Light-Dependent Reaction

Break down of light-dependent reaction, cyclic and non-cyclic photophosphorylation and relevant definitions.

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NCP Step One:

  • Light energy is absorbed by electrons in PSII.
  • Light energy excites electrons in chlorophyll.
  • The electrons move to a higher energy level, i.e they have more energy.
  • The high energy electrons movealong the electron transport chain to PSI.
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NCP Step Two:

  • The excited electrons from PSII must be replaced.
  • Light energy splits a water molecule (photolysis) into protons (H+ ions), electrons and O2. This is where the O2 in photosynthesis comes from.
  • The reaction is H2O -------> 2H+ + 1/2O2
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NCP Step Three:

  • The excited electrons lose energy as they move along the electron transport chain. 
  • This energy is used to transport protons INTO the thylakoid membrane so that the thylakoid has a higher concentration than the stroma. 
  • This forms a proton concentration gradient (pH gradient) across the membrane. 
  • Protons move down their concentration gradient through an enzyme known as ATP synthase. (CHEMIOSMOSIS)
  • The proton motive force causes the ATP synthase to rotate, and the active site to become accessable.
  • ADP and inorganic phosphate (P) are then combined to make ATP.
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NCP Step Four:

  • Light energy is absorbed by PSI, which excites the electrons to a higher energy level.
  • The electrons are transferred to NADP along with a proton from the stroma (moved during chemiosmosis) to form reduced NADP (NADPH)
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Cyclic Photophosphorylation:

  • Only uses PSI.
  • Electrons from chlorophyll do not reduce NADP but return to the photosystem via electron carriers.
  • Electrons are recycled and flow repeatedly through PSI.
  • Does not produce NADPH or O2.
  • Produces ATP.
  • Used in guard cells to open/close stomata.

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  • The making of ATP from ADP and Pi, in the presence of light.
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Electron Carriers:

  • Molecules that transfer electrons.
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Electron Acceptors:

  • Chemicals that accept electrons from another compound. They are reduced while acting as an oxidising agent.
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