Ciencias
Stomata: The Pores a Leaf Opens and Shuts
Stomata are pores on the underside of a leaf, each between two guard cells. They open for carbon dioxide and close to save water, and cannot do both.
Stomata
Stomata are tiny pores in the surface of a leaf, mostly on the underside, that open and close to let gases in and out.
- También llamado
- Stoma (singular), Leaf pores
- Dónde lo encuentra el alumnado
- Grade 8 science, when leaf structure is first drawn and labelled, and again in Grade 10 for transpiration and the rate of photosynthesis.
La respuesta corta
Stomata are microscopic pores in the surface of a leaf, mostly on the underside, each formed by a pair of guard cells. They open to let carbon dioxide diffuse in for photosynthesis and close to limit water loss, so a plant is continually trading food against drying out.
Un ejemplo
guard cells turgid → stoma open; guard cells flaccid → stoma closed
Water enters the guard cells by osmosis and they swell. Because the wall on the side facing the pore is thicker than the outer wall, swelling makes each guard cell curve away from its partner, and the gap between them opens. When water is short the guard cells lose water, go limp and straighten, and the gap closes. The pore has no muscle and nothing pushes it — the shape of the two cells does all the work.
One pore, two guard cells
A stoma is not a hole in a single cell. It is the gap between two curved guard cells sitting side by side in the epidermis, and it exists only while those cells hold it open. The word is singular stoma and plural stomata, so a leaf has many stomata and one of them is a stoma — a small point, but it is marked.
Guard cells are unusual among epidermal cells in containing chloroplasts, which is part of how they respond to light. The rest of the leaf surface is covered by a waxy cuticle that lets nothing through, so essentially all gas exchange and almost all water loss happen at these pores.
The trade-off the plant cannot escape
Photosynthesis needs carbon dioxide, and the only route in is through an open stoma. But an open stoma is also an opening for water vapour to escape from the wet surfaces inside the leaf. The plant cannot take in one without losing the other, and the guard cells are where that choice gets made.
So the pattern of opening follows what there is to gain. Stomata are generally open in daylight, when photosynthesis is running and carbon dioxide is being used up, and closed at night, when there is nothing to gain by letting water out. In drought they close during the day too, which stops photosynthesis and is exactly why plants stop growing in dry conditions rather than merely wilting.
The water leaving through them is not wasted. Evaporation from the leaf pulls the transpiration stream up through the xylem, carrying dissolved mineral ions from the roots with it. Anything that speeds evaporation — bright light, high temperature, wind, dry air — speeds transpiration too.
Why they are mostly underneath
The lower surface of a leaf is shaded and cooler than the upper one, which is in the sun. Water evaporates more slowly there, so putting most of the stomata on the underside gets the gases in and out at a lower cost in water. The upper surface is left to the cuticle and to catching light.
Two observations confirm the arrangement. A water lily has its stomata on the upper surface instead, because the lower one is pressed against water and would gain nothing from a pore. And in the classic experiment, a leaf coated in petroleum jelly on its underside loses far less mass than one coated on top, because the coating has blocked the surface where nearly all the pores are.
Preguntas frecuentes
What do stomata do?
They control the exchange of gases between the leaf and the air. Carbon dioxide diffuses in for photosynthesis and oxygen diffuses out, and water vapour is lost through the same openings. Because they can open and close, the plant can adjust the rate rather than being stuck with whatever the conditions impose on it.
Why are stomata mostly on the underside of a leaf?
Because the underside is shaded and cooler, so less water evaporates through a pore there than through one on the sunlit upper surface. The plant gets the carbon dioxide it needs at a lower cost in water. Leaves that float, such as a water lily's, reverse the arrangement for the obvious reason.
What are guard cells?
The pair of curved cells that form each stoma and control whether it is open. When water enters them by osmosis they become turgid and bend apart, opening the pore; when they lose water they go flaccid and the pore closes. Their inner walls are thicker than their outer walls, which is what makes them bend rather than simply swell.
Do stomata close at night?
In most plants, yes. With no light there is no photosynthesis, so there is nothing to gain from letting carbon dioxide in and no reason to lose water. Some desert plants do the reverse, opening at night and storing carbon dioxide until daylight, which saves a great deal of water in very hot conditions.
How can you see stomata?
Paint a thin layer of clear nail varnish onto the underside of a leaf, let it dry, peel it off with sticky tape and look at it under a microscope. The varnish takes an impression of the surface, and the stomata appear as small openings between pairs of guard cells. Counting them per field of view compares one leaf with another.
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