Guard cell

In the syllabus and the exam

Guard cells are examined as the pair of specialised, bean-shaped cells that surround each stoma and control whether it is open or closed, linking this term directly to both leaf structure and osmosis. Candidates must know that guard cells, unlike most surrounding epidermal cells, contain chloroplasts and can photosynthesise, and that their unique uneven wall thickness (thinner outer wall, thicker inner wall) is precisely what causes them to bend into a curved shape and open the stoma when they become turgid with water. Structured questions often ask candidates to explain, step by step, what happens as guard cells lose water and become flaccid: the cells straighten out again and the stoma closes, reducing water loss when conditions are dry. This makes guard cell questions a common site for testing understanding of osmosis, since water movement into and out of guard cells follows a water potential gradient. A frequent mistake is describing guard cells as simply opening and closing without explaining the underlying change in turgidity and wall thickness, or reversing which wall is thicker, which leads to an incorrect explanation of the bending mechanism. An additional detail sometimes tested is that guard cells respond not only to water availability but also to light, generally opening in daylight when photosynthesis requires carbon dioxide and closing at night, which links guard cell behaviour to the daily rhythm of gas exchange in a leaf. This is also why a wilted plant left in the dark overnight often recovers some turgidity by morning even before watering, since reduced transpiration through partially closed stomata allows the existing water in the plant to redistribute more evenly.

Example

Explaining stomatal control, a candidate writes: 'When guard cells absorb water and become turgid, they bend outward because their inner walls are thicker than their outer walls, causing the stoma between them to open.'

Related terms

StomaOsmosisCuticle

Source:SRC-DSKP-EN