# A student investigates how light intensity affects the rate of photosynthesis in a pondweed plant. As light intensity increases, the rate of photosynthesis increases up to a certain point, after which it levels off. Explain why the rate of photosynthesis increases as light intensity increases, and why the rate eventually stops increasing even when light intensity continues to rise.

> OCR A-Level Biology B — Advancing Biology (H422) — 4.1 Photosynthesis · Explain · 5 marks

> Photosynthesis is an endothermic reaction that takes place in the chloroplasts, converting carbon dioxide and water into glucose and oxygen using light energy. The overall equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

## Mark scheme (5 marks)

1. Light energy is required/used for photosynthesis to occur
2. As light intensity increases, more light energy is available so photosynthesis can occur at a faster rate, producing more glucose (and oxygen)
3. At high light intensities the rate levels off because another factor is limiting / becomes a limiting factor
4. The limiting factor could be carbon dioxide concentration (or water availability)
5. Even with more light energy available, the rate cannot increase further because the supply of carbon dioxide (or water) is insufficient to sustain a faster rate of reaction

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-investigates-how-light-intensity-9dfbf11a) · Published by Druglandscape Ltd.