# Explain how the closure of stomata in a leaf is brought about when a plant experiences water stress.

> OCR A-Level Biology A (H420) — 5.5 Plant and animal responses · Explain · 4 marks

> Plants growing in arid conditions frequently experience water stress — a state in which the rate of water loss from leaves exceeds the rate of water uptake from the roots. Under these conditions, guard cells respond to the hormone abscisic acid (ABA) to reduce further water loss.

## Mark scheme (4 marks)

1. Water stress triggers the release of abscisic acid (ABA) from mesophyll cells
2. ABA binds to receptors on guard cells, causing potassium ions (K⁺) to leave the guard cells via ion channels
3. Loss of K⁺ reduces the solute concentration inside the guard cell, raising the water potential of the guard cell
4. Water leaves the guard cells by osmosis, the cells lose turgor and the stomatal pore closes

## Key terms

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

## 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-how-the-closure-of-stomata-26358b2e) · Published by Druglandscape Ltd.