temperature affects the enzyme action in the Calvin cycle, carbon dioxide
concentration affects the enzyme action of RUBISCO and slows the Calvin cycle and light intensity
affects the light dependent reaction
...

The rate of photosynthesis is determined by the limiting factor – the factor in the shortest supply
and therefore the factor with the least favourable value
...
For example, at low light intensities, the amount of light
that hits the plant is the limiting factor of photosynthesis
...
When the light intensity increases, the rate of photosynthesis increases, but after a certain
point, the rate will stop increasing no matter how much the light intensity is increased by
...

The rate of photosynthesis is measured in two main ways: volume of carbon dioxide taken up and
volume of oxygen produced
...
This means that as the light intensity increases, the rate will also increase (shown by the
increase in oxygen production and carbon dioxide uptake)
...
At this point, no net gas exchange takes place, and it is
known as the light compensation point
...

Carbon dioxide is usually the limiting factor of photosynthesis, as it makes up such a tiny proportion
of air
...
In commercial greenhouses, farmers enrich the air with carbon dioxide in order
to increase the rate of photosynthesis and hence the growth rate of the plants
...
The rate of photosynthesis is directly proportional to temperature – between 0O C
and 25O C, the rate of photosynthesis will double for every 10O C temperature increase
...
Temperature doesn’t affect the photochemical light dependent
reaction, so the fact that photosynthesis is affected by temperature suggested to scientists that the
reaction had a chemical component as well
...
In this case, the oxygen produced is measured
...
This would
affect the results as oxygen would be measured that didn’t come from the plant
...

Potassium hydrogencarbonate is used as an additional source of carbon dioxide
...

The only source of light in the room is the source that produces the light at the set
intensity
...

The plant is kept in the dark for 2 hours before the experiment, so the rate of
photosynthesis slows and stops and no oxygen is being produced before the experiment
starts
...
ATP is the usable form of energy in the body
...
Aerobic respiration uses oxygen and produces carbon dioxide,
while producing large quantities of ATP
...

It produces lactic acid (lactate) in animals and ethanol and carbon dioxide in plants
...

The first stage of both aerobic and anaerobic respiration is glycolysis
...
It breaks down a single glucose molecule into two pyruvate molecules
...
The first stage is the phosphorylation of glucose
...
The energy from the hydrolysis is used to attach the phosphate
groups to the glucose molecule to form glucose-1,6-diphophate
...

2
...

3
...
Each triose phosphate molecule loses a hydrogen,
which is accepted by the hydrogen carrier NAD to form NADH
...
The oxidised triose phosphate is then converted into pyruvate
...

The total yields of glycolysis are:




Two molecules of ATP (four molecules are produced in the conversion of oxidised TP to
pyruvate but two were used to phosphorylate the glucose)
Two molecules of NADH
Two molecules of pyruvate

In aerobic respiration, the pyruvate molecules then go on to take part in the link reaction and Krebs
cycle
...
This insures that glycolysis can continue as, without the NAD, the TP cannot be
oxidised
...
All the enzymes
required for glycolysis are present in the cytoplasm
...
This produces many hydrogen atoms which are then taken to the
electron transport chain to produce ATP through oxidative phosphorylation
...

An important feature of both respiration and photosynthesis is the use of coenzymes
...
g
...

The coenzymes NAD+, FAD2+, and NADP+ are used in respiration and photosynthesis
...

In the link reaction, pyruvate is oxidised to acetate
...
It is oxidised to acetate,
which produces a carbon dioxide molecule as the pyruvate loses a carbon
...
The oxidation of pyruvate is done by removing a hydrogen
from the pyruvate, which reduces NAD+ to NADH
...
The link reaction produces acetyl coenzyme A, one NADH and one CO2 from every
pyruvate molecule
...
The citrate then passes through a series of intermediary
reactions in which it is oxidised several times and loses two carbons
...

Yields of the Krebs cycle:





Two molecules of CO2 are produced
One molecule of ATP is produced by substrate level phosphorylation
Three molecules of NADH
One molecule of FADH2

The Krebs cycle has many important features
...
It also produces hydrogen in the many oxidation reactions that are carried by hydrogen
carriers (NAD and FAD) to the electron transport chain for use in oxidative phosphorylation
...
The Krebs cycle is also a source of intermediate compounds used by cells in
other reactions, as many intermediary compounds are produced in the breakdown of citrate
...
It uses the hydrogen collected
by coenzymes in glycolysis, the link reaction and the Krebs cycle to produce lots of ATP
...
The mitochondria have
a double membrane
...
This increases the surface area of the inner membrane, so many enzymes and proteins
involved in the electron transport chain can attach to the inner membrane and the reactions can
take place more quickly
...
The coenzymes NADH and FADH2 attach to the inner membrane and release the hydrogens
that they carry
...

2
...
They travel is a series of redox reactions, similar the electron
transport chain in photosynthesis
...

They energy release is used to phosphorylate ADP to ATP in substrate level phosphorylation
...

3
...

4
...
These protein channels are ATPase enzymes
...
This force is used to
phosphorylate ADP to ATP
...

5
...
In this way, oxygen acts as
a final electron acceptor
...
This would cause all the reactions of aerobic respiration apart from
glycolysis to cease, resulting in death, as not enough ATP would be produced to sustain the
organism
...
However, as the reactions of aerobic respiration
are much lengthier, anaerobic respiration is used when a quick, fast burst of energy is required
...
This means that the link reaction, the Krebs cycle and the electron
transport chain cannot be used to produce ATP, as their function relies on the presence of oxygen as
the final electron acceptor
...
However, the
NADH produced by glycolysis must be converted back into NAD+ in order for glycolysis to continue
...

Both animals and plants carry out anaerobic respiration, but with different products
...
The pyruvate loses a carbon and two oxygens to form
carbon dioxide and is further reduced by NADH to form ethanol
...

Animals also undergo anaerobic respiration during strenuous exercise
...
However, the muscles must continue to work (e
...
when the animal is evading a
predator)
...
The pyruvate is reduced to lactate by NADH to
reform NAD+
...

Lactate is either converted to glycogen or converted back into pyruvate to enter the reactions of
aerobic respiration
...
The only ATP
producing process that takes place in anaerobic respiration is glycolysis, which produces 2 molecules
of ATP from one glucose molecule
...
This means that anaerobic respiration can only be sustained for a short period
of time
...
They are made up of producers, consumers and decomposers, and are split into trophic
levels
...
Photoautotrophs are autotrophs that use light to synthesise chemical energy by
photosynthesis
...
Chemoautotrophs use a source of
carbon that is different to the source used by the photoautotrophs
...
Autotrophs are called producers because they capture chemical energy and are the
entry point of the energy into an ecosystem
...
Primary consumers are those that feed on producers, secondary
consumers feed on primary consumers etc
...
They absorb the soluble products of this
breakdown to supply their own nutritional and metabolic needs
...

Food chains can be constructed which show the flow of energy through an ecosystem marked by the
different consumers
...
There are considerable energy
losses between each trophic level
...
To reflect this, food
chains are combined to form food webs, which show the interaction between different species
...


Energy Transfer in Food Chains
The Sun is the source of energy for all plants and therefore entire ecosystems
...
This is due to the energy losses that occur
between each trophic level
...
There are
four main reasons for this:






90% of the Sun’s light is actually reflected back into space by dust and clouds or absorbed by
the atmosphere
...


Only 10% of the chemical energy produced by plants is then converted into chemical energy by
primary consumers
...
This is due to their digestive systems and diet, which
are more efficient than those of herbivores (primary consumers)
...

Not all of the organism is digested
Energy is lost in waste products such as faeces and urine
Energy is used in respiration for movement and homeostasis
...
They are therefore technically more energetically inefficient and must
consume more energy to supply these processes
...
Most food chains only have 4 or 5 trophic levels
...
The total biomass of a population at higher trophic levels is lower than that of a population at
lower trophic levels, again due to the lack of energy available to sustain them
...
However, they provide no quantitative
information, which is useful for calculating energy losses
...
There are three types of ecological pyramid: pyramid of numbers, pyramid of
biomass and the pyramid of energy
...

As there are usually less individuals the higher up you go in a food chain (due to the losses in energy
at each trophic level) the data collected produces a pyramid
...
The first is that individuals of any species are counted as the
same as each other: e
...
a single oak tree is counted as the same as a single aphid
...
The second disadvantage is that some
species are so numerous that they cannot be represented on the same scale as other species,
leading to difficulties in visually representing the data
...

The pyramid of biomass details the biomass present at each trophic level
...
The measurement of biomass is difficult as the fresh mass of organisms is relatively easy to
assess
...
Dry mass is more accurate as the water is
removed, but this involves killing the organism so only a small sample can be taken, which may not
be representative
...
Biomass measurements are also made at a particular time and
therefore don’t take into account any changes over time or seasonal differences
...
They measure the energy
stored at each trophic level
...
They are also measured
over time, usually a year, and therefore account for seasonal differences in energy storage
...
The flow of energy
is measured in kJ m-2 year-1
...
In the case of crops, they are designed to maximise the amount of
light energy that plants convert into biomass
...

Gross productivity is the total amount of light energy that a plant converts into chemical energy
...

Net productivity = gross productivity – respiratory losses
...

Agricultural ecosystems have a greater ENERGY INPUT than natural ecosystems, which makes their
productivity greater
...
This energy is used to remove limiting factors of
photosynthesis
...
This means that the crop doesn’t have to
compete for resources such as light, nitrates and water
...

- Fertilisers are added to the soil, which increase the nitrate and mineral ion concentration
...

-Pesticides and herbicides are added
...
Pesticides kill organisms that feed off the plants, reducing the surface area of the leaf
and therefore the rate of photosynthesis
...
The results are then given to farmers who can
take suitable measures to increase productivity in their fields
...
For example, at very high concentrations, nitrate ions can
damage the plant by denaturing enzymes
...


Pesticides
Pests are organisms that damage agricultural ecosystems and compete with humans for food
...
They also can cause disease in livestock
and decrease the growth rate of livestock
...

 Pests are part of food webs and other, NATURAL, ecosystems
...

There are many different types of pest control
...

Pesticides = chemicals that kill pests
Pesticides must meet four requirements before they are used:







They must be SPECIFIC
...
The pesticide should not act
on the crop, which would damage and poison it, or any natural predators of the pest that
help to keep the population in check
...

The pesticide must BIODEGRADE after use
...
However, it must also be chemically stable
...
Pesticides can often only be used for a short period of time, as
pests can become resistant to the pesticide through natural selection
...

The pesticide must not accumulate
...
It can also be dangerous for consumers of the
crop
...
For a biological control to function, the pest must not be eradicated as this would leave
no food for the control to eat
...
The biological control aims to keep the pest population at an
acceptable level, where its effects are minimal
...
Once a population is established, it can
remain in the ecosystem indefinitely
...


However, the disadvantages to biological controls are that they take much longer to have an
effect than pesticides, by which time the crops may have already become irreversibly damaged
...

Integrates pest control systems combine chemical, biological and physical controls to maximise
the efficiency of pest control while doing as little damage possible to the environment
...
Due
to energy losses at each trophic level, conventional (organic) agriculture is less productive overall, as
energy from the food the livestock are given is lost in respiration to provide ATP for movement and
homeostasis
...

Intensive farming removes external factors that may cause excess energy usage, meaning more
energy from food is converted into biomass and hence growth
...

The main methods of intensive rearing are:









Restricting movement so less energy is expended in muscle contraction
...

Controlled feeding so that the right type and quantity of food is ingested by the livestock
...

The livestock are protected from predators so that there is no energy loss to other animals in
the food chain
...

