# A student is investigating the effect of light intensity on the rate of photosynthesis in pondweed. Explain how the student should make their investigation reliable and what they should do to control variables.

> OCR GCSE Biology A: Gateway Science (J247) — B7 Practical skills · Explain · 4 marks

> A student places a piece of pondweed in a beaker of water and moves a lamp to different distances from the beaker. They count the number of oxygen bubbles produced per minute as a measure of the rate of photosynthesis.

## Mark scheme (4 marks)

1. Repeat each measurement (at each distance / light intensity) and calculate a mean
2. Keep the temperature constant / controlled throughout the investigation
3. Keep the concentration of carbon dioxide constant (e.g. by using the same volume / concentration of sodium hydrogen carbonate solution)
4. The independent variable (distance of lamp / light intensity) must be the only variable that changes, so all other variables must remain the same

## Key terms

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

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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-is-investigating-the-effect-7daf1a54) · Published by Druglandscape Ltd.