How leaves balance gas exchange with water loss
Tiny pores called stomata allow gases to move between a leaf and the air. Opening them lets carbon dioxide enter for photosynthesis, but also allows water vapour to escape. Guard cells adjust the openings, helping the plant balance its need for carbon dioxide against the risk of losing too much water.
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One opening serves competing needs
Most leaves contain stomata, pores bordered by specialised guard cells. Changes in the guard cells' water content and shape alter the size of the opening.
An open pore allows carbon dioxide to diffuse into the leaf. At the same time, water vapour can leave the moist internal spaces. Closing the pore limits water loss but can also restrict the carbon dioxide available for photosynthesis.
The plant faces a trade-off rather than a permanently ideal setting. Light, internal carbon dioxide, water availability and other signals influence stomatal behaviour.
- Open pores support gas exchange.
- Water vapour escapes through the same route.
- Guard cells adjust the opening as conditions change.
Patterns vary among plants. Some plants adapted to dry environments take in much of their carbon dioxide at night, reducing the water cost under suitable conditions.