Antibiotics are used to prevent the spread of disease through the animals to prevent any
loss of energy due to loss of livestock
...


Intensive farming advantages:
Intensive farming is much more productive than organic farming and therefore has a higher yield,
producing more food at a higher rate
...
Intensive farming is also a more efficient use
of space, as less is used to rear the livestock, leaving more available for natural ecosystems
...
The livestock are therefore treated with antibiotics
...
Hormones and supplements
given to the livestock to increase their growth rate can cause harmful side effects and damage the
health of the livestock
...

The conditions in which the livestock are kept in are poor, so they cause the animals distress
...
Intensive farming emits lots of

pollution
...
Factory farms also consume lots of fossil fuels in
order to heat and house the livestock, which pollutes the atmosphere further
...
The population therefore has a reduced ability to adapt to disease or a
change in environment
...
The energy enters the ecosystem from
sunlight and is then passed through a food web where it is lost as heat and other waste products
...

Nutrients are a finite resource and so must be recycled
...
The source of most
carbon containing compounds in living organisms comes from carbon dioxide
...
The consumers incorporate the carbon
containing organic compounds from the plant
...

When organisms die, saprobiotic microorganisms consume them
...
The enzymes break down the complex molecules into simple
molecules, which the saprophytes absorb by diffusion
...


The main way in which carbon is removed from the atmosphere is through photosynthesis, but there
are other processes
...
Peat bogs also are a store of carbon, as the conditions are
too acidic for the enzymes of saprophytes to function, leading to a build up in undigested organic
matter
...
They form
carbon containing sediment which develops into limestone, chalk and coal
...

Combustion of fossil fuels and deforestation has released lots of carbon dioxide
...
Many trees are also burned, releasing more carbon dioxide
...
At night, the
concentration is higher as plants are not photosynthesising due to the lack of light
...

The concentration of carbon dioxide during winter is higher than in summer due to the lower rates
of photosynthesis
...

At the understory and forest floor, the concentration of carbon dioxide is higher as less light
penetrates this layer so the rate of photosynthesis is lower, whereas at the canopy, the
concentration is low
...
It helps to
keep the temperature of Earth warm enough to sustain life
...

Sunlight is reflected back into space by the atmosphere, absorbed by the atmosphere or reaches
Earth
...

Greenhouse gases in the atmosphere absorb the reflected sunlight and re-emit it, effectively
trapping it in the biosphere
...
The bonds in these molecules absorb the
radiation
...
Carbon dioxide is the principal greenhouse
gas as it has a very long residence time and is produced at such a high rate by living organisms
...
It is produced when decomposers
break down organic matter and by bacteria in the guts of consumers
...
It is not 100% certain
that this rise is due to increased greenhouse gas emissions, but what is certain is the positive
correlation between atmospheric carbon dioxide concentration and global temperatures
Consequences of global warming:










Unpredictability of the weather, with changes in climate, seasons and temperature
...
Organisms that occupy specific niches will
migrate to new areas when climate change makes their original habitat inhospitable
...
Some species may adapt to
live in the new conditions, but the speed at which climate change takes place may make this
difficult
...

Thermal expansion causes rise in sea levels which leads to flooding in low lying areas
...
Crops will be difficult to grow
in drought stricken areas
...
Only xerophytic plants will be able to survive
...

Life cycles and behaviours of insects will change
...
This will increase the variety of
insect pests that affect northern crops and increase the number of tropical diseases that
affect northern areas
...

To measure temperature, the proportion of radioactive oxygen isotopes in fossil shells is
measured
...


The proportion of the sea water was determined by polar ice caps, which themselves were
determined by global temperature
...
The age of the ice is measured and the % carbon dioxide in its air bubbles is also
measured, therefore the age of the ice shows the % carbon dioxide at that time
...
The source of nitrogen for plants is soil nitrates and the source of nitrogen for
animals is the plants themselves
...

Nitrogen gas is very stable
...
Nitrates are absorbed
from the soil by active transport into the root hair cells
...

Nitrates, as a nutrient, must be recycled through the ecosystem
...
This is important as the only source
of nitrogen available in natural ecosystems is from decomposition, and nitrate concentration is
usually a limiting factor of photosynthesis
...
This releases ammonia from nitrogen containing
compounds such as urea, faeces and flesh
...

This is the stake where nitrogen is transferred from the living to the non-living part of the
ecosystem
...
This is a two step process
...
This reaction is exogonic, and the bacteria use the energy
released in the reaction to fuel their own metabolic processes
...
Farmers plough their fields to keep them aerated in order to
supply the bacteria with sufficient oxygen for the reaction
...

NH4+  NO2NO2-  NO3NITROGEN FIXATION:
Nitrogen gas is converted to ammonium ions by a select group of organisms, known as nitrogen
fixation bacteria
...

There are two types of nitrogen fixers: free living and mutualistic
...


The nitrogen is released when they die and are broken down
...
They do the same thing as free living nitrogen fixers but give
some of the amino/nucleic acids they produce to the host plant, and are in turn supplied with
carbohydrates by the plant
...
There is less oxygen for the aerobic nitrifying bacteria so their population
decreases, leading to a decrease in nitrate production
...
They convert nitrates into nitrogen gas, which is lost to
the atmosphere, further decreasing the nitrate concentration
...


Uses of fertilisers
Fertilisers are substances that increase the nutrient content of the soil that plants grow on in
order to increase the rate at which they grow
...
Fertilisers also
contain magnesium, potassium and phosphorus which are also essential for plant growth
...
In natural ecosystems, plants take in nitrates from the soil
...
Consumers that eat plants and other consumers release
nitrogen upon death, as well as in urine and faeces
...

In agricultural ecosystems, the plants are taken away from the soil in which they are grown and
are consumed by animals and humans
...
As a result, the nitrogen is removed from the soil and not
returned
...

Fertilisers are therefore added to the soil in order to boost the nitrate concentration
...

Natural fertilisers are made up of organic substances that contain nitrogen, such as the dead
remains of plants and animals, excretory products (manure) and animals waste (bone meal)
Artificial fertilisers are made up of inorganic compounds obtained from mineral deposits in
rocks
...

Fertilisers increase productivity as it ensures that nitrate and mineral ion concentration is not a
limiting factor for photosynthesis
...

These are essential for making proteins and DNA
...


Environmental consequences of fertiliser use
Nitrogen increases productivity by increasing growth
...
This results in an increase leaf surface area, which leads to greater light and CO2
absorption that increases the rate of photosynthesis
...







Nitrogen fertilisers reduce species diversity in ecosystems
...
Soils with high nitrate
concentration favour fast growing plant species that out-compete slower growing species
...
The result is weeds and grasses outcompeting other species such as wild flowers and other grasses, leading to low biodiversity
...
Leaching is the process by which nutrients leave the soil
...
They then enter
watercourses
...
High nitrate concentration can reduce the oxygen carrying capacity of
haemoglobin in babies and can increase the risk of stomach cancer
...

Eutrophication is a natural process that takes place when the nitrate concentration of a
natural aquatic ecosystem increases exponentially and ceases to be a limiting factor of
photosynthesis
...
In natural aquatic ecosystems, the nitrate concentration of the environment is low and
is therefore the limiting factor of photosynthesis for plants and algae
2
...

3
...
Algae
grow at the surface of the water, in algal bloom
...
Algal bloom prevents light from reaching aquatic plants that live deeper in the water
...

5
...

6
...
As their numbers
increase, the oxygen in the ecosystem is gradually depleted as the oxygen
concentration of the water is not high enough to support the saprophytes and the
other organisms in the ecosystem
...

7
...

8
...
Anaerobic organisms further decompose dead
organic matter, releasing nitrates and toxic compounds which make the water putrid
...
Although the overall climate may remain the same, the
ecosystem itself can be altered by the behaviour and actions of the organisms that live within it
...

There are two types of succession: primary and secondary
...
The land may become available due to:






Glacial retreat that exposes bare rock and moraines
Volcanic eruption that creates new land masses
Sand being formed into dunes by the wind and sea
Lakes and ponds forming due to land subsiding
Silt and mud deposits forming in river estuaries

The first stage of primary succession is the colonisation of the new environment by pioneer species
...
Pioneer species often have adaptations
that allow them to survive and reproduce in these environments, such as:






Production of large quantities of seeds or spores that are dispersed by the wind, so that they
can reach many different isolated environments
Rapid germination of theses seeds and spores so they don’t go dormant, which increases
their chances of survival
Ability to photosynthesise, as light may be the only resource available to them
Ability of fix nitrogen, as there will be very little soil nitrates in the new environment
Tolerance to extreme conditions

Pioneer species lay the foundations for new species to move into the new environment, as they
make the environment less hostile
...
They move onto bare
rock
...
When the lichen die, they are
broken down by saprophytes and nitrates are released, which increases the soil fertility and make
the environment more hospitable
...
These
species help to build up the soil by dying and releasing nitrates
...

Animals also contribute to succession, with new animal species moving into the ecosystem due to
the new habitats and food sources provided for them by plants
...
The final stage
of succession is when the ecosystem is balanced, and there is little or no new introduction of new
species
...
The dominant species are determined by
the overall environmental conditions
...


Common features of succession:






Environment become less hostile, with a plentiful supply of nutrients and habitats
Greater biodiversity due to increased numbers of food sources, habitats and an increasingly
hospitable environment
...
Biodiversity falls as the climax community
approaches as dominant species move in and out-compete other species, which are then
lost from the community
...
The process by with the ecosystem re-establishes a climax
community is the same as primary succession, except that it takes place much faster, as plant spores
and seeds remain in the soil, nutrients are not completely lost, and animals and plants move into the
ecosystem from the surrounding environment by migration
...


GLACIAL SUCCESSION
Retreating glaciers expose gravel deposits called moraines
...
Ecosystems become established on moraines by primary
succession
...
Pioneer stage
...
They fix nitrogen,
which is essential as moraines contain very little nitrogen
...
When they die, they decompose to form nutrient rich humus
...
Dryas stage
...
Dryas are small herbaceous plants that help to further stabilise the
soil layer in the moraine using their roots
...

3
...
Alder are small, shrub like trees
...
In winter, they shed their leaves which
decompose and add nutrients to the humus, further increasing the soil fertility
...
Spruce stage
...
Over a period of
time, the spruce out-competes the alder, which is lost from the ecosystem
...
Conservation also helps to maintain biodiversity when it is threatened by human activities
such as deforestation and pollution
...
Ecosystems help to maintain our atmosphere and climate
...

Ethical
...

Economic
...


In order to conserve certain habitats, succession must be managed
...
When an ecosystem approaches the final stage of succession and the climax
community is reached, dominant species move in and out-compete other species, leading to a
loss of biodiversity
...

For example, moorland in the UK is conserved by preventing succession from occurring, which
would result in the moorland becoming deciduous oak woodland, the climax community in
lowland UK
...
After
the land was used it was abandoned, and proceeding to undergo secondary succession,
ultimately ending in oak woodland
...

Moorland is managed in order to conserve it and the species that inhabit it
...


Genetic inheritance
Inheritance of different genes can have significant effects on a population of organisms
...
This is how
populations evolve through natural selection
...

GENOTYPE: The genetic constitution of the organism, detailing the alleles that an organism has
...
For example, the genotype of an
organism may determine that a human child can grow to be 6 feet tall
...

However, environmental factors, such as lack of available food and nutrients for growth, could
decrease the maximum height of the child
...
This mutation may be inherited if it occurs in the gametes
...
The phenotype of an
organism is determined by both the genotype and the environment
...
g
...
Any change to the phenotype that is not a result of a change to the genotype is
a modification and is not inherited, so the height change to the child is not a result of a change to
the genotype and isn’t therefore inherited
...

