A student investigates the effect of light intensity on the rate of photosynthesis in a pondweed. As light intensity increases, the rate of photosynthesis increases, but above a certain light intensity the rate no longer increases. Explain why the rate of photosynthesis increases as light intensity increases, and explain why the rate stops increasing at high light intensities.

Pearson Edexcel A-Level Biology A (9BI0) — 3.1 Photosynthesis · Explain · 5 marks · View as Markdown

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

Photosynthesis is the process by which plants use light energy to produce glucose from carbon dioxide and water. The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen.

Model answer (5 marks)

Light supplies the energy that drives the light‑dependent reactions of photosynthesis. As the intensity of light increases, more photons reach the chloroplasts, so more energy is available for the photochemical reactions and the rate of photosynthesis rises.

At very high light intensities the rate no longer rises because another factor becomes limiting. Usually the concentration of carbon dioxide (or sometimes temperature) limits the Calvin cycle. Even if more light is supplied, the enzymes cannot fix carbon dioxide fast enough, so the overall rate of photosynthesis plateaus.

Examiner tips

  • Use the word ‘light provides the energy’ to show understanding of the role of light.
  • Explain that more light = more photons = higher rate, then state the limiting factor (CO₂ or temperature).
  • Mention that the rate stops increasing because the limiting factor restricts the process.
  • Keep the answer concise – 5 marks can be achieved with 3–4 short sentences.

Common mistakes

  • Saying ‘light intensity is too high for the plant’ – the problem is a different limiting factor, not the light itself.
  • Using vague terms like ‘too much light’ without explaining the limiting factor.
  • Failing to mention that the limiting factor could be CO₂ or temperature – examiners expect one of these options.

Mark scheme (5 marks)

  1. Light provides the energy needed for photosynthesis to occur
  2. As light intensity increases, more light energy is available, so the rate of photosynthesis increases
  3. At high light intensities another factor becomes limiting
  4. The limiting factor is carbon dioxide concentration OR temperature
  5. Even if light intensity is increased further, the rate cannot increase because the limiting factor restricts the process

Key terms in this question

photosynthesis · light intensity

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