Explain how the light-dependent reactions of photosynthesis produce the molecules required for the Calvin cycle.

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)

Light energy is absorbed by chlorophyll in photosystems I and II, exciting electrons to a higher energy level.
Water is split at photosystem II (photolysis), releasing O₂ and providing replacement electrons and protons.
Excited electrons flow through the electron transport chain, creating a proton gradient that drives ATP synthesis by ATP synthase.
At photosystem I, electrons reduce NADP⁺ to NADPH using protons; the resulting ATP and NADPH are then used in the Calvin cycle to fix and reduce CO₂.

Examiner tips

  • Use the word ‘excited’ for electrons, ‘photolysis’ for water splitting, mention O₂ as a by‑product, and state ATP synthase drives ATP formation.
  • Show the sequence: PSII → water → O₂ + electrons → ETC → ATP, then PSI → NADP⁺ → NADPH.
  • Link ATP and NADPH directly to the Calvin cycle.

Mark scheme (4 marks)

  1. Light energy is absorbed by photosystems (I and II) / chlorophyll and used to excite electrons to a higher energy level
  2. Water is split (photolysis) at photosystem II, releasing oxygen as a by-product and supplying replacement electrons / protons (H⁺)
  3. Excited electrons pass along the electron transport chain, driving ATP synthesis via chemiosmosis / ATP synthase
  4. NADP⁺ is reduced to NADPH at photosystem I using electrons and protons, and NADPH / ATP are then available for the Calvin cycle to fix / reduce CO₂

Key terms in this question

light-dependent reactions · Calvin cycle

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