GENE: A sequence of nucleotide bases on a DNA molecule that code for a particular polypeptide, by
way of mRNA
...
E
...
a gene may code for the production of a polypeptide
that forms an enzyme
...
This position of a gene on a chromosome is the locus
...
Alleles of a gene all code for the same general physical
characteristic but provide variation within that characteristic
...
g
...
Only one
allele can be present on each chromosome, as the chromosome only has one available locus for a
specific allele
...

Homologous chromosomes are chromosomes that have genes on the same loci as each other
...

Homozygous organisms are organisms that have the same alleles on both their chromosomes
...

Homozygous recessive organisms have two recessive alleles on their homologous chromosomes
...


Recessive alleles are only expressed in homozygous recessive organisms
...

CODOMINANCE: where tow alleles are not dominant or recessive and exert an equal influence on
the physical characteristics of an organism, so the result is a blend of feature of the two alleles
...
g
...

MULTIPLE ALLELES: where there are more than two alleles that code for a characteristic, However,
as diploid cells only have two homologous chromosomes, an organism can only possess two of the
three or more alleles for the characteristic, as all the available loci are occupied
...
Monohybrid inheritance can be use to
study Gregor Mendel’s experiments and how the law of segregation was formulated
...
ONLY ONE ALLELE IN A PAIR CAN BE PRESENT IN A SINGLE GAMETE
...
The resultant two cells each divide again,
splitting the chromosomes and ensuring that the resultant gametes only have a single allele from the
pair
...
Pea pods come in two different
colours: green and yellow
...
Pea plants with green pods that are bred with one and other to
consistently produce green pod offspring are pure breeding
...
Pea plants with yellow pods that are bred together to consistently
produce yellow pod offspring are also pure breeding
...

When pure breeding green pod plants are crossed with pure breeding yellow pod plants, the
offspring will all have green pods but will have a heterozygous genotype (Gg) and be carriers for the
recessive yellow pod gene
...

A test cross is used to determine the genotype of an organism
...
The plant is crossed with a
yellow pod plant, which must have the homozygous recessive genotype
...

Homozygous: produces all green pod plants
Heterozygous: produces 50% green pod and 50% yellow pod
...
22 of these pair carry the same genes as each
other (albeit different alleles), and have these genes on the same loci
...
There are two types: an X chromosome and a Y chromosome
...
Therefore, female gametes will
all contain one X chromosome, whereas half of all male gametes will contain an X chromosome and
half will contain a Y chromosome
...
This means that the sex of an organism determines the expression
of the physical characteristics of the gene
...

Genetic disorders that are sex linked and carried on the X chromosome are more likely to affect
males than females
...
The X
chromosome is much larger than the Y chromosome
...
If there is a recessive allele present on the
X chromosome, there is a much higher likelihood that it will be expressed by males, as the Y
chromosome is less likely to have a homologous portion that contains a dominant allele, which
would override the expression of the recessive allele
...

The X chromosome carries many genes, one of which is the recessive allele that results in
haemophilia
...
It is often fatal if untreated
...
The current treatment for this is to provide
sufferers with donated blood containing the required clotting factors
...
The recessive allele is carried on the X chromosome
...
In male individuals, if their X chromosome carries the
recessive haemophilia allele, they will suffer from haemophilia as they lack the dominant allele on
the Y chromosome to override the expression of the recessive allele
...
Although it is possible to have female haemophiliacs, the majority of females that are
homozygous recessive for haemophilia die as foetuses
...

XHXH = female that is homozygous dominant
...
He possesses the haemophilia allele on his X chromosome
and lack the homologous portion on his Y chromosome that contains the dominant clotting factor
allele
...

They are therefore both expressed, resulting in a blend of features from the two alleles
...
There are two alleles for petal colour in snap dragons, one for
red and one for white
...
The allele for white petals codes for the production of a non-functional enzyme that
doesn’t produce any pigment, leading to white leaves
...
If a plant
possesses both of these alleles, the resultant petals will be pink (blend of features) as the plant will
have just enough pigment enzyme to produce pink flowers
...
This results in variation within a population as,
although individuals can only have two alleles on their two gene loci, different individuals may have
different combinations of alleles
...
Gene I (immunoglobulin gene)
has three alleles
...
As an individual can only possess
two alleles, due to there being two homologous chromosomes and therefore only two gene loci,
different blood groups arise
...
This is arranged left to right from
most dominant to most recessive
...
g
...
Sometimes, the gene pool is used to refer to all the alleles of a particular gene in a population
...
These two alleles
can appear in different proportions in each organism (due to different genotypes arising) however
an individual can still only possess two of these alleles
...
In a population of 100 individuals,
therefore, there are 200 alleles in the gene pool of this gene
...
One way to calculate
and use allelic frequency is if we reduce the total number of alleles of a particular gene in a
population to 1
...
If all the individuals in the population are heterozygous, then the allelic
frequency of the dominant allele is 0
...
5
...

p2 + 2pq + q2 = 1
In this case, p = the dominant allele and q = the recessive allele
In a normal population, p + q = 1
...

AA + Aa + aA + aa = 1
The Hardy-Weinberg equation is only accurate according to the Hardy-Weinburg principle
...
Also, as these conditions are not met, the
principle itself does not hold and the allelic frequencies and dominant : recessive allele proportions
do change from generation to generation
...
One of
these conditions is that no natural selection takes place
...

Natural selection increases the proportion of alleles that give advantageous characteristics in a
population
...
In a population, there are always more organisms than can be supported by the resources
available in their habitat
2
...
This maintains the constant size of the population
...
Each population has a gene pool, with a wide variety of alleles
...

4
...

5
...

6
...
Gradually, the population of organisms without
advantageous alleles decreases and the population with the advantageous alleles increases
...
The alleles possessed by the organisms without the advantageous alleles are lost from the
population and the alleles possessed by the advantaged individuals increase within the
population
...
The allelic frequency of some alleles
increases and the allelic frequency of other alleles decreases
The environment in which a population lives determines which physical characteristics are
advantageous and therefore which alleles are advantageous and more likely to be passed on
...

Before the Industrial Revolution, the peppered moth was white with grey speckles
...
The camouflage protected it from predators, so
individuals that possessed this allele were more likely to survive and reproduce, passing the allele on
to the next generation
...

After the Industrial Revolution, trees and plants were stained black with soot
...
The black moths are now more likely to survive and pass on their alleles to the next
generation, whereas the white moths are less likely to survive and their alleles are gradually lost
from the population
...
An environmental change occurred which resulted in an allele that was
previously advantageous becoming a disadvantage, and a previously disadvantageous allele
becoming advantageous
...
The resultant normal distribution curve that forms shows individuals in
a population whose characteristics vary about the mean, due to the different alleles they possess
...
E
...
fur length
...
This is the mean
fur length
...

There are two types of selection: directional and stabilising
...
This is a result of
an environmental change that changes the preferred characteristics, and changes the overall
characteristics of the population
...
Some individuals, which fall on either side of the
mean, will possess characteristics that allow them to be better adapted to the new environmental
conditions
...
Over time, the mean moves to the right or left, depending on
which phenotype is selected for, as more organisms will possess the advantageous phenotypes
...
g
...
At 10 degrees, the optimum fur length is 30mm
...
If environmental conditions change, and the average temperature drops to 3
degrees, individuals with longer fur will have advantageous phenotypes and be more likely to
survive
...
Individuals with longer fur will be more likely to survive and reproduce,
so the mean fur length will gradually increase
...
This happens when the
environmental conditions remain constant, and therefore the characteristics of the population
remain the same
...
g fur length
...
If
the temperature remains constant, individuals at either side of the mean (with longer or shorter fur
lengths) are at a competitive disadvantage and are less likely to survive and reproduce
...


Speciation
Speciation is where two different species arise from one
...
They all belong to the same gene pool, and there is a flow of alleles
between all the members of the population
...
This is because the will be no flow of alleles between the
two populations
...
As there is no allele flow, the alleles in each population will change but stay within the
population, and not be passed on to individuals in the other population
...

Many factors cause the isolation of different populations
...

1
...
The species is made up of a single population and
gene pool, and there is a flow of alleles between all the individuals in the population
...
Climatic changes and deforestation cause the size of the forest to shrink, and the forest to
be split into two smaller forests, each many miles apart
...
Climatic changes result in one of the forests becoming wetter and more humid, and another
forest becoming hotter and drier
...
There are now two populations of snakes, one in the hot forest and one in the wet forest
...

5
...
The snakes in the
hot forest adapt for better survival in hot, dry conditions and the snakes in the wet forest
adapt for better survival in wet, moist conditions
...

6
...
This, coupled
with the changing allelic frequency of each population due to succession, results in the gene
pools of each population changing to such an extent that the two populations develop into
different species of snake that are unable to breed and produce fertile offspring
...
The ability and efficiency of responding to internal or external changes
increases the chances of survival for the organism
...
This increases their chances for survival as they can absorb more light and
increase their rate of photosynthesis
...
As certain responses and the efficiency of these responses increases the survival chances of
individuals, there is selective pressure on certain responses, and organisms able to respond in the
most advantageous way to stimuli have a higher chance of survival and reproduction
...
Once
the stimulus is detected, the information in the form of a nerve impulse is sent to a coordinator
...

The coordinator processes the information from the receptors and stimulates effectors to illicit a
response
...

Muscles can contract, producing movement, and glands can secrete hormones which have a
widespread internal effect
...

A taxes is a movement response to a directional stimulus
...
Negative stimuli result in negative taxes, whereas positive stimuli
result in positive taxes
...
Bacteria move away from their own waste products in a
negative chemotaxis as the waste products are toxic and would damage the bacteria, decreasing
their survival chances
...
The more
negative the stimulus, the more the movement pattern changes
...
In negative conditions (light, hot, dry) the insects will move very fast in many different
directions (or in one single direction)
...
Once they have found favourable conditions,
they move mar slowly and change direction less often (or more often but in smaller movements)
...

A tropism is a growth movement in plants in response to a directional stimulus
...

E
...
plant shoots grow toward light in a positive phototropism so that they can absorb more light and
increase their rate of photosynthesis
...
This increases their chances of growing into soil and hence
finding mineral ions and water
...


Nervous Control
The nervous system is made up of several divisions, each of which co-ordinates, receives or
stimulates
...

The central nervous system is made up of the brain and spinal cord
...
The peripheral nervous system is made up of the pairs of nerves that
originate from either the brain or spinal cord
...

The peripheral nervous system is made up of two different types of neurones: sensory neurones and
motor neurones
...
The motor neurones transmit
nervous impulses that originate in the central nervous system to effectors, in order to illicit a
response
...
The somatic (voluntary) nervous system carries impulses generally to
skeletal muscle
...
The autonomic nervous system carries nerve
impulses to other effectors such as smooth muscle, cardiac muscle, organs and glands
...
You cannot decide to stimulate these effectors
...
The sympathetic nervous system
stimulates effectors
...
The parasympathetic nervous system inhibits
effectors
...
The
sympathetic centre stimulates effectors by sending nerve impulses down the sympathetic nervous
system
...
The enteric division extends to the digestive tract, gall bladder and
pancreas
...
Although the enteric division can function on its own, it is
normally controlled by the sympathetic and parasympathetic systems
...
In the case of the withdrawal reflex, the stimulus is
negative and could damage the tissues of the organism, so the organism withdraws the vulnerable
body part quickly and without thinking
...
g
...
This reflex follows a reflex arc
...
The stimulus (hot object) is detected by thermoreceptors in the skin
2
...
The nervous impulse passes from the sensory neurone into the relay neurone in the spinal
cord across synapses

4
...

