Photophosphorylation.

An overview of Non-cyclic and cyclic photophosphorylation. ATP, reduced NADP and O2 are produced.

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LIGHT ENERGY EXCITES THE ELECTRONS IN CHLOROPHYLL

  • Light energy is absorbed by Photosystem II
  • The light excites the electrons in chlorophyll, and these electrons move to a higher energy level.
  • The high energy electrons move along the electron transport chain to Photosystem I
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PHOTOLYSIS OF WATER PRODUCES PROTONS (H+ ions), EL

  • As the excited electrons from the chlorophyll leave photosystem II to move along the electron transport chain, they have to be replaced.
  • Water is split into oxygen, protons and electrons by light (this is called photolysis).

Overall reaction:

H2O----->2H+ and 1/2 O2

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ENERGY FROM THE EXCITED ELECTRONS MAKES ATP

  • The excited electrons lose energy as they move along the electron transport chain.
  • Protons are transported into the thykaloid membranes by using the energy from the excited electrons. This means that the thykaloid membrane has a higher concentration of protons than the stroma, and a proton concentration gradient is formed.
  • Protons move down the concentration gradient, and into the stroma; via ATP synthase (an enzyme)
  • ATP is formed from this energy from the combination of ADP and an inorganic phosphate. 
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REDUCED NADP IS GENERATED

  • Light energy is absorbed by photosystem I
  • This excites the electrons to a higher energy level
  • The electrons are transferred to NADP along with a proton (from the stroma)
  • This forms reduced NADP
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CYCLIC PHOTOPHOSPHORYLATION

  • Only uses photosystem I
  • Electrons from the chorophyll molecule aren't passed onto NADP but are passed back to photosystem I via electron carriers (hence cyclic)
  • The electrons are recycled
  • No NADP or O2 are produced; only small amounts of ATP
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