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Production of ATP & Reduced NADP

Biology: AQA A Level, topic 5.1.3

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.

Electron transport chain

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).
The proton gradient

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.
Chemiosmosis

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.
Reduced NADP

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.
Photolysis

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.
Non-cyclic photophosphorylation

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.

Test yourself on Production of ATP & Reduced NADP

  • Stages in production of ATP via the ETC

  • Which of the following statements are true of non-cyclic photophosphorylation?

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