Plants keep their insides steady too. A leaf needs carbon dioxide for photosynthesis, but every opening that lets carbon dioxide in also lets water vapor out. Water moving up from the roots cannot always keep up, so plants use osmosis to control their leaf pores, in a feedback loop.
Key idea
Stomata are pores in the leaf, each between two guard cells. When water is plentiful, guard cells swell (become turgid) and bow apart, opening the pore. When water is short, they go limp and the pore closes. This balances carbon dioxide intake against water loss.
Why it works
Guard cells open by osmosis. In light, with water available, they pump in potassium ions; water follows by osmosis, the cells swell, and because their inner walls are thicker, they bend apart. In a drought, the hormone abscisic acid (ABA) signals the guard cells to let potassium out; water leaves, the cells go slack and the pore shuts. Less water lost is the response that reverses the problem: negative feedback.
Worked example: A dry afternoon
On a hot, dry afternoon, a plant's soil is drying out. What happens to its stomata, and what does that cost?
- Water is short, so ABA builds up and guard cells lose water.
- The stomata close, saving water.
- But carbon dioxide can no longer get in.
Answer: The stomata close; the plant saves water but photosynthesis slows.
Worked example: Where are the stomata?
Many plants have more stomata on the underside of their leaves. Why might that help?
- The underside is shaded and cooler.
- Less water evaporates there.
Answer: It lets carbon dioxide in while losing less water.
Plants in dry places have other ways to save water: thick waxy coatings, fewer stomata, and, in plants such as cactuses, opening stomata only at night, when it is cooler, and storing the carbon dioxide until daytime.
Common mistake
Stomata are often described as holes that let oxygen out. They are mainly a trade-off: open, they let carbon dioxide in for photosynthesis, and they also let water vapor out. Oxygen does leave through them, but controlling water loss is why the plant opens and closes them.