Explain how cyclic photophosphorylation differs from non-cyclic photophosphorylation in terms of the electron pathway and the products formed.

IB DP Biology Higher Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

In non‑cyclic photophosphorylation electrons are taken from water by PSII, pass through the electron transport chain to PSI and finally reduce NADP⁺ to NADPH. In cyclic photophosphorylation electrons are taken from PSI, returned to the electron transport chain and do not reach PSI again. Non‑cyclic photophosphorylation therefore produces NADPH and releases O₂ from water. Cyclic photophosphorylation produces no NADPH and no O₂. Both pathways generate a proton gradient that drives ATP synthesis, but cyclic photophosphorylation yields only ATP, providing extra ATP when the cell needs it without extra NADPH.

Examiner tips

  • Use the word ‘source’ for water in non‑cyclic, ‘cycle back’ for PSI in cyclic.
  • Mention both NADPH and O₂ as products of non‑cyclic, and state their absence in cyclic.
  • Highlight that ATP is produced in both, but only cyclic gives extra ATP.
  • Keep the answer brief and use the exact terminology from the mark scheme.

Common mistakes

  • Confusing the source of electrons (water vs PSI)
  • Saying cyclic produces NADPH or O₂
  • Not noting that both pathways produce ATP

Mark scheme (4 marks)

  1. In non-cyclic photophosphorylation, electrons are sourced from the photolysis of water and passed from photosystem II through an electron transport chain to photosystem I, then on to NADP⁺; in cyclic photophosphorylation, electrons from photosystem I are cycled back to the electron transport chain without involving photosystem II or water.
  2. Non-cyclic photophosphorylation produces NADPH, whereas cyclic photophosphorylation does not produce NADPH.
  3. Non-cyclic photophosphorylation produces oxygen as a by-product of photolysis, whereas cyclic photophosphorylation does not produce oxygen.
  4. Both pathways produce ATP via the flow of protons through ATP synthase / chemiosmosis, but cyclic photophosphorylation produces only ATP, making it useful when the cell requires additional ATP without additional NADPH.

Key terms in this question

cyclic photophosphorylation · non-cyclic photophosphorylation

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