A student places a green plant on a windowsill in bright sunlight. Explain why the rate of photosynthesis in the plant may increase at first but then stop increasing even if light intensity continues to rise.

OCR GCSE Biology A: Gateway Science (J247) — B1.4 Photosynthesis · Explain · 4 marks · View as Markdown

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

Plants carry out photosynthesis using light energy, carbon dioxide, and water. The rate at which photosynthesis occurs can be affected by several limiting factors.

Model answer (4 marks)

1. Light energy is required for photosynthesis.
2. Increasing light intensity initially increases the rate of photosynthesis.
3. After a point, the rate no longer rises because another factor becomes limiting.
4. For example, the concentration of carbon dioxide may become the limiting factor, so further light does not increase the rate.

Examiner tips

  • Use the word "initially" to show the change, then explain the limiting factor.
  • Mention a specific limiting factor such as CO₂ or temperature.
  • Keep each point brief and use exam‑style wording.
  • Include the reason why the rate stops rising (other factor limits).

Common mistakes

  • Confusing light as the only limiting factor, ignoring CO₂ or temperature.
  • Using vague phrases like "something else" instead of naming a factor.
  • Writing a long paragraph instead of concise points.

Mark scheme (4 marks)

  1. Photosynthesis requires carbon dioxide and water as raw materials (as well as light energy)
  2. Increasing light intensity increases the rate of photosynthesis initially
  3. The rate stops increasing because another factor becomes a limiting factor
  4. Named example of a limiting factor other than light, e.g. carbon dioxide concentration or temperature

Key terms in this question

photosynthesis

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