5
...

6
...

Reflex actions are very important for many reasons
...
This means that the brain is
not overloaded with information and doesn’t have to constantly formulate identical
responses to different situations
...

The brain does still have some awareness and control over the reflex action, so that it can be
overridden if need be
...
The heart rate must be controlled in order to respond to the oxygen and nutrient
demands of cells in different scenarios
...
The control centre in the CNS for the heart rate is the
medulla oblongata
...

Two different receptors detect stimuli that affect heart rate: chemoreceptors and baroreceptors
...
Chemoreceptors in the carotid artery detect pH changes in the blood
...
When an organism is exercising, the muscle cells are respiring at a higher rate and are
therefore producing CO2 at a higher rate
...
CO2 dissolves in blood plasma to form carbonic acid, which dissociates, releasing H+ ions
...
This
pH fall is detected by the chemoreceptors
...
The chemoreceptors send nerve impulses to the heart rate increase centre of the medulla
oblongata
...
The medulla oblongata sends nervous impulses to the SAN via the sympathetic nervous
system, causing the heart rate to increase
...
The increases heart rate increases blood flow, causing CO2 to be removed from the blood at
the lungs
...

7
...

8
...

Baroreceptors detect pressure changes in the blood
...
The medulla
oblongata sends nerve impulses to the SAN via the parasympathetic nerves, causing the heart rate
to decrease and blood pressure to return to normal
...
The baroreceptors send
nerve impulses to the heart rate increase centre of the medulla oblongata
...


Receptors – the Pacinian Corpuscle
Receptors are units of cells that detect stimuli – changes in the internal or external environment
...
There are two types of receptor: primary and secondary
...
The substance that reacts to the stimulus is incorporated with the
sensory neurone
...
The
Pacinian corpuscle is a primary receptor, while the rod and cone cells of the eye are secondary
receptors
...
As nerve impulses are basically waves of electrical energy, the
receptors transduce the energy of the stimulus into electrical energy
...
A generator potential is the depolarisation of
the axon membrane
...

The Pacinian corpuscle detects changes in mechanical pressure
...
The corpuscle is
most numerous in the hands, feet and external genitalia, and in the joints where it is used to detect
directional changes
...
The membrane of the neurone contains stretch mediated sodium ion channels, which
open and close according to the flexibility of the membrane
...
Mechanical pressure deforms the malleable layers of tissue and gel, causing the membrane
of the neurone to stretch
...
When resting, the sodium ion channels are too narrow to allow any sodium ions to diffuse
into the cell through them
...
When the membrane is stretched, the ion channels open and sodium ions diffuse into the
cell across the membrane
...

4
...


Receptors – Cone and Rod Cells
Receptors only respond to specific stimuli, so the body has many different receptors that respond to
their different respective stimuli
...
Rod cells and
cone cells are secondary receptors as they detect stimuli and create a generator potential which is
transferred to a bipolar cell before the sensory neurone
...
Rod cells
respond to lower light intensities and are therefore more sensitive to light
...
This depolarization causes
them to constantly release the neurotransmitter glutamine
...
In light,
the pigment is activated and the sodium ion channels close, causing the release of glutamate to
cease
...
This change in light intensity causes action potentials to be sent to the brain, which
processes the information to form an image
...
Rod cells contain the pigment rhodopsin
...
Rod cells show retinal convergence,
which further helps them to respond to low light intensities
...
At very low light intensities, it is unlikely that this threshold value will be achieved by one
cell, as a single rod cell is unlikely to absorb all the necessary light energy to create the generator
potential
...
Collectively,
at low light intensities, they can overcome the threshold value and create a generator potential
...
However, retinal convergence results in low visual acuity, as many rod
cells are attached to one neurone, so only one nerve impulse is created, no matter how many
different sources of light there are
...

There are three different types of cone cell, each of which responds to a different wavelength of
light
...
Cone cells only respond to
high light intensities, which is why we cannot see bright colours in the dark, as only rod cells are
stimulated
...
This means that a higher threshold value is required to create a
generator potential from a cone cell
...
An advantage of the lack of retinal convergence is the higher visual acuity of
cone cells
...

The distribution of rod and cone cells throughout the retina changes
...


At the peripheries of the retina, the concentration of the rod cells increases and the concentration of
the cone cells decreases
...
Each cell type
arises as a result of differentiated stem cells
...
As some genes are switched off, specialised
cells lose the ability to carry out certain tasks, and so rely on other cells that can perform this
function to carry it out for them
...
g
...
Epithelial cells must be present to secrete digestive
enzymes to break down food and to absorb glucose into the bloodstream
...

The two main systems that coordinate the body are the nervous system and the endocrine system
...
The response produced is rapid
and short-lived
...

The endocrine system is made up of many different glands and organs that produce hormones
...
Target cells have receptors on their cell
membranes that the hormone binds to, stimulating changes within the cell by activating enzymes
...
The
effect of hormonal release is much slower that nervous stimulation and long-lived
...
E
...
almost all body
cells have insulin receptors and therefore almost all body cells are affected by the release of insulin
...
There are more localised
systems that coordinate responses at a cellular level
...
These are
substances released from cells that have an effect on cells in their immediate vicinity
...
These are released when cells
are damaged or in response to allergens such as pollen
...
It
causes blood vessels to dilate, increasing blood flow to the affected area, and increases the
permeability of capillaries
...
The symptoms of this are swelling, itching and redness
...
They also affect blood pressure and
neurotransmitters, which in turn affects pain sensation
...
To
coordinate these responses, they use auxins – plant growth factors
...
They
also affect any cell in their vicinity (not just target cells) and the growth factor can affect the cell that
originally released it
...


IAA (indoleacetic acid) is a growth factor
...
The elongation
is a growth movement, so a plant releases the IAA when it wants to grow towards a stimulus (e
...

light)
1
...

2
...

3
...

4
...


Structure of the Neurone
A mammalian neurone (nerve cell) is made up of distinct units that are adapted to transmitted
nervous impulses around the body
...
This contains the nucleus and organelles of the neurone
...

Branching off from the cell body are dendrons
...
Dendrons and dendrites carry nerve impulses towards the cell body
...

The axon in the long extension from the cell body
...

Schwann cells are wrapped around some neurones
...

The myelin sheath is formed from the membranes of Schwann cells
...
Not all neurones have a myelin
sheath, but those that do transmit nerve impulses faster than those that don’t
...
They are gaps in the myelin
sheath where there is no electrical insulation as the Schwann cell membranes don’t cover
these axon areas
...

Sensory neurones have a very small cell body that occurs roughly half way down the axon, in a small
membrane enclosed bubble
...

Motor neurones have a large cell body that occurs at one end of the axon
...

Interneurones/intermediate neurones have a very large cell body and many dendrons and dendrites
...


The Nerve Impulse
The nerve impulse is an electrical impulse that is transmitted along the action potential
...
The
reversal is between two states: resting potential and action potential
...
Their movement is controlled by the axon membrane
...

The Na+ and K+ ions must therefore diffuse through intrinsic protein channels in the cell
membrane
...
Some channels are gated – they open and close according to different cellular
conditions
...
g
...
Ligand gated channels open when certain ligands (e
...

neurotransmitters) bind to the protein channel and cause a conformational change in the
protein, leading to the opening of the channels
...

The sodium-potassium pump is a protein complex that actively transports Na+ ions out of
the cell and K+ ions into the cell
...


The resting potential:
1
...

2
...
There is an overall loss of positive ions from the axon interior
...
Na+ ions start to diffuse back into the axon and K+ ions diffuse out of the axon through the
open protein channels
...
However, at resting potential, most of the gated sodium ion channels are closed, whereas
most of the gated potassium ion channels are open
...
This means that Na+ ions are prevented from moving back into the axon, and potassium are
allowed to move out of the axon, leading to a loss of positive ions from the axon interior
...
The exterior of the
axon is very positive as there is a high concentration of Na+ and K+ ions
...
At a certain point, the diffusion of K+ out of the axon slows as an electrical gradient slows
the rate of diffusion
...
They therefore stay within the axon, which has an overall negative
charge
...
The movement of ions balances and equilibrium is established where there is no net
movement of ions
...


The action potential forms as a result of a generator potential
...
This is the
generator potential
...
This change causes voltage gated sodium ion channels
to open further along the membrane, so sodium ion channels depolarise the next section of the
membrane
...

1
...
Energy of a stimulus causes sodium gated channels to open and the potassium channels
to close
...
They diffuse from a high concentration to a low concentration
and from a positively charged area to a negatively charged area
...

3
...
This change is detected by sodium voltage gated channels further down the
membrane, which open as a result, causing more Na+ ions to diffuse into the axon
...
Once a potential of +40mV in the axon interior is established, the sodium voltage gated
channels close and the potassium voltage gated channels open
...

5
...
The rate is
so high that the axon interior becomes hyperpolarised (-75mV)
6
...
This slightly increases the overall charge to the
normal -65mV of resting potential
...
The action potential
itself is not moving, it is being created along the axon membrane by the movement of sodium
ions across the membrane through protein channels
...

After the axon has been depolarised in one region, this causes the next region to become
depolarised, which therefore causes the action potential to continue along the membrane
...

Passage of action potential along an unmyelinated axon:
1
...
The concentration of potassium ions on the inside is greater than on the
outside
...
A stimulus causes the depolarisation of the axon membrane
...

3
...
Sodium ions
therefore diffuse into the next region of the axon, resulting in depolarisation, and
continuing the flow of the action potential along the axon
...
This is an example of positive feedback, where the
increase in sodium ion influx causes a further increase in sodium ion influx as it triggers
more sodium voltage gated channels to open
...
The region behind the action potential undergoes repolarisation
...
The outflow of
potassium ions causes the axon to repolarise in this region
...

Passage of action potential along a myelinated neurone:
The action potential in a myelinated neurone forms and moves along the axon in the same way: the
depolarisation causes the reversal of the electrical potential of the membrane, which triggers more
sodium voltage gated channels to open
...
This is because the axons of myelinated
neurones are surrounded by Schwann cells, which gives them their myelin sheath
...
Along the axon are nodes of Ranvier
...
Action potentials
can therefore form at the nodes of Ranvier as the reversal of the electrical potential can only occur
at these points
...


Speed and Control of the Action Potential
The size of the action potential is always the same
...
A certain stimulus
must exceed a threshold value of a neurone in order to create an action potential
...
If the energy of the
stimulus exceeds the threshold value, an action potential is created
...
The size of the
stimulus is determined by the frequency of the action potentials that reach the CNS: the higher the
frequency, the larger the stimulus
...

The speed of the action potential varies:




The myelin sheath increases the speed of the action potential, as the action potential travels
along the axon by saltatory conduction
...

The diameter of the axon
...
Ion leakage makes
the electrical potential harder to maintain, so the action potential travels faster in wide
axons
...
A faster diffusion rate causes the action potential to form more quickly and
therefore travel faster
...
A higher respiration rate produces ATP at a higher rate, which is used
by the sodium-potassium pump
...


The passage of the action potential is controlled by the refractory period
...
The region
just behind the action potential prevents the continued influx of sodium ions by closing the sodium
voltage gated channels and opening the potassium voltage gated channels
...

The refractory period helps to control the action potential by making sure that the action potential
only travels in one direction
...
Also, the refractory period ensures that discrete impulses are produced, by making
sure that the action potentials are separated out along the axon
...


