Explain why the rate of photosynthesis in a plant does not continue to increase indefinitely as light intensity increases, even when carbon dioxide concentration 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 a shade-adapted plant to progressively higher light intensities in a controlled environment chamber. At low light intensities, the rate of photosynthesis rises steeply with each increase in light intensity. However, above a certain light intensity, the rate of photosynthesis levels off and remains constant despite further increases in light intensity.

Model answer (4 marks)

At light saturation the light‑dependent reactions are already producing ATP and NADPH at their maximum rate, so further light cannot increase their output.
The Calvin cycle is now the limiting step; its enzymes (e.g. Rubisco) are saturated and work at Vmax.
Because ATP and NADPH cannot be used any faster, the overall rate of photosynthesis cannot rise further.
Thus, beyond the light‑saturation point, light is no longer limiting and the rate plateaus.

Examiner tips

  • Use the phrase ‘light saturation point’ and ‘light‑dependent reactions’. Mention the Calvin cycle as the limiting step and cite enzyme saturation (Vmax).
  • Show the logical sequence: light → ATP/NADPH → Calvin cycle limitation → plateau.

Common mistakes

  • Confusing light saturation with CO₂ saturation. Failing to mention the Calvin cycle or enzyme saturation. Using vague terms like ‘too much light’ without explaining the biochemical limit.

Mark scheme (4 marks)

  1. Above the light saturation point, light is no longer the limiting factor
  2. The rate of the light-independent reactions (Calvin cycle) limits overall photosynthesis
  3. ATP and NADPH (reduced NADP) produced in the light-dependent reactions cannot be used any faster by the Calvin cycle
  4. Enzyme-controlled reactions in the Calvin cycle are saturated / working at Vmax, so no further increase in rate is possible

Key terms in this question

light intensity

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