Guard cells & the stomata (CIE A-level)

This lesson describes the mechanism by which the guard cells open and close the stomatal apertures in response to changes in environmental conditions. The PowerPoint and accompanying resources have been designed to cover points 1, 2 and 3 as set out in topic 14.2 of the CIE A-level biology specification (for assessment in 2025 - 2027).

The lesson begins with a plant biology check, where the students have to spot the structure which isn’t found in a leaf, and then to spot the three leaf cells. This reveals the palisade and spongy mesophyll cells, but most importantly the guard cells. In a change to the normal, this lesson considers the function of the guard cells first before the structure is considered, and an understanding of the relationship between the two is embedded. Students will understand that there’s a balance between the opening of the stomatal aperture to allow carbon dioxide to diffuse in for photosynthesis, with the closing to reduce transpiration losses. The students’ knowledge of photosynthesis and other related topics are challenged throughout, and answers to all of the questions are found in the PowerPoint to allow the students to assess their progress. The final part of the lesson describes the mechanism by which the guard cells open the stomata, including details of ATP, potassium ions and the movement of water by osmosis.