Explain why increasing the concentration of carbon dioxide in a greenhouse can increase the rate of photosynthesis, and describe what happens to the rate of photosynthesis if light intensity then becomes a limiting factor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers sometimes pump extra carbon dioxide into their greenhouses to improve crop yields.
Model answer (4 marks)
Increasing the concentration of CO₂ increases the rate of photosynthesis because CO₂ is a reactant in the light‑independent reactions and, when it is low, it limits the overall rate. By pumping more CO₂ into the greenhouse the plants can fix carbon more quickly, so the rate rises.
If light intensity then becomes the limiting factor, the rate of photosynthesis will no longer increase when CO₂ is added; it will remain constant or increase only very slightly because the light‑dependent reactions are now the bottleneck.
If light intensity then becomes the limiting factor, the rate of photosynthesis will no longer increase when CO₂ is added; it will remain constant or increase only very slightly because the light‑dependent reactions are now the bottleneck.
Examiner tips
- Use the term ‘reactant’ and ‘limiting factor’ exactly as in the mark scheme
- Show that CO₂ is limiting first, then explain the effect of light as the new limiting factor
- Keep the answer concise – 4 marks only
Common mistakes
- Confusing CO₂ with light as the limiting factor
- Using vague terms like ‘more light’ or ‘more CO₂’ without linking to the limiting factor concept
- Writing a long paragraph instead of a short, point‑by‑point answer
Mark scheme (4 marks)
- Carbon dioxide is a reactant in photosynthesis
- Increasing CO₂ concentration increases the rate of photosynthesis (because CO₂ was previously the limiting factor)
- A limiting factor is a variable that limits the rate of a particular reaction
- If light intensity is a limiting factor, the rate of photosynthesis will not increase further / will remain constant even if CO₂ is increased
Key terms in this question
photosynthesis · limiting factor · carbon dioxide · light intensity
Related
- Revision notes: Photosynthesis →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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