# A student investigates the effect of light intensity on the rate of photosynthesis in pond weed. As light intensity increases from low to high, the rate of photosynthesis increases, then levels off at a maximum rate. Explain why the rate of photosynthesis increases as light intensity increases, and suggest one reason why the rate levels off at a maximum even when light intensity continues to increase.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.1 Photosynthesis · Explain · 5 marks

> Pond weed is submerged in water and exposed to different light intensities. The number of oxygen bubbles produced per minute is used as a measure of the rate of photosynthesis. As light intensity increases, the bubble count increases, but eventually stays constant despite further increases in light intensity.

## Mark scheme (5 marks)

1. Light provides the energy for photosynthesis
2. More light means more energy available, so the rate of photosynthesis increases
3. More glucose and/or oxygen is produced as light intensity increases
4. The rate levels off because another factor becomes limiting
5. A named limiting factor — carbon dioxide concentration or temperature (or amount of chlorophyll)

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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-the-effect-of-03d35cf7) · Published by Druglandscape Ltd.