5.1.3

Production of ATP & Reduced NADP

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Production of ATP and Reduced NADP

During the light-dependent reaction, ATP and reduced NADP are produced. In this process electrons are transferred down the electron transfer chain and protons pass across the membranes of chloroplasts.

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Electron transport chain

  • Light energy excites electrons in chlorophyll and the electrons are moved to a higher energy level (they are high-energy electrons).
  • High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
  • Electron carriers are proteins located in the thylakoid membranes.
  • When high-energy electrons are released they are transferred along a chain of electron carriers.
  • The series of electron carriers is called the electron transport chain (ETC).
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The proton gradient

  • As the electrons move down the ETC, they lose energy.
  • This energy pumps protons from the stroma into the thylakoids. The protons are being transported against their concentration gradient and this requires energy.
  • As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane because the concentration of protons inside the thylakoids is greater than in the stroma.
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Chemiosmosis

  • The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme.
  • As protons diffuse through the ATP synthase, energy is released.
  • This energy converts ADP and inorganic phosphate to ATP.
  • This process is called chemiosmosis.
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Reduced NADP

  • When light energy is absorbed, high-energy electrons are released.
  • Some electrons are transferred directly to NADP. They are not passed along the ETC.
  • The electrons react with a proton in the stroma to produce reduced NADP.
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Photolysis

  • Electrons can be replaced by photolysis.
  • In this process, light energy splits water into protons, electrons and oxygen.
  • The electrons can then replace those released when light is absorbed.
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Non-cyclic photophosphorylation

  • Non-cyclic photophosphorylation produces reduced NADP and ATP.
    • Reduced NADP is produced when light is absorbed by PSI.
  • Electrons are not recycled in non-cyclic photophosphorylation.
  • Both types of photophosphorylation produce ATP
  • The ETC is located in the thylakoid membrane.

Jump to other topics

1Biological Molecules

2Cells

3Substance Exchange

4Genetic Information & Variation

5Energy Transfers (A2 only)

6Responding to Change (A2 only)

7Genetics & Ecosystems (A2 only)

8The Control of Gene Expression (A2 only)

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