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GCSE Biology · key term
Specialised cells that control the opening and closing of a stoma.
Used by AQA, Edexcel and OCR
Specification 4.2.3.2 · Paper 1
Pairs of cells around stomata that control whether the pores are open or closed.
Revise: The Mechanisms of Transpiration and TranslocationA stoma is a pore in the leaf surface, surrounded by two guard cells. When the guard cells open the pore, carbon dioxide can diffuse into the leaf for photosynthesis and oxygen can diffuse out. Water vapour can escape through the same opening.
Specification 6.11B · Paper 2
Cells beside a stoma that change shape to open or close the pore.
Revise: Leaf StructureGuard cells beside each stoma change shape to open or close it: opening allows gas exchange but also lets water vapour escape. This is why a leaf's gas-exchange structure also affects water loss.
Specification B2.2i · Paper 1 (Foundation), Paper 3 (Higher)
Revise: Factors affecting water uptakeIn brighter light, guard cells cause the stomata to open, allowing carbon dioxide to enter for photosynthesis. Open stomata also provide a route through which more water vapour can escape. Evaporation and diffusion out of the leaf therefore increase, raising the rate of transpiration.
Do not describe stomata as cells. They are pores, each controlled by surrounding guard cells.
Do not confuse stomata, which are pores, with guard cells, which control whether the pores are open.
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