Light dependent stage: cyclic and non-cyclic photophosphorylation

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9 Terms

1
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Where does the light dependent stage of photosynthesis occur?

In the thylakoid membrane.

2
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What are the 2 paths that electrons can follow?

  • Non-cyclic photophosphorylation

  • Cyclic photophosphorylation

3
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What is photophosphorylation?

The addition of a phosphate ion to ADP, using light energy.

4
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Summarise non-cyclic photophosphorylation.

The electrons from PSII pass to PSI and are then passed to the final electron acceptor NADP; as they pass along the electron transport chain, ATP is synthesised.

5
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Summarise cyclic photophosphorylation.

Excited electrons from PSI pass from the electron acceptor, through electron carriers and then pack to PSI. This produces ATP.

6
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Describe the stages in cyclic photophosphorylation.

  1. Light energy is absorbed by photosystem I, which excites electrons in the chlorophyll a molecules in the reaction centre.

  1. The electrons are emitted and picked up by an electron acceptor, which passes them down a chain of electron carriers back to PSI.

  1. The energy released as electrons pass through the electron transport chain phosphorylates ADP to ATP.

7
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Describe the stages in non-cyclic photophosphorylation.

  • Light energy is absorbed by chlorophyll a molecules in the reaction centres of PSI and PSII.

  • Electrons become excited and are released from the chlorophyll a molecules.

  • The electrons released from PSII are accepted by an electron acceptor and passed along carriers in the membrane.

    As the electrons pass along carriers, they lose energy; this energy is used to pump H+ ions, allowing ATP to be produced.

  • The electrons released from PSII are finally accepted by the chlorophyll a molecule in PSI.

  • The electrons released from PSI are accepted by an electron acceptor and passed to NADP which is reduced.

  • The end products formed are oxygen, ATP and reduced NADP.

8
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What is photolysis?

The splitting of water using light energy, producing H+ ions, electrons and oxygen.

Photolysis is important in non-cyclic photophosphorylation as the electrons released from the splitting of water replace the electrons lost from PSII.

9
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What happens to the ATP and reduced NADP produced in the light dependent stage?

The ATP and NADPH2 are used in the next stage of photosynthesis – the light independent stage (the Calvin Cycle).

Some oxygen is used in respiration but during the day the rest diffuses out of the leaf through the stomata.