Explain why photosynthesis in a C3 plant eventually stops when a plant is kept in complete darkness for an extended period.

IB DP Biology Standard 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 darkness the light‑dependent reactions cannot take place because no light energy is absorbed by photosystems.
As a result ATP and NADPH are not produced and the existing stores are gradually depleted.
Without ATP and NADPH the Calvin cycle cannot operate – the G3P produced in the reduction phase cannot be regenerated.
Consequently RuBP is not regenerated, CO₂ fixation stops and the cycle cannot be sustained, so photosynthesis ceases.

Examiner tips

  • State that light‑dependent reactions stop in darkness first. Explain the chain: no ATP/NADPH → no G3P → no RuBP regeneration → cycle stops. Use the exact terms: light‑dependent reactions, ATP, NADPH, Calvin cycle, G3P, RuBP.

Common mistakes

  • Confusing dark reactions with light reactions. Failing to mention ATP and NADPH depletion. Using vague phrases like "energy stops" instead of specific processes.

Mark scheme (4 marks)

  1. In darkness, the light-dependent reactions cannot occur / no light energy is absorbed by photosystems
  2. ATP and NADPH are not produced / are depleted
  3. Without ATP and NADPH, the Calvin cycle cannot proceed / GP cannot be reduced to triose phosphate (G3P)
  4. RuBP is not regenerated, so CO2 fixation cannot continue / the cycle cannot be sustained

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