Explain why the rate of photosynthesis in a plant cell does not continue to increase indefinitely as the concentration of carbon dioxide in the surrounding air is raised, even when light intensity and temperature are kept constant.

OCR A-Level Biology A (H420) — 5.6 Photosynthesis · Explain · 4 marks · View as Markdown

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

A researcher exposes isolated mesophyll cells to increasing concentrations of carbon dioxide whilst maintaining a fixed light intensity and a constant temperature of 25 °C. Above a certain carbon dioxide concentration, the rate of oxygen evolution stops increasing and remains constant.

Model answer (4 marks)

1. The rate of photosynthesis is limited by a factor other than CO₂.
2. Because light intensity is fixed, the light‑dependent reactions produce a fixed amount of ATP and NADPH.
3. The Calvin cycle can only use the ATP and NADPH that are available, so its rate is capped.
4. Therefore, once CO₂ exceeds the amount that can be fixed by the available ATP/NADPH, the overall rate of photosynthesis cannot increase further.

Examiner tips

  • Use the word ‘limit’ to show understanding of a limiting factor.
  • Show the link between fixed light intensity and a fixed supply of ATP/NADPH.
  • Explain that the Calvin cycle is the step that becomes rate‑determining.
  • Conclude that photosynthesis plateaus once CO₂ is no longer the limiting factor.

Common mistakes

  • Claiming that the light‑dependent reactions are unlimited, ignoring the fixed light intensity.
  • Saying the rate stops because of CO₂ toxicity, not because of ATP/NADPH limitation.
  • Using vague terms like ‘energy’ instead of ATP and NADPH.

Mark scheme (4 marks)

  1. Identifies that a factor other than carbon dioxide is limiting the rate
  2. Links fixed light intensity to a fixed rate of the light-dependent reactions
  3. Explains that the Calvin cycle is limited by the supply of ATP and reduced NADP from the light-dependent reactions
  4. Concludes that carbon dioxide fixation / overall rate of photosynthesis therefore cannot increase beyond this maximum

Key terms in this question

rate of photosynthesis · light intensity

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