Photosynthesis

• 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.

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.