Edexcel Biology Unit 4 Notes

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Biology Notes
Photosynthesis ­ 6CO2 + 6H20 + (Energy/ATP) C6H12O6 + 6O2
Autotrophic organisms make their own food e.g. by photosynthesis.
Heterotrophic organisms eat other organisms as their source of food.
ATP ­ Adenosine triphosphate
ATP is the molecule that provides the energy that is needed within an organism to allow it to carry
out its metabolic reactions. It releases energy by breaking the third phosphate group to produce
ADP (Adenosine diphosphate) and an inorganic phosphate molecule. This is catalysed by the enzyme
The reaction is also reversible so energy can be used to produce ATP and the same enzyme catalyses
the reverse reaction.
Production of ATP
ATP is produced from ADP and an inorganic phosphate group. This requires energy which is produced
by many redox reactions throughout an electron transport chain. One electron acceptor/carrier
accepts an electron and is therefore reduced. It then passes on the electron to the next acceptor so
it is oxidised and the next acceptor is reduced. These reactions release a small amount of energy
each which is then used to produce the bond between ADP and phosphate to produce ATP.
Structure of a chloroplast
The thylakoids contain
chlorophyll which is a mix of 5 different
photosynthetic pigments which absorb
different wavelengths of light. This is
where the light dependent reaction
Stacks of thylakoids are called
granum (grana = plural). They are
connected to other grana by stroma
lamella which keeps the structure of the
chloroplasts rigid.
The fluid matrix inside the
chloroplast is called the stroma and it
contains the enzyme RUBISCO (used in
independent stage). This is where the
light independent reaction occurs.
Chloroplast adaptations
Chloroplast envelope keeps reactants very close to their reaction sites.
Thylakoids have a large surface area to maximise light absorption.

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Stroma contains all the enzymes etc. for light independent reaction to take place.
Lots of ATPase is present in the thylakoid membranes to produce ATP.
Light dependent reaction - first step in the production of glucose ­ takes
place on thylakoid membranes. These contain photosynthetic pigments
such as chlorophyll a, chlorophyll b and carotene.
It has a few main roles:
1. Produce ATP so that the energy can be used to produce glyceraldehyde 3-phosphate
from glycerate-3-phosphate in the light independent reaction.
2.…read more

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This breaks down into 2 molecules of glycerate-3-phosphate almost immediately.
3. Glycerate-3-phosphate is reduced to glyceraldehyde 3-phosphate (a 3 carbon sugar) by
NADPH. Energy for this reaction is provided by ATP.
4. The sugar is then used to produce organic molecules such as glucose.
5. Glucose can then be used to produce starch so that it can be stored and doesn't affect the
osmotic concentration of the cell (since it is insoluble).
6.…read more

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Since more plants are present, there is a higher plant biodiversity and the plants are able to provide
for more animals hence the biodiversity of animals in the habitat also increases.
Secondary succession
Secondary succession is when succession occurs on land that already has soil but the previous habitat
was destroyed by a flood, forest fire etc. Since soil is already present, plants such as flowering plants
and shrubs can grow providing their seeds were preserved in the soil.…read more

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Biotic factors
Predation ­ predators kill prey hence increased predators will cause a decrease in prey but if
the predator numbers increase too much they will compete (intraspecific competition) and
their numbers will decrease due to lack of food.
Finding a mate ­ if a male can't find a female they will not be able to reproduce and produce
offspring hence they will become extinct in extreme circumstances.…read more

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Investigating populations and abiotic factors
Abundance ­ the number of individuals of one species in an area. This can be measured by counting
the number of organisms of a species or by using percentage cover (for plants) i.e. the percentage of
area they cover.
Distribution ­ where the species is within a habitat.
Random sampling
1. Choose an area to sample within the entire area being investigated at random.
2.…read more

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Carbon sinks ­ these are places where carbon is removed from the
atmosphere and locked up
Oceans ­ These are carbon sinks and they store a large proportion of the earth's CO2.
Phytoplanktons use the CO2 to photosynthesise and crustaceans use it to make their shells.
This means there is always a higher concentration of CO2 in the atmosphere so more CO2 can
be dissolved into oceans continuously.…read more

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If the rings are wide, this suggests that the year was particularly warm and vice versa for thin
rings. However, temperature isn't the only thing that affects tree ring growth. If the ring is
wide, this could suggest better rainfall, CO2 levels, more sunlight etc. Not ONLY temperature.
Pollen in peat bogs
Peat bogs are made up of organic material and are cool, acidic and anaerobic hence they preserve
pollen as enzymes are not able to decompose them in these harsh conditions.…read more

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Preventing increased carbon emissions
Biofuels ­ these are fuels made from renewable biological resources (biomass). They are
carbon neutral as they only release the carbon the plants took in during photosynthesis
when the fuels are burned. However, CO2 is released when the biofuels are transported
hence they aren't completely carbon neutral.…read more

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Different types of sympatric speciation ­ pre-zygotic
Mechanical isolation ­ this is when the physical reproductive organs of two sets of
organisms is different hence they no longer fit together so reproduction is not possible.
Temporal isolation (time) ­ organisms may be fertile during different seasons. These
seasons may not be synchronised between two groups of organism and hence they are
unable to reproduce with each other.…read more


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