Concise, pretty summary notes of AQA A-Level Biology Topic/Unit 5 - Energy transfers in and between organisms.
Used to get an A* in A-Level AQA Biology.
Concise and well-written notes.
Topic 5: Energy transfers in and between organisms
Photosynthesis
Reaction in which light energy is used to produce glucose in plants.
The process requires water and carbon dioxide, with the products being glucose and oxygen.
There are 2 stages of photosynthesis, these are the light dependent stage and the light independent
stage.
The rate of photosynthesis is determined by carbon dioxide concentration, light intensity as well as
temperature.
Chloroplasts are the site of photosynthesis and are adapted to photosynthesis:
-> Contains stacks of thylakoid membranes called grana which provides a large surface area for
attachment of chlorophyll, electrons and enzymes.
-> Network of proteins in the grand hold the chlorophyll in a very specific manner to absorb the
maximum amount of light.
-> The granal membrane has ATP synthase channels embedded allowing ATP to be synthesised as
well as being selectively permeable allowing the establishment of a proton gradient.
-> Chloroplasts contain DNA and ribosomes allowing them to synthesise proteins needed in the light
dependent reaction.
Light dependent reaction
1. Photons of light hit chlorophyll molecules in PSII causing the electrons to become excited - photoionisation. The charge separation from this drives the
process of photolysis.
2. Photolysis is splitting of water with light. One molecule of water requires 4 photons of light to split. When water is split it produces 1 molecule of
oxygen, 4 protons and 4 electrons. The oxygen either naturally diffuses out through the stomata or is used in respiration. The 4 electrons replace
those lost from the chlorophyll, whilst the protons move into the stroma, later creating a proton gradient.
3. The excited electron then moves down a series of protein complexes. At one of the complexes the energy from the electron is used to pump 4 protons
from the stroma to the thylakoid space.
4. The electron then moves down the chain further to PSI. Here more photons of light are absorbed causing the electron to move back up to a high
energy level.
5. The electron then moves along the chain to another complex where the electron combines with a proton to form a hydrogen atom. This is then used to
reduce NADP, forming reduced NADP.
6. The pumping of protons across the membrane means that there is now a greater concentration of protons in the thylakoid space than the stroma. As a
result a proton gradient forms with a high concentration in the thylakoid space and a low concentration in the stroma. The protons move across the
membrane by diffusion through a protein known as a stalked particle. The movement of these proteins drives the process of photophosphorylation. The
enzyme ATP synthase phosphorylates ATP from ADP and Pi.
Light independent reaction
1. Carbon dioxide fixation - carbon dioxide that has diffused in through the stomata is fixed with ribulose diphosphate (RuBP) in carboxylation. The
enzyme Rubisco is needed in order to do this. A 6 carbon sugar is formed first, however this is very unstable and therefore forms 2 molecules of
glycerate-3-phosphate.
2. Reduction phase - 2 molecules of glycerate-3-phosphate contain a -COOH group and is therefore an acid. The reducing power of reduced NADP
therefore reduces the glycerine-3-phosphate, with energy being provided by ATP. This therefore forms 2 molecule of triose phosphate. All of the
NADP from the light dependent reaction has now been used with only some of the ATP being used.
3. Regeneration of RuBP - 5 molecules of triose phosphate are used in order to regenerate 3 molecules of ribulose biphosphate. The remaining amount
of ATP from the light dependent stage is now used.
4. Organic molecule production - 2 molecules of triose phosphate can combine to form the intermediate hexose sugar fructose 1,6 bisphosphate where
after it forms molecules of glucose.
6 turns of the Calvin cycle are required in order to produce 1 molecule of glucose per molecule of CO2.
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