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Photosynthesis

Non-cyclic and cyclic photophosphorylation

  • 3 things to know
  • Component 1

Overview

Route taken by an electron in non-cyclic photophosphorylation, what becomes of each of the three products of photolysis and how reduced NADP is formed.

What you need to know

Answer each one before you reveal it.

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Key term

State what becomes of each of the three products of photolysis.

Key term

What is photolysis?

Key term

Write the equation for the photolysis of water.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Which photosystems take part in non-cyclic photophosphorylation?

    Model answer

    Both photosystem I and photosystem II.

  2. 2

    Describe the route taken by an electron in non-cyclic photophosphorylation.

    Model answer

    It leaves photosystem II, is captured by an acceptor and passes down a chain of carriers to photosystem I, where light re-excites it before it is passed to a second acceptor and used to reduce NADP.

  3. 3

    What is photolysis?

    Model answer

    The splitting of a water molecule by light energy, catalysed by an enzyme associated with photosystem II.

  4. 4

    Write the equation for the photolysis of water.

    Model answer

    2H₂O gives 4H⁺ + 4e⁻ + O₂.

  5. 5

    State what becomes of each of the three products of photolysis.

    Model answer

    The electrons replace those lost from photosystem II, the protons contribute to the gradient across the thylakoid membrane and are used to reduce NADP, and the oxygen diffuses out as a waste product.

  6. 6

    Why is water described as the ultimate source of the electrons in non-cyclic photophosphorylation?

    Model answer

    The electrons that end up in reduced NADP are never returned to a photosystem, so they must be continually replaced, and photolysis of water is what supplies them.

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