Stomata and gas exchange
Plants exchange gases through tiny pores called stomata, mostly on the underside of leaves. In daylight, carbon dioxide diffuses in for photosynthesis and oxygen diffuses out. At night, oxygen goes in for respiration. Water vapour leaves through the stomata during transpiration.
How stomata open and close
Each stoma is surrounded by two guard cells. Their inner walls are thick and the outer walls thin, so when they take in water and become turgid they bend apart and the stoma opens. When they lose water they become flaccid and the stoma closes.
Theories of stomatal opening
| Theory | Idea |
|---|---|
| Starch–sugar | In light the starch in guard cells is changed to sugar, which lowers their water potential; water enters and the cells swell. At night the sugar is changed back to starch. |
| Potassium ion (active) | Light makes the guard cells pump K⁺ ions in. This lowers the water potential, so water enters by osmosis and they swell. |
| Abscisic acid | When the plant is short of water, the hormone abscisic acid causes K⁺ ions to leave the guard cells; water follows and the stomata close. |
Try itStomatal opening
The pore opens wider as light gets brighter. Water loss follows the pore width.
Key terms
- Stoma
- A pore in the leaf surface for gas exchange.
- Guard cell
- One of a pair of cells that controls the stoma.
- Turgid
- Full of water and firm.
- Flaccid
- Having lost water and soft.
Summary
- Gases enter and leave leaves through stomata.
- Guard cells open the pore when turgid and close it when flaccid.
- Theories: starch–sugar, potassium ion pumping, and abscisic acid.
- Stomata open in the light and close in the dark or in drought.