Explain why the rate of photosynthesis in a C3 plant eventually reaches a plateau when light intensity is increased under conditions of constant, non-limiting carbon dioxide concentration and temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Increasing light intensity raises the rate of the light‑dependent reactions, producing more ATP and NADPH.
2. The Calvin cycle uses these ATP and NADPH to fix and reduce CO₂.
3. At high light, the Calvin cycle enzymes (e.g. RuBisCO) become saturated or CO₂ supply is limited, so further light cannot increase carbon fixation.
4. Consequently the overall photosynthetic rate plateaus, giving a flat section on the light‑response curve.
2. The Calvin cycle uses these ATP and NADPH to fix and reduce CO₂.
3. At high light, the Calvin cycle enzymes (e.g. RuBisCO) become saturated or CO₂ supply is limited, so further light cannot increase carbon fixation.
4. Consequently the overall photosynthetic rate plateaus, giving a flat section on the light‑response curve.
Examiner tips
- Use the word ‘saturated’ or ‘limited’ to show understanding of enzyme activity; link light‑dependent and independent reactions; keep answer concise; include key terms like ATP, NADPH, RuBisCO.
Common mistakes
- Confusing light intensity with CO₂ concentration; not mentioning the Calvin cycle; using vague terms like ‘more energy’ without specifying ATP/NADPH.
Mark scheme (4 marks)
- As light intensity increases, the light-dependent reactions produce more ATP and NADPH (reduced NADP), so the rate of the light-dependent reactions increases.
- The Calvin cycle (light-independent reactions) requires the ATP and NADPH produced by the light-dependent reactions to fix and reduce carbon dioxide.
- At high light intensities, the Calvin cycle enzymes (such as RuBisCO) become saturated / the supply of CO2 or enzyme activity becomes the limiting factor, so the rate of carbon fixation cannot increase further.
- Therefore, even though more light is available, the overall rate of photosynthesis cannot increase beyond this maximum, producing a plateau on a graph of rate versus light intensity.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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