Stoma

In the syllabus and the exam

Stoma (plural stomata) is central to the leaf structure and function topic, where candidates study how a leaf exchanges gases with the surrounding air and controls water loss at the same time. Most stomata are located on the lower epidermis of the leaf, an arrangement candidates are often asked to explain, since it reduces direct exposure to sunlight and therefore limits excessive water loss compared with placing the pores on the upper surface. Exam questions commonly test the dual role of stomata: they allow carbon dioxide in and oxygen out during photosynthesis, and they are also the main site through which water vapour escapes during transpiration, meaning stoma questions frequently connect the leaf structure topic to both photosynthesis and transpiration. A closely related and heavily tested idea is that each stoma is bordered by a pair of guard cells, which change shape to open or close the pore in response to light and water availability, so candidates should never describe a stoma as opening or closing by itself without reference to its guard cells. A frequent error is confusing the stoma, the pore itself, with the guard cells that control it, or assuming stomata remain open at all times regardless of environmental conditions. Candidates should also know that the total number and distribution of stomata can vary between plant species and even between leaves of the same plant grown in different light conditions, a detail sometimes tested through practical questions comparing leaf peels or epidermal impressions under a microscope. This is also why practical work sometimes asks candidates to estimate stomatal density by counting pores visible in a microscope field of view on a nail-varnish peel, a skill that links the theory of stomatal structure directly to a quantitative laboratory technique.

Example

Explaining gas movement in a leaf, a candidate writes: 'Carbon dioxide diffuses into the leaf through open stomata for photosynthesis, while oxygen produced during photosynthesis diffuses out through the same pores.'

Related terms

Guard cellCuticleTranspiration

Source:SRC-DSKP-EN