Explain how non-cyclic photophosphorylation results in the production of ATP and NADPH in the light-dependent reactions of photosynthesis.

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)

1. Light energy is absorbed by chlorophyll in PSII, exciting electrons to a high‑energy state.
2. The excited electrons are replaced by electrons from water; H₂O is split, releasing O₂ and protons into the thylakoid lumen.
3. The electrons travel through the electron transport chain (plastoquinone, cytochrome b₆f, plastocyanin), pumping protons across the membrane and creating a proton gradient that drives ATP synthase to produce ATP.
4. The electrons reach PSI, are re‑excited by light, and are transferred via ferredoxin to NADP⁺, together with protons, forming NADPH through ferredoxin/NADP⁺ reductase.

Examiner tips

  • Use the 4 key stages: PSII excitation, water splitting, electron transport & chemiosmosis, PSI re‑excitation and NADPH formation.
  • Show the flow of electrons and the role of the proton gradient explicitly.
  • Mention the by‑product O₂ and the enzyme ferredoxin/NADP⁺ reductase.
  • Keep the answer concise – one sentence per point is enough for full marks.

Common mistakes

  • Confusing cyclic with non‑cyclic photophosphorylation – forgetting that water is split only in the non‑cyclic pathway.
  • Omitting the proton gradient/ATP synthase step or mis‑labeling the electron carriers (e.g., calling plastocyanin a plastoquinone).

Mark scheme (4 marks)

  1. Light energy is absorbed by photosystem II (PSII), causing photoactivation/excitation of chlorophyll and the release of high-energy electrons.
  2. Water is photolysed, releasing electrons to replace those lost from PSII, with oxygen released as a by-product and protons (H⁺) produced in the thylakoid lumen.
  3. Excited electrons pass along the electron transport chain (plastoquinone, cytochrome b6f complex, plastocyanin), releasing energy that is used to pump protons across the thylakoid membrane, creating a proton gradient that drives ATP synthase (chemiosmosis) to produce ATP.
  4. Electrons are re-energised at photosystem I (PSI) using light energy and ultimately reduce NADP⁺ (with protons) to form NADPH via the enzyme ferredoxin/NADP⁺ reductase.

Key terms in this question

non-cyclic photophosphorylation · NADPH

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