Structure and Function of Synapses
Synapses are junctions between neurones
...
They use chemical messengers
called neurotransmitters to trigger an action potential in the next neurone in the sequence
...
The
presynaptic neurone is the only neurone that produces neurotransmitter, which ensures that
the action potential continues moving in one direction
...
When resting, the neurotransmitter is stored in synaptic vesicles
...
The neurotransmitter
binds to ligand gated ion channels on the membrane of the postsynaptic neurone, which triggers
and action potential to form
...
Many postsynaptic
neurones can connect to a single presynaptic neurone via the synapse, which allows for the
nerve impulse to be transmitted through many different neurones and therefore reach many
different parts of the body or CNS
...
This allows for many nerve impulses from different
receptors or the CNS to produce a single nerve impulse and therefore a single response
...

This means that the presynaptic neurone must release a set amount of neurotransmitter to illicit
an action potential in the postsynaptic neurone
...
Neurones can overcome this problem by
summation:




Spatial summation is where many presynaptic neurones converge at one synapse with
one postsynaptic neurone
...

Temporal summation is where a single presynaptic neurone releases neurotransmitter
many times over a short period
...


Some synapses are inhibitory, which means that they inhibit the formation on an action potential at
the postsynaptic neurone
...
This causes an influx of Cl- ions, which hyperpolarises the axon
...


Transmission across a Synapse
Neurotransmitters transmit action potentials between neurones by causing an action potential to be
created in the postsynaptic neurone
...
Acetylcholine is a common neurotransmitter
that is used principally in the stimulation of skeletal muscle
...
The arrival of the action potential into the synaptic knob of the presynaptic neurone causes
calcium ion channels in the presynaptic membrane to open
...
The influx of calcium ions into the synaptic knob causes synaptic vesicles to fuse with the
presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis
...
Acetylcholine diffuses across the synaptic cleft and binds to receptor sites of the sodium
ligand gated ion channels in the postsynaptic membrane
...
These sodium ion channels open and sodium ions diffuse into the neurone, depolarising it
and creating an action potential
...
Acetylcholinesterase is an enzyme that is present in the synaptic cleft
...
They diffuse back across the membrane
and into the presynaptic neurone, where they are recycled
...

6
...
The
acetylcholine is then stored in the synaptic vesicles
...

Drugs affect the nervous system by targeting the synapses
...
They
are two main classes of drug: excitatory and inhibitory
...

They do this in three main ways:




Mimicking the effects of a neurotransmitter by behaving as one
...

Inhibiting the enzyme that breaks down neurotransmitter
...


As there are excitatory synapses and inhibitory synapse, drugs may work in different ways
...
An
excitatory drug that increases the production of a neurotransmitter at an inhibitory synapse will
actually inhibit the nervous system as less action potentials will be created
...
There are three types of muscle: cardiac, smooth and skeletal (striated)
...
Both cardiac and smooth muscle are not under somatic control
...
It brings about movement by
pulling on bones, causing them to move
...

Skeletal muscle is arranged to be able to contract powerfully and be strong
...

Many myofibrils run parallel to each other in the muscle fibre, which means that the collective force
produced by every sarcomere is large
...
In this way, many sarcomeres can be packed into a single tissue, so that are large
amount of force can be produced
...
Muscle cells fuse
during embryonic development, and share cytoplasm (sarcoplasm) and nuclei
...
The
sarcoplasm contains large quantities of mitochondria and endoplasmic reticulum for ATP and protein
synthesis
...
Microfilaments
are protein filaments
...




Actin is a thin, fibrous filament that consists of two filaments wrapped around each other
...


Two other proteins are present in sarcomeres that play a role in muscle contraction:




Tropomyosin is a fibrous strand that wraps around the actin filament
...
When the muscle is stimulated to
contract, the calcium ions released bind to the troponin, causing it to change shape and
move tropomyosin off the binding site, allowing the myosin crossbridge to form
...







The I band (isotropic) are the light bands on either side of the sarcomere
...

The A band (anisotropic) are darker bands in the middle of the sarcomere
...

The H zone is a section in the middle of the sarcomere which is light as only myosin is
present



The Z lines on either side of the sarcomere are what separate one sarcomere from the
other
...

Slow twitch muscle fibres contract slowly and less powerfully over long periods of time
...
The calf muscles or spinal erector muscle are examples of slow twitch muscles as they contract
almost constantly to keep the body upright
...






They respire aerobically, as this releases more energy and prevents the build up of lactic
acid
...

They have many mitochondria for ATP production
...
It is similar to haemoglobin in
that is consists of a Fe2+ molecule that binds to oxygen
...


Fast twitch fibres contract powerfully over short periods of time
...
The quadriceps muscles have both
fast twitch and slow twitch fibres
...
They
respire anaerobically as often the intensity of the exercise means that oxygen cannot reach the
muscles fast enough for the reactions of aerobic respiration to take place
...

They have lots of enzymes associated with anaerobic respiration
They have a store of phosphocreatine, which is basically a store of phosphate that is used to
phosphorylate ADP to ATP, so more ATP can be produced
...
There are
many neuromuscular junctions spread throughout the muscle to ensure that the contraction of the
muscle is rapid and powerful
...
Many
junctions mean that the entire muscle can be stimulated simultaneously
...
The division of the muscle
into motor units gives the nervous system more control over the degree of contraction
...
When
only a small amount of force is needed, only a small number of motor units are stimulated
...

1
...
The increased
permeability to calcium ions causes the synaptic vesicles to fuse with the presynaptic
membrane
...
Acetylcholine is released into the synaptic cleft, which it diffuses across to bind to the
sodium ion channels in the membrane of the muscle fibre
...
This causes sodium ions to diffuse into the muscle fibre, creating an action potential
...
This causes
muscle contraction
...
This model states that the
myosin heads form crossbridges with binding sites on the actin molecule
...

The process of muscle contraction is a continuous process but can be split into three main stages:
stimulation, contraction and relaxation
...
An action
potential is created in the muscle fibre at the neuromuscular junction
...
The action potential travels deep into the muscle fibre through a system of T tubules
...

2
...
As the smooth endoplasmic reticulum acts as a store of calcium ions and actively
takes them up, calcium ions diffuse into the sarcoplasm down the concentration gradient
...
The calcium ions in the sarcoplasm bind to the troponin molecules on the action filaments
...

4
...
As the binding
site is no longer blocked, the myosin heads can now form crossbridges with the actin
filaments
...
Once the myosin heads are attached to the filament, they change shape, pulling the actin
filament horizontally, and overlapping the myosin and actin filaments
...

6
...

7
...
The
energy released causes the myosin heads to revert to their original position with an ADP
molecule attached
...

Relaxation:
1
...
The energy of this is obtained from the hydrolysis of
ATP
...
The reabsorption of the calcium ions decreases their concentration in the sarcoplasm, which
causes the troponin to revert to their original conformation and shift the tropomyosin
filament back into position, blocking the myosin binding site
...
As the binding sites are blocked, the myosin heads cannot form crossbridges with the actin
filaments so muscle contraction ceases
...
At rest, the sarcomere can be divided into distinct sections due to the colour that each
section appears to have
...

When contracted:





The size of the H-zone decreases as the actin filaments are pulled over the myosin filaments
...
This is due to how
the actin filaments are pulled horizontally over the myosin filaments
...

The size of the A-band remains the same
...
This discounts
the theory that the filaments themselves shorten when the muscle contracts
...
As there are around 100,000 sarcomeres per muscle fibre,
this equates to a considerable energy demand
...
Most of
this energy is obtained from the respiration of pyruvate in the reactions of aerobic respiration
...
As a result, anaerobic respiration is used instead
...
It acts as a store of phosphate that is combined with ADP to form ATP
...
Internal conditions in this case refer
to the tissue fluid that bathes the cells
...
Our internal conditions are monitored and controlled by our brain
...
The response
can involve many different physical units of the body: muscles, hormones and behavioural changes
are all involved
...
g
...
This
means that any changes to this factor, causing it to deviate from the set point, will be resisted and
counteracted by homeostatic mechanisms in the body
...
Enzymes that catalyse reactions that are essential for survival, such as
the reactions of aerobic respiration, must continue to function
...

Blood glucose is also an important factor that must be maintained at a constant level
...

This sets up a water potential gradient, so water from cells moves by osmosis into the
surrounding environment
...
If the blood
glucose concentration is too low, then the water potential of the tissue fluid falls, which sets
up a water potential gradient between the cells and the tissue
...
Also, if the blood glucose is too low, there is
insufficient glucose for cells to use in respiration, so the rate of ATP production falls
...
This results in increased survival
chances of the organisms
...

The maintenance of internal conditions is controlled by control mechanisms in the body
...

1
...
The set point is monitored by receptors, which detect any deviation from the set point and
send this information to the controller
3
...
The controller then produces a response to the deviation
4
...

5
...
They send this information to
the controller, which receives this information and so stops stimulating the effectors
...
If information from only one receptor was analysed, the
perception by the controller may be inaccurate and so any response produced may be detrimental
to the organism
...


Thermoregulation
Thermoregulation is the process by which the body maintains a constant internal temperature
...

Heat is gained by:



Producing metabolic heat as a result of respiration
Gaining heat from the environment through conduction, convection or radiation

Heat is lost by



Sweating – the evaporation of water requires heat energy, so heat energy is lost in this
reaction
...


Conduction, in this case, is where heat is transferred from solid to solid
...
Convection is
where fluids heat up and expand and move, carrying the heat to other areas
...
This causes the organism to gain heat,
as the electromagnetic waves are very high energy
...
Ectotherms obtain most of their body heat
from the external environment (reptiles and amphibians) and are cold blooded, whereas
endotherms obtain most of their heat internally, as a byproduct of respiration
...

Ectotherms control their heat by:






Absorbing infrared radiation from the sun
...

Absorbing heat from hot rocks and surfaces
...

Colouration improves the amount of heat absorbed
...
Reptiles in colder countries generally have darker
colourations than those is warmer countries

Endotherms have many mechanisms to control heat
...
This allows them to survive in a much greater variety of environments
...

Heat is conserved by:











A low surface area to volume ratio
...
Animals also have other compositional adaptations, such as a thick layer of
insulating fat, fur, and small extremities to further decrease their surface area
...
Arterioles near the surface of the skin constrict, reducing their diameter
...
As blood carries heat, less
heat is lost through the skin
...
Hair muscles contract causing skin hairs to stand up
...
Muscles contract rapidly, causing heat to be produced in respiration
...

Behavioural mechanisms, such as finding shelter, basking in the sun or huddling together are
triggered
...
A large surface area to volume ratio is useful in very hot
environment, as metabolic heat is lost through the skin
...
Arterioles near the skin expand and increase the blood flow to the skin
...

Hair muscles relax as insulation isn’t needed
...
The evaporation of water is an endothermic process and therefore
requires heat energy
...

Decrease in metabolic rate and activity levels
...


The body temperature is controlled by the hypothalamus in the brain
...
The
hypothalamus has two centres: a heat loss centre and a heat gain centre
...
The hypothalamus itself also monitors the temperature of the blood
passing through it
...
Skin receptors may
register a decrease in temperature, but the hypothalamus may detect that the core temperature is
above normal, so no action is taken
...


Hormones and the Regulation of Blood Glucose
Blood glucose is regulated by the interacting hormones that are released from the pancreas and
other glands, such as the adrenal glands
...

Hormones are released by glands and organs into the blood stream when these organs are
stimulated
...
Target cells are cells which
have receptors on their membranes that are complementary to the hormones
...

Hormones have two different modes of action
...
The
second messenger model works as follows:
1
...
It is released by glands and organs into the blood
stream
...

