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Photosynthesis The Light Reaction. Photosynthesis takes place in the chloroplasts in areas known as photosystems Photosystems - complexes containing the pigments where light energy is harvested Chlorophyll is the main pigment in the photosystems of eukaryotic plants

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photosynthesis the light reaction
PhotosynthesisThe Light Reaction
  • Photosynthesis takes place in the chloroplasts in areas known as photosystems
    • Photosystems - complexes containing the pigments where light energy is harvested
      • Chlorophyll is the main pigment in the photosystems of eukaryotic plants
    • The light reaction takes place in the thylakoid membranes
      • The thylakoid membranes are populated by two types of photosystems (PS I & PS II). The two work together to produce ATP & NADPH (8 steps in the non-cyclic flow of electrons)
photosystem ii
Photosystem II
  • When a photon is absorbed by a pigment it causes an e- to move from its ground state to an excited state.
    • The electron (because it is unstable in its new configuration) immediately returns to its ground state releasing the stored energy.
    • photon energy is transferred from pigment to pigment (light harvesting complex) until it reaches the reaction center
    • The energy is then harvested in a reaction center where it is transferred to an electron acceptor.
  • Water is split
    • 2 H+ are produced
      • Moved into the inner thylakoid space where they provide the energy to produce ATP
    • 2 e- are given back to the P680
    • O2 is produced from the split water
  • The excited e- is passed through an electron transport chain (ETC) to PS I
    • The exergonic "fall" of electrons provides energy for the synthesis of ATP along with the proton (H+) motive force created by the movement of H+ across the membrane
photosystem i
Photosystem I
  • PS I absorbs a photon and donates an e-
    • 2 electrons from PS II fill in the space (electron hole)
    • Electrons captured by the reaction center are transferred down a 2nd ETC
    • Enzyme NADP+ reductase transfers the 2 e- to NADP+ to NADPH (requires 2 e-)
      • NADP+ is an electron carrier
      • NADPH is then transferred to the Calvin Cycle where it will be used to produce glucose
summary
Summary
  • Step 1
    • Light absorbed by PS II splits water
      • 2 e- to ETC to PS I
      • H+ moved to create gradient to form ATP
      • O2 released
  • Step 2
    • Light absorbed by PS I reenergizes e- from PS II
      • Energized e- passed to ETC & is used to produce NADPH
        • 2e- + NAD+ g NADPH
        • NADPH transferred to stroma to the Calvin Cycle
  • H+ from inner thylakoid membrane passed through ATP Synthase to make ATP
    • ADP + Pi g ATP