A scientist is studying wheat plants growing in two different fields. In Field A, the wheat plants are grown in conditions where carbon dioxide levels are kept artificially high. In Field B, the wheat plants are grown under normal atmospheric carbon dioxide levels. Both fields receive the same amount of light and water. Explain why the wheat plants in Field A are likely to produce a greater biomass than those in Field B.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Photosynthesis is an endothermic reaction that takes place in the chloroplasts, converting carbon dioxide and water into glucose and oxygen using light energy. The glucose produced can be used by the plant to build new biological molecules, contributing to biomass.
Model answer (5 marks)
1. Photosynthesis requires carbon dioxide as a reactant.
2. In Field A the higher CO₂ concentration increases the rate of photosynthesis.
3. A higher photosynthetic rate produces more glucose.
4. The extra glucose is used to synthesise new biological molecules, supporting growth.
5. Consequently, wheat plants in Field A will develop greater biomass than those in Field B.
2. In Field A the higher CO₂ concentration increases the rate of photosynthesis.
3. A higher photosynthetic rate produces more glucose.
4. The extra glucose is used to synthesise new biological molecules, supporting growth.
5. Consequently, wheat plants in Field A will develop greater biomass than those in Field B.
Examiner tips
- Use the exact phrase ‘rate of photosynthesis’ to show understanding of the mechanism.
- Show the causal chain: CO₂ → rate ↑ → glucose ↑ → biomass ↑.
- Keep the answer concise – one sentence per point.
- Use UK spelling (e.g., ‘biomass’).
Common mistakes
- Confusing light intensity with CO₂ concentration as the limiting factor.
- Failing to mention that glucose is used for growth rather than just produced.
- Using vague terms like ‘more energy’ instead of ‘more glucose’.
Mark scheme (5 marks)
- Photosynthesis uses carbon dioxide (as a reactant)
- Higher carbon dioxide concentration increases the rate of photosynthesis
- More glucose is produced (as a result of faster photosynthesis)
- Glucose is used to build new biological molecules / used for growth
- Therefore greater biomass is produced in Field A
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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