# Explain how the concentration of carbon dioxide affects the rate of photosynthesis in a C3 plant.

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

> C3 plants fix carbon dioxide via the Calvin cycle using the enzyme RuBisCO. At current atmospheric CO₂ concentrations, RuBisCO operates below its maximum reaction rate.

## Mark scheme (4 marks)

1. CO₂ is fixed by RuBisCO / combined with RuBP to form glycerate-3-phosphate (G3P) in the Calvin cycle
2. Increasing CO₂ concentration increases the rate of carbon fixation / increases the rate of the Calvin cycle
3. At low/limiting CO₂ concentrations, CO₂ availability is the limiting factor, so increasing CO₂ increases the rate of photosynthesis
4. Above a saturating CO₂ concentration, another factor (e.g. light intensity or RuBisCO enzyme concentration) becomes limiting, so the rate plateaus / no further increase occurs

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-concentration-of-carbon-a8a2f9f7) · Published by Druglandscape Ltd.