Guard cell structure and function

Guard cells are a pair of specialised, bean-shaped epidermal cells that surround each stoma, mostly on the lower surface of a leaf. By changing shape, they control whether the stoma is open or closed, and so control gas exchange and water loss.

Parts and functions

How structure suits function

The wall of a guard cell is not evenly thickened: the inner wall, next to the stoma, is thicker and less elastic than the thin outer wall. When a guard cell absorbs water and becomes turgid, the thin outer wall stretches more than the thick inner wall, so the whole cell is forced to bow outward into a curved shape, pulling the two guard cells apart and opening the pore.

When the guard cells lose water and become flaccid, they straighten out again and the pore closes.

A structure-to-function answer needs a feature, a consequence and a link to the stoma opening. The adaptations below follow that pattern.

• The wall is unevenly thickened, with a thin elastic outer wall and a thick rigid inner wall, so the outer wall stretches more when the cell is turgid and the cell curves to open the pore.
• The two cells are bean-shaped and arranged as a pair, so when both curve outward a gap, the stoma, is left between their inner walls.
• Guard cells contain chloroplasts and can actively take up solutes such as potassium ions in the light, so they lower their own water potential and draw water in by osmosis to become turgid.
• The large vacuole can take in or release a large volume of water quickly, so the cell can change from turgid to flaccid and open or close the stoma as conditions change.
• The cell membrane is partially permeable, so water moves in or out by osmosis whenever the solute content of the cell is changed, driving the whole opening and closing mechanism.

Related processes

Guard cells control transpiration. When the stomata are open, water vapour diffuses out of the leaf through the pores, so the rate of transpiration depends heavily on how wide the guard cells hold the stomata.

In hot, dry or windy conditions the guard cells lose turgor and close the stomata, which reduces water loss and helps prevent wilting.

Guard cells also control gas exchange for photosynthesis. In daylight the stomata open, letting carbon dioxide diffuse into the leaf for photosynthesis and oxygen diffuse out; at night, when there is no photosynthesis, most stomata close.

This is why guard cells respond to light and why they contain chloroplasts, unlike surrounding epidermal cells.

The opening and closing mechanism is a direct application of osmosis and turgor. The guard cells actively change their solute content, which changes their water potential; water then enters or leaves by osmosis, making the cells turgid or flaccid.

This links guard cells back to osmosis, water potential and plant cell turgidity studied earlier in transport.

Common labelling errors

Worked question

Question. Diagram 1 shows a stoma on the lower epidermis of a leaf during the day, with the pore open. Structures X are the two cells on either side of the pore.

(a) Name structures X and the pore between them. (b) Explain how structures X change to open the pore in the morning.

(c) State one feature present in X that most other epidermal cells lack. (d) On a hot, dry afternoon the pore closes.

Explain how this benefits the plant.

Model answer. (a) X are the guard cells; the pore is the stoma. (b) In light the guard cells actively take up solutes (for example potassium ions), which lowers their water potential; water enters by osmosis and they become turgid; the thin outer wall stretches more than the thick inner wall, so each cell curves outward and the pore opens.

(c) Chloroplasts. (d) The guard cells lose water and become flaccid, so they straighten and the pore closes; this reduces water loss by transpiration and helps prevent the plant from wilting.

Source:SRC-DSKP-EN

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