# Explain how changes in carbon dioxide concentration affect the rate of the Calvin cycle in a C3 plant.

> IB DP Biology Higher Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks

## Mark scheme (4 marks)

1. CO₂ is fixed by RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) combining CO₂ with RuBP to form glycerate-3-phosphate (G3P/3-PGA)
2. Higher CO₂ concentration increases the rate of carboxylation / RuBisCO activity, so more G3P is produced per unit time
3. At very low CO₂, RuBP accumulates / cannot be regenerated into G3P efficiently, so the cycle slows; NADPH and ATP from the light reactions are not consumed and may limit/feedback on light reactions
4. Above a CO₂ saturation point, the rate no longer increases because RuBisCO / another factor (e.g., available RuBP, ATP, NADPH) becomes limiting

## Key terms

- [Calvin cycle](https://www.gradenine.co.uk/glossary/calvin-cycle)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-changes-in-carbon-dioxide-6c97049c) · Published by Druglandscape Ltd.