# Stomata

Tiny holes on the underside of leaves that let gases in and out. Guard cells open and close them.

Biology (school) · Part 02: Moving materials, enzymes and energy · Card 10 of 11 · Last updated 2026-10-19

## Analogy

Little doors in the leaf.

## In practice

Carbon dioxide enters leaves through stomata, and water vapour leaves through them.

## Practice

### Flashcards

- **What controls the opening of stomata?** Guard cells.

## Questions people ask

### How do guard cells open and close stomata?

When guard cells take in water by osmosis, they swell and bend apart, opening the stoma. When they lose water, they go limp and the gap closes. Plants usually open stomata in the day to take in carbon dioxide and close them when water is short to avoid drying out.

### Why are stomata mostly on the underside of leaves?

The underside is shaded and cooler, so less water evaporates through the openings. If most stomata were on the sunny top surface, the plant would lose water much faster. Having them underneath still allows carbon dioxide to enter for photosynthesis while keeping water loss lower.

## Related concepts

- [Transpiration](https://studyvisually.com/learn/science/biology-school/biology-school-part-2/biology-school-part-2-concept-11)

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