Explain why the rate of photosynthesis in a C3 plant does not continue to increase indefinitely as carbon dioxide concentration rises, even when light intensity and temperature are kept constant at optimal levels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Researchers studying crop yield noticed that supplying extra carbon dioxide to a greenhouse increased photosynthesis rates up to a point, after which no further increase was observed despite continued carbon dioxide enrichment.
Model answer (4 marks)
RuBisCO, the enzyme that fixes CO₂ in the Calvin cycle, becomes saturated when the CO₂ concentration is high – all of its active sites are occupied.
The regeneration of RuBP, the CO₂ acceptor, is limited to a fixed maximum rate; when RuBisCO is saturated, more CO₂ cannot be used because RuBP is the limiting substrate.
The light‑dependent reactions supply ATP and NADPH to the Calvin cycle at a fixed maximum rate; when RuBisCO is saturated, the cycle cannot use the extra CO₂ because the energy carriers are limiting.
Consequently, further increases in CO₂ do not raise the rate of carbon fixation or overall photosynthesis, so the rate plateaus.
The regeneration of RuBP, the CO₂ acceptor, is limited to a fixed maximum rate; when RuBisCO is saturated, more CO₂ cannot be used because RuBP is the limiting substrate.
The light‑dependent reactions supply ATP and NADPH to the Calvin cycle at a fixed maximum rate; when RuBisCO is saturated, the cycle cannot use the extra CO₂ because the energy carriers are limiting.
Consequently, further increases in CO₂ do not raise the rate of carbon fixation or overall photosynthesis, so the rate plateaus.
Examiner tips
- State that RuBisCO saturation limits CO₂ fixation
- Explain that RuBP regeneration is fixed
- Mention ATP/NADPH supply limits the cycle
- Show the logical chain leading to a plateau
Common mistakes
- Confusing light‑dependent reactions with the Calvin cycle
- Saying CO₂ is always limiting instead of RuBisCO saturation
- Omitting the role of RuBP regeneration
Mark scheme (4 marks)
- RuBisCO becomes saturated with carbon dioxide / all active sites of RuBisCO are occupied
- The supply of RuBP becomes limiting / RuBP is regenerated at a fixed maximum rate
- ATP and reduced NADP from the light-dependent reactions are being supplied at a fixed maximum rate, limiting the Calvin cycle
- Therefore a further increase in CO2 cannot increase the rate of carbon fixation / rate of the Calvin cycle, so overall photosynthesis rate plateaus
Key terms in this question
carbon dioxide concentration · rate of photosynthesis · light intensity · C3 plant
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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