# A student is investigating the effect of light intensity on the rate of photosynthesis in pondweed. The student places a lamp at different distances from the pondweed and counts the number of oxygen bubbles produced per minute. The student notices that when the lamp is very close to the pondweed, the rate of bubble production stops increasing even when the lamp is moved closer. Explain why the student must control variables in this investigation, and suggest two variables that must be kept constant and explain why each one must be controlled.

> OCR A-Level Biology B — Advancing Biology (H422) — 1.1 Practical skills assessed in a written examination · Explain · 5 marks

> A student places a piece of pondweed in a boiling tube of water. A lamp is placed at measured distances from the boiling tube. At each distance, the student counts the number of oxygen bubbles produced per minute as a measure of the rate of photosynthesis.

## Mark scheme (5 marks)

1. Variables must be controlled so that any change in the rate of photosynthesis can be attributed only to the change in light intensity / so that it is a fair test
2. Temperature must be kept constant (e.g. using a water bath or keeping the lamp wattage the same)
3. Because temperature affects enzyme activity / the rate of photosynthesis, so if it changed it would act as a limiting factor / alter the rate independently of light intensity
4. Carbon dioxide concentration must be kept constant (e.g. by adding sodium hydrogen carbonate to the water)
5. Because carbon dioxide is a reactant in photosynthesis / a limiting factor, so if its concentration changed it would alter the rate independently of light intensity

## Key terms

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

## 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-is-investigating-the-effect-4fbe62d3) · Published by Druglandscape Ltd.