2
...
This binding activates an enzyme within the cell which produces a new
molecule
...

3
...

Glucagon, adrenaline and thyroxin all work in the same way, following the second messenger model
...
Adrenaline is released in the adrenal glands and travels in the blood stream
...
Adrenaline reached target cells and binds to the complementary receptors on the
membranes of these cells, forming an adrenaline-receptor complex
...
This activates the enzyme adenylate cyclase, which synthesises cyclic AMP from ATP
...
cAMP acts as the second messenger, and activates the enzyme glycogen phosphorylase,
which carries out glycogenolysis and deactivates enzymes that carry out glycogenesis
...

Blood glucose is regulated in order to keep the blood glucose level high enough to supply all
body cells with sufficient glucose for respiration
...
The blood glucose changes throughout the day as
a result of glucose influx via food and glucose expenditure, as work is done by cells (exercise or
mental work)
...
The pancreas is made up of mostly exocrine
cells, which produce digestive enzymes in a fluid called pancreatic juice
...
Throughout the pancreas, there are groups

of cells called the islets of Langerhans
...
Alpha cells produce glucagon and beta cells produce insulin
...
When this fall is detected, they
release insulin
...






Insulin changes the tertiary structure of glucose protein channels in the cell membrane,
making the cell more permeable to glucose
...

Increases the number of glucose protein channels in the cell membrane, so that more
glucose can be transported into the cell
...


This results in a fall in the blood glucose concentration and a return to the set point
...

Alpha cells detect a fall in the blood glucose concentration, and release glucagon as a result
...
Liver cells
are the only body cells with glucagon receptors, so these are the only cells that can produce a
response
...


As a result, the glucose concentration rises and returns to the set point
...

Adrenaline also affects blood glucose
...
Adrenaline binds to body cells and activates glycogen phosphorylate, for glycogenolysis,
and inactivates the enzymes that carry out glycogenesis and the conversion of glucose into fat
...
As a
result of the negative feedback, the system is self regulating and automatic
...
This can affect the water potential of the cellular environment and can be
fatal
...
This may be due to the immune system
attacking the beta cells in the pancreas, leaving the body unable to detect rises in blood glucose
concentration or produce insulin to lower glucose concentration
...
The symptoms are usually obvious: high blood glucose level, increased
thirst and hunger, glucose in urine, need to urinate excessively, tiredness, genital itching, weight loss
and blurred vision
...
A reduced ability to respond to insulin
arises from a change in tertiary structure of the glycoprotein insulin receptors on body cells
...
The blood glucose concentration stays high
...
The symptoms are less severe than type 1, so they develop
slowly are more likely to go unnoticed
...

Type 1 diabetes is controlled by regular injections of insulin
...
Insulin is
taken with meals, mimicking the increase in insulin production that normally occurs after a meal due
to an increase in blood glucose
...
If the amount of insulin injected is too high, the blood glucose
concentration will decrease too much, which leads to low blood glucose and unconsciousness as the
cells have insufficient glucose for respiration
...
This can be complemented by taking
small insulin injections or drugs that stimulate insulin production
...


Feedback mechanisms
In almost all homeostatic control in the body, a feedback loop is established that helps to maintain
the set point
...
There are
two types of feedback mechanisms: positive and negative
...
This is by far the most common form of feedback
...
Two examples are thermoregulation and
the regulation of blood glucose
...
g
...
This information is sent to the heat loss centre of the hypothalamus
...
This involves vasodilation and increased sweating, as well as a decrease in metabolic rate
and a relaxation of body hairs
...
This
change is detected by thermoreceptors and sent back to the hypothalamus
...

In the regulation of blood glucose, a change in blood glucose (e
...
a rise) is detected by the beta cells
in the islets of Langerhans
...
As a result, the blood glucose concentration falls back to normal
...

In positive feedback, the feedback causes the corrective measures carried out to remain turned on,
which causes an even larger deviation from the set point
...
One example of positive feedback
is in the transmission of the action potential
...
This increases the permeability of the axon membrane and
ensures that the action potential is transmitted rapidly down the axon
...
In typhoid fever, thermoregulatory systems in the body break
down, causing an uncontrollable rise in temperature which can be fatal
...
However, only humans and some primates have a menstrual cycle, which
is where the uterus lining and some blood is shed between each cycle
...
Two of these hormones (FSH and LH) are
produced by the pituitary gland, and the other two are produced in the ovaries (oestrogen and
progesterone) by the follicles and corpus luteum
...
It stimulates follicles in the
ovary, which contain an egg each, to mature and develop
...

Oestrogen is produced in the ovaries by the follicles
...
In small quantities, it inhibits the
production of FSH and LH, but in larger quantities it stimulates the production of FSH and LH,
especially LH
...

Progesterone is produced by the corpus luteum in the ovaries
...
At day one, the uterus lining breaks down
...

2
...
This causes the follicles in the
ovaries to develop
...

3
...
In small
amounts, oestrogen inhibits the production of FSH and LH, in an example of negative
feedback
...

4
...
The quantity of oestrogen continues to
rise until it reaches a certain point, at which it stops inhibiting the production of FSH and LH
and stimulates the production of these two hormones (positive feedback)
...
This causes a surge in the concentrations of FSH and LH in the blood
...
The LH also causes the follicle
that released the egg to develop into the corpus luteum
...

6
...
It also helps to maintain the uterus lining in order to receive the
fertilised egg
...
If the egg isn’t fertilised, the corpus luteum breaks down
...


8
...
LH
and FSH are no longer inhibited and so FSH is released, causing the follicles to develop again
and the cycle to restart
...

LH  stimulates progesterone production and inhibits oestrogen production
Progesterone  inhibits LH/FSH production

RNA and the Genetic Code
DNA codes for proteins
...

Each codon corresponds to a different amino acid
...

As DNA is too big to leave the nucleus, and leaving the nucleus would risk damaging the DNA, it is
transcribed into a single stranded molecule known as messenger RNA (mRNA)
...
The mRNA base sequence is complementary to that of the DNA, so it is
not exactly the same
...
Here, it associates with the ribosomes and tRNA is used to synthesise the
protein that it codes for
...
Some
amino acids are coded for only be a single codon, but most have more than one
...
Instead they are placed at the
end of the base sequence for a particular protein and signal the ribosomes to stop
transcribing as the synthesis is finished
...

The code is non-overlapping, as each codon is read as distinct, three base sections
...


mRNA and tRNA are both made up of ribonucleic acid, instead of deoxyribonucleic acid
...
The nucleotides are molecules of
ribonucleic acid
...
The nitrogenous bases in RNA are the same as those in DNA except for thymine, which is
replaced in RNA with uracil
...

mRNA and tRNA differ in their structure and function, but are both made up of RNA
...
It is complementary the DNA section from which it
was transcribed
...
The fact that it is single stranded means that it’s nitrogenous bases are
exposed, which allows tRNA molecules with complementary anticodons to bind to the exposed
bases
...
The mRNA is easily broken down by enzymes in the cytoplasm, so proteins are
not continually transcribed when they are not needed
...
Hydrogen bonds
form between complementary bases within the molecule, which forms the clover shape
...
Each different tRNA molecule also has a
different anticodon
...

As the mRNA codes for a specific tRNA molecule, via the anticodon, the codon on the mRNA
therefore codes for the amino acid carried by this tRNA molecule
...


Polypeptide Synthesis – Transcription and Splicing
The functional molecules of all organisms are proteins, which are made up of polypeptide chains
...
There are
millions of different types of protein, each of which have a specific function in the body
...
The information that codes for different
polypeptides is contained in the DNA – the nucleotide sequence of the DNA molecule codes for
amino acids is a specific sequence
...
In
order to synthesise a polypeptide, the DNA must be transcribed into a molecule of pre-mRNA
...
Here, it acts as a
template to which complementary tRNA molecules attach, and the amino acids they carry are linked
to form the polypeptide chain
...
This is where the nucleotide sequence of
DNA is converted into a complementary sequence on a pre-mRNA molecule
...
First, DNA helicase acts on the region of DNA that is being transcribed
...
This
opens up the double helix and exposes the nucleotide bases in the DNA
2
...
It then moves down one of
the strands of the DNA, the template strand, using the base sequence of this strand to a
synthesise complementary molecule of pre-mRNA using the pool of nucleotide bases
present in the nucleus
3
...
This means that only 12 nucleotide bases are exposed at any
one time
...
When the RNA polymerase reaches a stop codon, it detaches from the DNA and releases
the completed molecule of pre-mRNA
...
DNA is made up of introns and exons
...
Introns can occur within genes, so when the RNA polymerase synthesises the molecule of premRNA, these non-coding base sequences are included in transcription
...

Enzymes therefore cut the introns out of the pre-mRNA and the exons join up to form the mRNA
molecule
...
This means that different polypeptides can be produced from the same initial premRNA molecule
...
The exons rejoin in a different sequence than that dictated by the gene, so a different
polypeptide is translated, which is non functional
...
It then
associates with the ribosomes to be translated
...
This process uses tRNA molecules to carry amino acids to the
ribosome
...
Each type has a unique anticodon on
one end of the molecule, which is complementary to a specific codon on the mRNA molecule
...
This means that each amino acid is transported by a specific tRNA molecule, which has
an anticodon that is complementary to a specific codon on the mRNA molecule
...
Therefore the codons code for amino acids by way of the tRNA
molecules
...
The mRNA migrates out of the nucleus and into the cytoplasm
...

2
...
It carries an amino acid
...
The tRNA molecule with the complementary anticodon to the second mRNA codon moves
into the ribosome and binds to the second codon
...

4
...

5
...

6
...

The tRNA is therefore allowed to go back to the amino acid pool to collect another amino
acid
...
Many ribosomes can move along a single mRNA strand, so that identical polypeptide chains
can be produced rapidly
...

8
...
This causes the
peptidyl transferase to finish the polypeptide by attaching a hydroxyl group to the final
amino acid
...

In summary, the DNA triplets in genes determine the codons on the mRNA molecule
...
The
sequence of the tRNA molecules therefore determines the sequence of amino acids in the
polypeptide
...
Since many of these proteins are enzymes, the
genes control the metabolic activities of the cell
...
Mutations that occur in body cells are
not passed onto the next generation, whereas mutations that occur in the formation of gametes
may be passed to the next generation
...

A mutation will change the nucleotide sequence of DNA
...

There are three types of mutation: addition, substitution and deletion
...
There are three
outcomes to this substitution which give three types of substitution mutation:






A nonsense mutation is where the substituted base results in the formation of a stop codon
...
When the mRNA is translated, the stop codon results in production of the
polypeptide stopping prematurely
...

A mid-sense mutation is where the substituted nucleotide base results in a new codon being
produced, that codes for a different amino acid
...

Depending on the role of the amino acid, this may not have much of an effect
...
However, if the original amino acid did
not have a very significant role in the formation of the tertiary structure, then the new
amino acid may not affect the function of the protein
...
This happens as the genetic code is degenerate, meaning
that a single amino acid can be coded for by more than one codon
...
It is a silent mutation as there is no evidence of the mutation in the protein
that the gene codes for
...
Addition mutations are where a new base is
added into the nucleotide sequence on the DNA
...
These mutations have a much greater effect on the overall sequence of the DNA
than substitution mutations
...
If a base is added or deleted, this causes ‘frame-shift’ where the triplets are shifted to the
left or write by one letter
...
If the mutation
occurs at the start of the nucleotide sequence, the entire sequence is changed
...


