AQA A2 biology

Summarised biology notes for unit 4 exam

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  • Created on: 13-04-11 22:39
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Ecosystem: All the organisms living in a particular area and all the
abiotic conditions.
Trophic levels:
Producer => primary consumer (herbivore) => secondary consumer
=> tertiary consumer
Decomposers: break down decaying material, releasing minerals that
can be absorbed by plants, e.g. fungi and bacteria and also
detritivores, earthworms.
Energy losses:
A large amount of the Sun's energy is reflected back into space
by clouds and dust or absorbed by the atmosphere.
Light may not hit a chlorophyll molecule.
Not all wavelengths of light can be absorbed and used for
Some of the organism is not digested or eaten.
Energy is lost is used up in respiration (respiratory loss) and
also lost directly from the body to the environment.
Net productivity= gross productivity ­ respiratory loss
Most food chains only have 4 or 5 trophic levels as there is
insufficient energy available to support higher than this. Also, the
total energy stored is less at each stage.
Pyramids of number: Shows the number of organisms in each trophic
No account is taken of size- so small numbers of big organisms or
large numbers of small organisms change the shape ­ stopping it
from being shaped like a pyramid.
Pyramids of biomass: Shows the amount of (dry) biomass at each
trophic level ­ kgm²
Organisms must be killed to measure biomass. => small sample
usually taken => may not be representative of population. Only
organisms present at a certain time are shown so seasonal differences
are not apparent. => Shape is nearly always pyramid shaped except
with plankton as a producer.

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Pyramids of energy: Shows the amount of energy available in each trophic
level - kgm²yr¹
Always pyramid shaped ­ most accurate representation of
energy flow e.g. compared to pyramid of biomass => two
organisms of same dry biomass may store different amounts of
energy. However, collecting data can be difficult and complex.
Agricultural Ecosystems: Agriculture tries to ensure that as much of the
available energy is transferred to humans.…read more

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Intensive farming: Involves converting the smallest possible amount of
food energy into the greatest quantity of animal mass.
The efficiency of energy conversion is increased so more
biomass is produced and productivity is increased.
Environment is kept warm to reduce heat loss from the body.
Predators are excluded so there is no loss to other organisms in
the food web.
Animals may be given feed that is higher in energy than their
natural food.…read more

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As succession continues, the ecosystem becomes more complex.
Species diversity increases and the abundance of each species
Climax community: The final seral stage, where the ecosystem is
supporting the most complex community of plants and animals it
can. ­ It won't change much more.
The Carbon Cycle: Shows how carbon moves through living organisms and
the non-living environment.
1. Carbon (Co2) is absorbed by plants
by photosynthesis, becoming carbon
compounds in plants.
2. Carbon is passed along the food
chain through consumption.
3.…read more

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Ammonification: Nitrogen Compounds (dead organisms) =>
Ammonium compounds.
Animal waste also contains nitrogen compounds which are also
converted into ammonium compounds by decomposers.
Nitrification: Ammonium compounds => Nitrogen Compounds
These nitrogen compounds can then be used by plants. Nitrifying
bacteria changes nitrites into nitrites and then another nitrifying
bacterium converts the nitrites into nitrates.
Denitrification: Nitrates (soil) => Nitrogen gas
The denitrifying bacteria use nitrates in the soil to carry out
respiration and produce nitrogen gas. This occurs in anaerobic
conditions, e.g. in waterlogged soils.…read more

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Phenotype: The characteristics of an organism i.e. the expression of it
genetic composition.
Dominant allele: Always expressed in the phenotype even when
there's only one copy.
Recessive allele: An allele whose characteristic is only expressed in
the phenotype if two copies are present.
Co-dominance: Where two alleles both contribute in the phenotype of
an organism.
Monohybrid inheritance is the inheritance of a single characteristic
(gene) controlled by different alleles.
Monohybrid crosses show the likelihood of certain genotypes being
inherited by the offspring.…read more

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Allele Frequency is affected by differential reproductive success:
The frequency of an allele within a population can change when
the allele codes for a characteristic that affects the chances of an
organism surviving.
Therefore, not all individuals are as likely to reproduce as each
other. ­ Differential reproductive success.
Individuals that have this allele are more likely to survive,
reproduce and pass on their genes (incl. the beneficial allele).
This means a greater proportion of the next generation inherit
the beneficial allele.…read more

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The two groups will have become two different species.
Photosynthesis converts light energy to chemical energy in the form of glucose.
Takes place in the chloroplasts.
Respiration: There are four stages in aerobic respiration: glycolysis, the
link reaction, the Krebs cycle and the electron transport chain.
Glycolysis: - the splitting of the 6C glucose molecule into two 3C pyruvate
Occurs in the cytoplasm of the cells. ­ Is an anaerobic process.
1.…read more

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NAD go into the last stage of
Krebs cycle: This is the addition of acetylcoenzyme A into a cycle of
oxidation-reduction reactions that yield some ATP and a large
number of electrons.
Takes place in the matrix of the mitochondria.
1. Acetyl CoA (2C) from link reaction combines with (4C)
Acetyl CoA(2C) + Oxaloacetate(4C) => Citrate (6C)
2. Citrate (6C) is decarboxylated and also dehydrogenation occurs.
­ The hydrogen is used to reduce NAD.
CO2 is removed as well as hydrogen.…read more

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The protons accumulate in the space between the 2
mitochondrial membranes before they diffuse back into the
matrix of the mitochondria through protein channels.
5. At the end of the chain, the electrons combine with these protons
and oxygen to form H2O.
Oxygen is the final acceptor of electrons in the electron
transport chain.
Without oxygen acting as the final acceptor of electrons, the H+
ions and electrons would `back up' along the chain and
respiration would come to a halt.…read more


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