OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What role do guard cells play in the function of stomata?

1. They absorb carbon dioxide

2. They control the opening and closing of stomata

3. They produce glucose during photosynthesis

4. They store water for the leaf

The correct answer is: They control the opening and closing of stomata

Guard cells are specialized cells located on either side of the stomata, which are small openings on the surface of leaves. Their primary function is to regulate the opening and closing of these stomatal pores, thereby controlling gas exchange and transpiration in plants. When guard cells take up water, they swell and bend outward, causing the stomata to open. This allows carbon dioxide to enter the leaf for photosynthesis while also enabling water vapor to be released. Conversely, when guard cells lose water, they become flaccid and close the stomata, reducing water loss from the plant, particularly during periods of drought or high temperatures. This regulation is crucial for maintaining homeostasis in the plant by balancing the need for carbon dioxide with the conservation of water. The other options do not accurately describe the function of guard cells. For example, while carbon dioxide absorption is an important process that occurs when the stomata open, it is not the role of the guard cells themselves to absorb it. Similarly, glucose is produced during photosynthesis in chloroplasts, not by the guard cells, and while water is essential for the plant, guard cells do not store it; rather, they respond to water levels to regulate stomatal movement.