A-Level Biology October/November 2024 Q4(c): Plants can carry out cyclic photophosphorylation and non-cyclic photophosphorylation du…

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  A structured answer is best for comparison questions. The following outlines the key similarities and differences between the two processes.

  Similarities:

  • In both processes, light energy is absorbed by chlorophyll, leading to the photoactivation of electrons, raising them to a higher energy level.
  • These excited, high-energy electrons are passed along an electron transport chain (ETC) embedded in the thylakoid membranes.
  • The movement of electrons along the ETC releases energy, which is used to pump protons across the thylakoid membrane. This establishes a proton gradient that drives the synthesis of ATP via the process of chemiosmosis.

  Differences:

  • Non-cyclic photophosphorylation involves both Photosystem I (PSI) and Photosystem II (PSII), whereas cyclic photophosphorylation only involves PSI.
  • In non-cyclic photophosphorylation, the photolysis of water occurs to replace electrons lost from PSII. This process also releases oxygen as a by-product. Neither photolysis nor oxygen production occurs in the cyclic pathway.
  • The final products differ. Non-cyclic photophosphorylation produces ATP, reduced NADP, and oxygen. Cyclic photophosphorylation only produces ATP; reduced NADP is not formed.
  • In the cyclic pathway, electrons excited from PSI ultimately return to the same photosystem. In the non-cyclic pathway, electrons from PSII are passed to PSI and are then accepted by NADP+, so they do not return to their starting point.

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  How the marks are awarded

  • B1 — Stating that photoactivation of chlorophyll occurs in both processes.
  • B1 — Identifying that energetic electrons move along an electron transport chain (ETC) in both pathways.
  • B1 — Stating that ATP is produced in both cyclic and non-cyclic photophosphorylation.
  • B1 — Correctly identifying that non-cyclic involves both PSI and PSII, while cyclic involves only PSI.
  • B1 — Stating that photolysis and the production of oxygen occur only in the non-cyclic pathway.
  • B1 — Stating that reduced NADP is produced in the non-cyclic pathway only.

  Common mistakes

  • Confusing which photosystems are involved in each pathway (e.g., saying cyclic uses PSII).
  • Incorrectly stating that cyclic photophosphorylation produces reduced NADP or oxygen.
  • Only describing the differences and failing to mention any similarities, thereby limiting the maximum marks achievable.
  • Using vague terms like 'energy is made' instead of specifying that 'ATP is produced'.

  Examiner tip: For 'compare and contrast' questions, structure your answer with explicit 'Similarities' and 'Differences' sections to ensure you address both parts of the question and maximise your marks.

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