# Explain why the rate of photosynthesis in a C3 plant decreases during a period of water stress, even when light intensity and carbon dioxide concentration remain constant.

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

> During hot, dry conditions, land plants close their stomata to reduce water loss by transpiration. Researchers observed that despite unchanged light intensity and atmospheric CO₂ levels, the net rate of photosynthesis fell significantly within 30 minutes of stomatal closure.

## Mark scheme (4 marks)

1. Stomatal closure reduces CO₂ entry into the leaf
2. Lower internal CO₂ reduces the rate of the Calvin cycle / carbon fixation
3. GP cannot be reduced at the same rate, so RuBP is not regenerated quickly enough
4. Reduced carbon fixation means fewer / less triose phosphate produced, so overall carbohydrate synthesis decreases

## Key terms

- [C3 plant](https://www.gradenine.co.uk/glossary/c3-plant)
- [water stress](https://www.gradenine.co.uk/glossary/water-stress)

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-rate-of-photosynthesis-d63d1b68) · Published by Druglandscape Ltd.