Light-Dependant stage (Thylakoids)
- Photosystem I is at a higher energy level then photosystem II.
- Light is absorbed by the chlorophyll in the two photosystems simultaneously.
- Photosystem I releases a pair of exited high energy electrons.
- These electrons are taken up by an electron acceptor.
- And then are transferred to the acceptor NADP.
- Because of the loss of electrons PSI (photosystem I) becomes unstable.
- When the light hits PSII a pair of electrons is released and are passed to an acceptor.
- They are then transferred through a series of carriers (down an energy gradient) to PSI therefore restoring stability.
- Whilst being transferred down, energy is being released, which is then combined with inorganic phosphate to produce ATP (2 molecules of it every time the cycle happens).
- However this loss of electrons makes PSII unstable.
- So the electrons lost are replaced by electrons released in the photolysis of water.
- Photolysis is the dissociation of water in light into oxygen, protons (H+ ions) and electrons.
- Stability is restored to PSII.
- The H+ ions are transferred to NADP to produce reduced NADP.
- The waste product is oxygen.
- Cyclic photophosphorylation occurs simultaneously to produce more ATP.
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Also known as the Calvin Cycle.
Occurs in the stroma.
- One molecule of CO2 (1C) is combined with Ribulose Bisphosphate (RuBP)(5C). This stage is called carbon fixation.
- An enzyme called RuBP Carboxylase catalyses this reaction.
- The unstable 6C compound immediately breaks down into two 3C molecules called Glycerate-3-Phosphate (GP).
- GP is reduced to form a 3C sugar Triose Phosphate (TP).
- Reduced NADP from the light-dependant stage provides hydrogen, ATP is hydrolysed to provide energy.
- 1/6th of the TP is built up into carbs like glucose and starch, these reactions require ATP.
- So, 5/6ths of the TP is converted to Ribulose Phosphate and is then finally phosphorylated into RuBP using phosphate from ATP.
The cycle has to turn 6 times to produce one molecule of 6C glucose.
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