Hey guys,

These are some notes on each stage of photosynthesis.

Hope they help!

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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Light-Independent stage

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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Freddy Fazbear


Very detailed

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