Mutations occur naturally and spontaneously during DNA replication
...
Although mutations are random, they occur
at a steady rate: 1-2 mutations per 100000 individuals per generation
...
These are chemical and physical agents that damage or change DNA,
increasing the rate of mutations
...
This includes UV radiation and gamma radiation
...
Free radicals can oxidise bases, altering their shape so that DNA
polymerase can no longer act on them
...

Chemicals that damage the DNA molecule and interfere with transcription
...
Nitrous acid removes an amine group to
cytosine in DNA, which changes it to uracil
...
Benzyprene,
found in tobacco smoke, adds a large group to guanine in DNA, making it unable to pair with
cytosine
...


Cell division is controlled by genes
...
Proto-oncogenes stimulate cell division and tumour suppressor genes
inhibit cell division
...
Cells are signalled to divide by growth factors that bind to
protein receptors on the cell membrane
...
Proto-oncogenes code for proteins involved in cell division
...
Oncogenes code for proteins that stimulate cell division
at a very high and uncontrolled rate
...

Oncogenes can code for excessive production of growth factor, which also causes the cell to
divide excessively
...
There are two tumour suppressor
genes
...
As there are two tumour suppressor genes, both must mutate in order for there to be a
cancer risk
...
This is also why cancer risk increases
with age
...


Totipotency and Cell Specialisation
All cells in the body have the same genes and can therefore, theoretically, produce the same
proteins
...
g
...
However, cells are very
different and produce different proteins in order to perform their particular function
...
This is because, although all cells contain the
same genes, different cells have different genes that are switched on
...
If it is switched off, the gene is not translated and the protein that it codes for is
not produced
...

Some genes are permanently switched on and expressed in all cells, such as the genes that code for
respiratory enzymes
...
For
instance, the gene that codes for insulin is permanently switched off in muscle cells but permanently
switched on in beta cells in the pancreas
...
For example, proteins that make up to
spindle fibres are only needed during cell division, so the genes that code for them are only switched
on when the cell enters cell division
...
An organism beings from a zygote (fertilised egg)
...
The zygote is therefore totipotent – it can differentiate into all cell types including
placenta and umbilical cells
...

Totipotent and pluripotent cells split and develop into every type of body cell, such as muscle cells
and epithelial cells, which are specialised to carry out a particular function
...
This is differentiation
...

In normal body cells, differentiation is an irreversible process: they will not be reversed back into
pluripotent cells
...
These cells are multipotent
– they can differentiate into a few different types of cell, usually a single tissue
...
Multipotent adult stem cells and pluripotent
embryonic stem cells are being used to treat a variety of conditions
...
Under suitable conditions, a plant cell can be taken and an
entirely new plant can be grown, as the cell can differentiate into any type of plant cell
...


Embryonic stem cells are pluripotent, so they have the greatest potential to treat disease
...
These tissues can
then be transplanted into a patient in order to treat their disease
...
An early embryo is cultured in a nutrient medium
...

2
...

3
...

4
...

5
...
For example, a differentiation factor could be added that causes one of
the groups to develop into nerve cells, which could be used to treat degenerative nervous
conditions
...

Controlling transcription:
In order for a gene to be translated, a transcriptional factor needs to bind to a specific site on the
DNA
...
This causes an mRNA
molecule to be produced, that carries the genetic code, which is translated into a polypeptide
...
When a gene is
switched off, the site on the transcriptional factor that binds to the DNA is occupied by an inhibitor
molecule
...
Different transcriptional factors bind to different regions of DNA, and therefore
stimulate the transcription of different genes
...

1
...

2
...

3
...

4
...

5
...


Preventing translation:
Even if a gene is transcribed into an mRNA molecule, the cell can still control whether the mRNA is
translated or not
...
These associate with
enzymes and then bind with complementary base sequences on the mRNA molecule, causing the
enzyme to break the mRNA into small pieces, which prevents its translation
...
An enzyme breaks long, double stranded RNA molecules into small sections of double
stranded RNA (siRNA)
2
...

3
...

4
...

5
...

siRNA can also be used by scientists to determine the roles of particular genes
...
siRNA is also used in medicine to treat diseases causes by genes, as the siRNA can
prevent the expression of these genes
...


Producing DNA Fragments
Many diseases are caused by the lack of the ability of an organism to produce certain chemicals
needed in metabolic reactions or homeostasis
...
Many of these
substances are proteins, or are made up of proteins and other non-protein groups, and enzymes
which produce these chemicals
...

In order to treat these patients, the substances that they lacked were extracted from other
organisms and introduced into the patient
...
An advantage of DNA
technology is the ability to produce pure substances that will not be rejected by the immune system
that can be given to the patient, using bacteria or viruses to produce them
...
DNA of two different organisms that is combined like this is called
recombinant DNA, and the organism itself is genetically modified
...
Isolation of DNA fragments that contain the desired gene
...
Insertion of the DNA fragments into a vector
...
Transformation (also called transfection) of a host organism – the transfer of the DNA into
the cells of the host organism
4
...
Cloning of host cells to produce the desired substance
...

Reverse transcriptase is an enzyme used by retroviruses to convert mRNA into DNA, reversing the
transcription process
...
A cell that readily produces the desired substance is identified (e
...
a beta cell in the
pancreas for insulin)
2
...

3
...
The DNA is known as complementary DNA (cDNA) as its base sequence is
complementary to the sequence of the mRNA
...
DNA polymerase is then used to produce the second strand of DNA, which forms a double
stranded DNA molecule which contains the desired gene
...
When viruses
enter a bacterial cell, they produce DNA that enters the nucleus of the bacteria
...
There are many different
types of restriction endonuclease
...
The
sequence at which a restriction endonuclease cuts is its recognition sequence
...

The restriction endonucleases cut the DNA in two ways: they either cut perpendicular to the DNA
stand, leaving two blunt ends, or they cut in a staggered fashion, leaving two ends with exposed
bases
...


In Vivo Gene Cloning – Insertion and Transformation
Once the fragments of DNA that contain the required genes are obtained, the next stage is to insert
them into vectors
...
In vivo means that the process takes place within a host
organism, whereas in vitro takes place externally of cells
...
The restriction
endonucleases used cut fragments of DNA that contain the required gene in a staggered pattern,
leaving exposed bases
...
The same restriction endonuclease is used to cut
the DNA so all the fragments produced will have sticky ends that are complementary to one and
other
...
As all
the sticky ends are complementary to one and other, the bases will pair up and form hydrogen bond
with one and other
...
This principle is used to
insert a DNA fragment into a vector
...
The DNA cannot be directly inserted
into the nucleus of the host cell as it would be rejected
...
Plasmids almost always carry genes for antibiotic resistance, so these are the genes that
are replaced by the genes inserted into the fragment
...
A restriction endonuclease is used to cut a DNA fragment that contains the required gene
from the DNA of a cell which synthesises the desired protein
...
The same restriction endonuclease is used to cut out a DNA fragment from a plasmid, which
contains a gene for antibiotic resistance
...
As the same restriction endonuclease is used, the sticky ends of the plasmid and the sticky
ends of the DNA fragment are complementary to one and other, so the DNA fragment
becomes incorporated into the plasmid loop
...
DNA ligase is used to bind the deoxyribose-phosphate backbone of the DNA fragment and
the plasmid DNA together, securely binding the fragment and the plasmid together
...

Not all plasmids take up the DNA fragment
...

The next stage is to transform the host cell by introducing the vectors into the cell
...
This causes the permeability of the bacterial membranes to increase, allowing the
vectors to enter the bacterial cell
...


Gene Cloning – Identification
There are two main steps to identifying usable bacteria: identifying which bacteria have taken up the
vector (plasmid) and then identifying which bacteria have taken up plasmids that contain the
required gene
...
For example, the R plasmid, which contains genes that code
for resistance to ampicillin and tetracycline
...
When the bacteria take up
these recombinant plasmids, they therefore gain the gene for ampicillin resistance which is present
on the plasmid
...

1
...

2
...

3
...
The bacteria that survive must be the
bacteria that took up the plasmids
...

This is done using gene markers
...

Antibiotic gene markers use the presence of a second gene that codes for antibiotic resistance on
the plasmid
...
When the required gene is inserted into the plasmid, the
tetracycline gene is cut out and removed from the plasmid
...
To identify
them, replica plating is used as the identification would result in them being killed by the antibiotic
...
Bacterial cells that are identified as having taken up the plasmids are grown in a culture by
spreading them thinly on a nutrient plate
...
The bacterial cells divide and grow to form colonies
...
Small samples of each colony are taken and placed in the exact same positions as the
original colony on a replica plate
...

4
...

5
...

Fluorescent markers are more rapid and don’t require replica plating
...
A gene from a jellyfish that produces a green fluorescent protein is added into the plasmid
loop
...
This gene is then cut and the DNA fragment that contains the required gene is added into
the loop in its place
...
The bacteria that have taken up the required gene containing plasmids will therefore not
fluoresce as they lack the gene that produces the required protein
...

The bacteria that have taken up the gene containing plasmids are identified visually by looking at
them through a microscope
...

Genes that produce enzymes can be used in the same way
...

1
...
The lactase gene is then cut out of the plasmid and the required gene inserted in its place
...
The bacteria that have taken up the right plasmids therefore lack the lactase enzyme, so
won’t turn the solution blue
...
This is called amplification – when a small quantity of DNA is built up
...
This is an enzyme which synthesises DNA from nucleotides, deoxyribose
and phosphate
...
DNA polymerase that is used in PCR is obtained from
bacteria that grow in hot springs
...

Primers, which are short sequences of nucleotides that have a complementary base
sequence to the first bases at either end of the two DNA strands from the DNA fragment
...


There are three stages to the polymerase chain reaction: separation of the DNA strand, annealing of
the primers, and the synthesis of DNA
...
DNA fragments, primers and DNA polymerase are placed in a vessel in the thermocyler,
which contains nucleotides
...
The high
temperature is used to break the hydrogen bonds of the bases in the DNA fragment
2
...
The temperature is reduced to 55 O C
...
The primers act as a starting molecule of DNA, from
which a new strand is built
...

3
...
The temperature is increased to 72O C, which is the
optimum temperature of DNA polymerase
...
Then the cycle restarts
and these two strands are copied
...
After 25 cycles, over a million
copies of DNA synthesised
...

The advantages of using PCR to clone genes over using in vivo techniques are that PCR takes much
less time and is much less complicated
...
PCR also
doesn’t require living cells, so no complex and lengthy culturing techniques are needed
...
First, in vivo techniques are
generally more accurate, precise and carry less risk of contamination that PCR
...
Also, if there
is a copying error in PCR, the contaminant DNA will carry on being copied by the temperature cycles
...
In vivo carries a very low risk of
contamination by other DNA molecules due to the specific restriction endonucleases used to cut out
genes
...

However, it is very easy for contaminant DNA to enter the thermocycler, which will then be copied
...
However, PCR is more useful for amplifying DNA for analysis, due to its speed and
efficiency
...
The ability to insert genes into different organisms speeds up this process exponentially
...

Increasing the nutrient content of plants
...

Making crops tolerant to herbicides
...
Antibiotics are compounds that kill bacteria that are
produced by other bacteria as a defence mechanism
...
Human
insulin is produced on mass scale using bacteria
...
In the past, pig insulin
was used, which had to be humanised
...
Using bacteria means that the human gene can be transplanted into the bacteria,
so the hormone doesn’t need to be humanised
...
Production of
enzymes used in the food industry also involves bacteria
...

Plants are modified for many reasons
...
Tomato plants produce an enzyme which causes them to soften, which speeds up the
decomposition process and decreases their shelf life
...
This means that the mRNA that
codes for the softening enzyme will be complementary to the enzyme transcribed from the inserted
gene
...

