A scientist investigates how the concentration of carbon dioxide in the air affects the rate of photosynthesis in a water plant. At very low carbon dioxide concentrations, the rate of photosynthesis remains low even when light intensity is increased. Explain why carbon dioxide concentration acts as a limiting factor in this situation, and describe what happens inside the chloroplasts during photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A water plant is kept in a sealed container with a very low concentration of carbon dioxide. Despite being placed under a bright lamp, the plant produces very little oxygen.
Model answer (4 marks)
Carbon dioxide is a reactant in the light‑dependent and light‑independent reactions of photosynthesis. When its concentration is very low, the amount of CO₂ that can be fixed into sugars is limited, so the overall rate of photosynthesis cannot increase even if light intensity is raised. In the chloroplasts, light energy absorbed by chlorophyll in the thylakoid membranes drives the conversion of CO₂ and H₂O into glucose and O₂. The CO₂ is fixed in the Calvin cycle, producing glyceraldehyde‑3‑phosphate that is then used to build glucose, while O₂ is released as a by‑product.
Examiner tips
- Mention CO₂ as a reactant and limiting factor; link low CO₂ to limited rate; state photosynthesis occurs in chloroplasts with chlorophyll; describe CO₂ + H₂O → glucose + O₂ using light energy.
Common mistakes
- Forgetting that CO₂ is a reactant; confusing light intensity with CO₂ concentration; omitting the role of chlorophyll or the chloroplasts; not naming the products (glucose and O₂).
Mark scheme (4 marks)
- Carbon dioxide is a reactant needed for photosynthesis to occur
- When carbon dioxide is in short supply it limits the rate of photosynthesis regardless of other factors
- Photosynthesis takes place in the chloroplasts, which contain chlorophyll
- Carbon dioxide and water are converted into glucose and oxygen using light energy
Key terms in this question
limiting factor · photosynthesis · chloroplasts
Related
- Revision notes: Photosynthesis →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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