Stoma structure and function

A stoma (plural: stomata) is a small pore in the epidermis of a leaf, bordered by a pair of guard cells. Stomata are the main gateway for gases and water vapour to move between a leaf and the air.

Because the guard cells can change shape, a stoma is an adjustable pore rather than a fixed hole. Questions on this structure usually test whether you can explain how the guard cells open and close the pore, and why a plant must balance gas exchange against water loss.

Parts and functions

How structure suits function

A stoma lets carbon dioxide in and oxygen out for photosynthesis, oxygen in and carbon dioxide out for respiration, and it is the main route by which water vapour is lost as transpiration. Every feature of the stoma serves one of two aims: efficient gas exchange and controlled water loss.

The guard cells make the pore adjustable, so the plant can change its rate of gas exchange and water loss to suit conditions such as light intensity and water supply.

• Unevenly thickened guard cell wall, the thin outer wall stretches more than the thick inner wall, so a turgid guard cell curves and pulls the pore open.
• Chloroplasts in the guard cells, they photosynthesise in light and build up solutes that lower the water potential, drawing in water so the pore opens by day.
• Small pores spread across the epidermis, total gas exchange is large while each opening loses little water.
• Most stomata on the shaded lower epidermis, the surface is cooler, so water loss by transpiration is reduced.
• Guard cells can close the pore, when water is scarce they lose turgor and shut, conserving water.
• A thin, moist inner surface behind the pore, carbon dioxide dissolves before diffusing into the mesophyll.

Related processes

By day, photosynthesis uses carbon dioxide faster than respiration produces it, so carbon dioxide diffuses into the leaf through open stomata and oxygen diffuses out. At night only respiration takes place, so oxygen diffuses in and carbon dioxide out.

The stoma is simply the pore through which these gases move by diffusion down their concentration gradients.

Water vapour also escapes through open stomata, and this loss is transpiration, which pulls the transpiration stream up the xylem. Whether a stoma is open depends on the turgor of its guard cells.

In light the guard cells photosynthesise and accumulate solutes such as potassium ions and sugars, so their water potential falls, water enters by osmosis, and the turgid guard cells curve apart to open the pore. In darkness or when the plant is short of water, the guard cells lose water, become flaccid and the pore closes.

Common labelling errors

Worked question

Question. The diagram shows a stoma bordered by its two guard cells. (a) Name the process by which carbon dioxide enters the leaf through an open stoma.

(b) Explain how the guard cells cause the stoma to open in bright light. (c) State one way in which the position of most stomata helps a leaf reduce water loss.

(d) A wilting plant closes its stomata. Explain how this helps the plant survive but also slows its growth.

Model answer. (a) Diffusion. (b) In bright light the guard cells photosynthesise and accumulate solutes, so their water potential falls; water enters by osmosis and the guard cells become turgid; because the inner wall is thicker and less elastic than the outer wall, each guard cell curves outward, so the pore opens.

(c) Most stomata are on the lower epidermis, which is shaded and cooler, so evaporation and water loss are reduced. (d) Closing the stomata reduces water loss by transpiration, so the plant conserves water and does not dry out; but carbon dioxide can no longer diffuse in, so the rate of photosynthesis falls, less glucose is made, and growth slows.

Source:SRC-DSKP-EN

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