Plants are also genetically modified to increase their yield by increasing their tolerance to certain
conditions
...
This means that only the
weeds and competing species will be killed by the pesticide, leaving the plant undamaged
...
Plants are also modified to increase their
tolerance to environmental conditions, diseases and pests such as insects
...

Animals are modified to increase the resistance of livestock to disease and to produce medicines
...
Genes that code for growth hormone production
are also added, which increases the growth rate and size of the organisms
...
Goats are modified to produce anti-thrombin in
their milk
...
Some individuals possess an allele that doesn’t code
for sufficient anti-thrombin, and are therefore at risk of blood clots
...

1
...
The human gene for normal anti-thrombin production is added into the fertilised egg
alongside the genes that code for proteins in the goats milk
...
The resulting goats that produce anti-thrombin are crossbred with other anti-thrombin
producing goats
...

4
...

Chickens are also used in this way
...
The resultant eggs from the chickens contain the
desired protein, which is then used in medicine
...


Gene Therapy
Gene therapy is used to treat diseases caused by genetic disorders, or diseases that are influenced
by genetic disorders
...
This means that functional proteins will be produced and the symptoms of the disease
will go
...
Cystic fibrosis is a genetic disorder caused by
a deletion mutation in a gene that codes for a membrane protein
...
It produces a membrane protein which
allows chloride ions to diffuse out of epithelial cells
...
Water
therefore moves from the cell by osmosis
...

Cystic fibrosis sufferers have a deletion mutation in the CFTR gene
...
As a result, the protein
produced lacks an amino acid
...
This means that the protein
doesn’t allow chloride ions to leave the cell, causing the epithelial membranes to be dry and the
mucus they produce to be thick and sticky
...

Breathing difficulties and less efficient gas exchange due to the increased congestion and
diffusion pathways in the lungs
Accumulation of the mucus in the pancreatic ducts, which impedes the release of pancreatic
enzymes and causes the formation of fibrous cysts
...


The mutated allele that causes cystic fibrosis is recessive, so two non-sufferers may be heterozygous
and produce offspring with cystic fibrosis
...
As the allele is recessive, it can be treated using gene therapy
...

Gene supplementation is where the normal allele is added into the DNA of the organism
alongside the defective allele
...


There are two types of gene therapy treatment:


Germ line therapy involves replacing or supplementing defective genes with normal alleles
in zygotes
...
It also means that, although it does pass on the
defective allele to its offspring as the defective allele isn’t removed, the offspring will not



suffer from the disease as they also inherit the normal allele
...

Somatic cell gene therapy targets only the cells of the affected tissues
...

Also, as cells in the affected tissues die without passing on the added genes, the treatment
has to be repeated at the rate at which the cells die
...
As this treatment has had limited success, the aim is to target undifferentiated
embryonic and adult stem cells that give rise to the mature tissues
...


There are two main methods of treating cystic fibrosis using gene therapy
...
These viruses cause colds and respiratory illnesses by
injecting their DNA into the epithelial cells of the lungs
...

1
...

2
...

3
...
The CFTR gene becomes
incorporated into the viral DNA
...
They are then extracted and introduced into the airways of the patient
...
The gene
supplements the defective gene, and the normal CFTR channel is produced
...

The lipid molecule is lipid soluble so it can pass across the membranes of the epithelial cells and
deliver the DNA
...

2
...

4
...

It is inserted into a plasmid using the same restriction endonuclease
...

The bacterial cells that have taken up CFTR containing plasmids are identified using gene
markers
...

5
...

6
...
They diffuse across the phospholipid bilayer and into the cell, where the plasmid is
released
...
It is an inherited
disorder where the sufferer cannot show cell mediated immunity or produce antibodies
...
This enzyme kills toxins that
would otherwise kill leucocytes, so white blood cells die as a result as the enzyme is non functional
...
The normal ADA gene is isolated from human tissue using restriction endonuclease
...
It is then inserted into a retrovirus (which attack the immune system by injecting DNA into T
cells)
3
...

4
...

As T cells only live for 6-12 months, the treatment must be repeated at intervals
...
This is because stem cells in the
bone marrow differentiate into T cells, so transforming them would provide lifelong protection
...

There are many concerns over gene therapy
...
They also can produce an immune response, which means that the treatment isn’t
effective and cannot be reused, as the virus would immediately be killed by the secondary response
...

Gene supplementation is short lived due to the lifespan of cells, and must be repeated at regular
intervals
...
Gene therapy is also only effective against disorders that involve a single defective
allele
...


Locating and Sequencing Genes
In order to carry out gene therapy and genetic screening, genes must be sequenced and located
...

DNA sequencing is used to determine the base sequence of a desired gene
...
For larger sequences, restriction mapping is used
...
These are nucleotides which cannot attach to the
next base in the sequence – they do not allow another nucleotide to be attached to them once they
are added into the chain and therefore are the last nucleotides in the sequence
...
To sequence DNA using the Sanger method:
1
...
This releases single stranded molecules of DNA into
the solution, each of which can act as a template strand for the synthesis of another,
complementary strand
...

2
...

3
...
E
...
adenine terminator
is added to test tube 1, thymine terminator is added to test tube 2 etc
...
DNA primers are added to each test tube
...

5
...

6
...
As the addition of nucleotides onto a new strand of DNA is a
random process, the addition of a terminator nucleotide is as likely as the addition of a
normal nucleotide
...
In each test tube, every strand will end with the same nucleotide, as the
terminator nucleotide will be the last nucleotide added into the new strand
...

Once the stands have been identified, the DNA is then separated out according to length by gel
electrophoresis
...
The
resistance of the gel means that the larger the fragments, the more slowly they move and the
shorter the distance that they move
...
If
radioactive primers were used, then photographic film is then placed over the gel for several hours
...

The shortest fragment will only have one nucleotide
...
The next fragment will have two nucleotides, which will be the first two
nucleotides in the base sequence
...
This is only useful for genes with base sequences of
less than 500 nucleotides
...

Restriction mapping involves cutting up a large section of DNA into smaller sections using restriction
endonucleases
...
The base sequence is then determined by reading off the fragments, from smallest to
largest
...
For
example, when a plasmid is sequenced, a single restriction endonuclease will cut the DNA at a single
recognition site, producing one peice of DNA
...
Depending on the restriction endonucleases used,
the lengths of the two strands will be different
...
For
example, with as plasmid with 100000 bases, if the recognition sites of two different restriction
endonucleases were 10000 bases apart, then the two strands produced would be 1000 and 9000
bases long respectively
...
The
fragments are then analysed, from smallest to largest, in the same way as in the Sanger method to
determine the base sequence
...
This is done using DNA probes
...
They are labelled
so that the position of the gene on the DNA molecule can be identified, either radioactively or
fluorescently
...

1
...

2
...

3
...
This is DNA hybridisation
...
The DNA is located using the radioactive or fluorescent marker attached to the probe
...
If the
mutation occurs in the gametes of the organism, the genetic defect will be passed onto the
offspring
...
If it is recessive, then it is only expressed if the organism has the homozygous recessive
genotype, and organisms that are heterozygous will not suffer from the disease but they will carry
the defective allele
...

1
...

2
...

3
...

4
...

5
...
If not, then the DNA probes will not be taken up by the
DNA
...
The DNA is then put onto photographic film
...
If the film isn’t exposed,
then the DNA hasn’t taken up the probes and therefore doesn’t contain the allele
...

If a patient finds out that they carry a particular mutated allele and are therefore a carrier for a
genetic disorder, they are taken to see a genetic counsellor for advice
...
In the case of genetic diseases, a couple will be genetically screened to see what the chances
are of their children having genetic diseases
...
Genetic counsellors also advice people if they find out that they
possess certain alleles that increase their risk of cancer
...
These individuals are more likely to get cancer, as both tumour
suppressor genes must mutate in order for cells to divide uncontrollably
...

In addition to detecting genetic diseases, genetic screening is also used to find oncogene mutations,
which help identify the type of cancer the patient has
...


Sickle Cell Anaemia
Sickle cell anaemia is a genetic disorder due do a single substitution in the DNA section that codes
for one of the four polypeptide chains that make up haemoglobin
...

A substitution mutation in the DNA section that codes for one of the polypeptide chains that make
up haemoglobin results in adenine beings swapped for thymine
...
The mRNA transcribed from this DNA section will therefore have a different codon than
normal mRNA transcribed from this gene
...
When the
mRNA is transcribed, the resultant polypeptide will be different as a different amino acid is coded for
by the changed codon
...
This small change
produces a sticky patch on the haemoglobin molecule
...
When haemoglobin S is not carrying oxygen (at low oxygen concentrations) the
sticky patches on the molecules adhere together, forming long insoluble fibres that distort the red
blood cells and make them into a crescent shape
...
Also, the sickle shaped red
blood cells may block the capillaries as their diameter is greater than that of the capillaries
...
There are therefore three different genotypes and phenotypes that can result
...
Normally, they have
no obvious symptoms, but at low oxygen concentrations (e
...
when exercising) they may
become tired more easily due to the increased sickle cell count in their blood
...

Individuals that are homozygous for haemoglobin S (HbSHbS) will suffer severely from the
disease as all their haemoglobin will be haemoglobin S
...


In regions of Africa where malaria is common, there is an elevated allelic frequency of the HbS allele
in the population
...
However, the plasmodium parasite that causes malaria reproduces in
red blood cells, and is unable to do so in sickle shaped red blood cells
...







Heterozygous individuals are selected for in regions where malaria is common, as the effects
of their sickle cell anaemia is mild and the allele affords them resistance to malaria
...

Individuals that are homozygous for haemoglobin A are selected against in regions where
malaria is common, as they lack the HbS allele that gives them malarial resistance
...

Individuals that are homozygous for haemoglobin S are selected against in regions where
malaria is present and isn’t present
...


Genetic Fingerprinting
Genetic fingerprinting is used to test to see how closely related two individuals or populations are,
as well as being used to determine if a DNA sample matches the DNA of an individual
...
Introns are non-coding sections of
DNA which contain repetitive sequences of bases called core sequences
...
The more closely two individuals are to each other, the more similar the core
sequences will be
...

Making a genetic fingerprint consists of five stages: extraction, digestion, separation, hybridisation
and development
1
...
As the DNA
quantity is usually small, it is amplified using PCR
...
Digestion: restriction endonucleases are used to cut the DNA into fragments
...

3
...
The DNA
fragments are then treated with alkali in order to separate the double strands into single
strands
...
Southern blotting involves placing a nylon membrane over the gel, and
then covering the membrane with absorbent paper
...
This transfers the DNA fragments into the
nylon membrane in the same positions as they were in the gel
...

4
...
The DNA probes
are each complementary to different core sequences and therefore bind to them
...
Development: the membrane is covered with a photographic plate which is exposed by the
radioactively labelled DNA probes (if they were fluorescently labelled they are identified
visually)
...

DNA fingerprints are then interpreted by checking to see if there is a match between two DNA
samples
...

DNA fingerprinting is used extensively in forensics, where it is used to determine if DNA taken at the
scene of a crime matches that of a suspect
...

However, the DNA may not be incriminating as the DNA could have been left there innocently, the
DNA could have been left there before the crime took place, and the DNA could have been left when
the person touched the victim
...


DNA fingerprinting is also used for paternity tests
...
The
banding patterns of suspected parents can then be compared with those of the child to see if there’s
a match
...
The more different the genetic
fingerprints of individuals in a population, the more genetically diverse the population is, whereas
the less different the genetic fingerprints of the individuals, the more similar the population is

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