Plant organ system: The role of stomata and guard cells are to control gas exchange and water loss.

The idea

Leaves are thin and flat to give a large surface area for diffusion, but that same surface lets water evaporate. Stomata are microscopic pores, mostly in the lower epidermis, each bordered by two guard cells. When guard cells take in water they swell and become turgid, bending apart so the pore opens; when they lose water they become flaccid and the pore closes. Open stomata allow carbon dioxide to diffuse in for photosynthesis and oxygen to diffuse out, while water vapour also escapes by transpiration. Closing them reduces water loss but also restricts gas exchange, so the plant balances photosynthesis against dehydration, for example shutting stomata in hot, dry, windy conditions.

What earns the marks

• Stomata are pores, mainly in the lower epidermis of leaves, through which gases diffuse.
• Carbon dioxide diffuses in through open stomata for photosynthesis and oxygen diffuses out.
• Water vapour is lost by transpiration through the same stomata.
• Guard cells surround each stoma and change shape by gaining or losing water.
• Turgid guard cells open the pore; flaccid guard cells close it.
• Opening stomata increases both gas exchange and water loss, so the two processes are linked.
• Closing stomata conserves water but limits carbon dioxide uptake and therefore photosynthesis.

Common mistakes and how to put them right

• Saying stomata are cells.

  Put it right: They are pores, and the guard cells are the cells that flank them.

• Claiming guard cells open stomata by shrinking.

  Put it right: They open when they become turgid after taking in water.

• Stating that stomata only let carbon dioxide in: oxygen and water vapour also move through them.

Exam tips

• Link each structure to its function in one sentence, for example pore for diffusion, guard cell for opening and closing.
• Use the terms turgid and flaccid when explaining how the pore changes.
• When asked about adaptation, mention the trade-off between gas exchange and water loss rather than describing only one side.