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

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

> A researcher exposes a shade-adapted plant to progressively higher light intensities in a controlled environment chamber. At low light intensities, the rate of photosynthesis rises steeply with each increase in light intensity. However, above a certain light intensity, the rate of photosynthesis levels off and remains constant despite further increases in light intensity.

## Mark scheme (4 marks)

1. Above the light saturation point, light is no longer the limiting factor
2. The rate of the light-independent reactions (Calvin cycle) limits overall photosynthesis
3. ATP and NADPH (reduced NADP) produced in the light-dependent reactions cannot be used any faster by the Calvin cycle
4. Enzyme-controlled reactions in the Calvin cycle are saturated / working at Vmax, so no further increase in rate is possible

## Key terms

- [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-38e094b6) · Published by Druglandscape Ltd.