11 Light dependant reaction

  • Created by: lee8444
  • Created on: 18-02-20 17:00

The making of ATP

  • Chlorophyll absorbs light energy
  • Boosts the energy of the electrons exciting them to a higher level
  • Electrons leave the chlorophyll
  • Chlorophyll molecule becomes ionised (photoionisation)
  • The electrons are taken up by an electron carrier
  • The chlorophyll has been oxidised as it has lost electrons
  • The electron carrier has been reduced as it has gained electrons
  • The electrons are passed along a chain of electron carriers in a series of redox reactions in the thylakoid membranes with each carrier at a slightly lower energy level than the one before
  • Some of the energy released is used to add an inorganic phosphate to an ADP to make ATP. This happens due to the chemiosmotic theory
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The chemiosmotic theory

  • Each thylakoid pumps protons/H+ ions from the stroma using carrier proteins called proton pumps into the thylakoid space
  • The energy to do this comes from the electrons released during photolysis
  • The protons produced from photolysis also increase the proton concentration inside the thylakoid space
  • This maintains the proton concentration gradient being higher inside the thylakoid space
  • These protons can only pass into the stroma through ATP synthase channels as the rest of the membrane is impermeable to protons
  • This changes the shape of the enzyme ATP synthase which catalyses the reaction creating ATP from ADP and an inorganic phosphate
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Photolysis of water

  • The electrons lost during photoionisation are replaced by electrons released from photolysis
  • The protons released are passed out of the thylakoid space through the ATP synthase channels and are taken up by an electron carrier called NADP
  • This reduces the NADP
  • The reduced NADP is the main product from the light-dependent stage of photosynthesis
  • This enters the light-independent stage of photosynthesis along with the electrons from the chlorophyll
  • Reduced NADP is a further potential source of energy
  • Oxygen produced from photolysis is either used for respiration or diffuses out through the stomata
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