# Explain why the rate of photosynthesis in a plant cell does not continue to increase indefinitely as the concentration of carbon dioxide in the surrounding air is raised, even when light intensity and temperature are kept constant.

> OCR A-Level Biology A (H420) — 5.6 Photosynthesis · Explain · 4 marks

> A researcher exposes isolated mesophyll cells to increasing concentrations of carbon dioxide whilst maintaining a fixed light intensity and a constant temperature of 25 °C. Above a certain carbon dioxide concentration, the rate of oxygen evolution stops increasing and remains constant.

## Mark scheme (4 marks)

1. Identifies that a factor other than carbon dioxide is limiting the rate
2. Links fixed light intensity to a fixed rate of the light-dependent reactions
3. Explains that the Calvin cycle is limited by the supply of ATP and reduced NADP from the light-dependent reactions
4. Concludes that carbon dioxide fixation / overall rate of photosynthesis therefore cannot increase beyond this maximum

## Key terms

- [rate of photosynthesis](https://www.gradenine.co.uk/glossary/rate-of-photosynthesis)
- [light intensity](https://www.gradenine.co.uk/glossary/light-intensity)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-why-the-rate-of-photosynthesis-39136104) · Published by Druglandscape Ltd.