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

Under what condition do stomata close automatically in plants?

1. During high humidity

2. When photosynthesis is active

3. When water supplies start to dry up

4. During the daytime

The correct answer is: When water supplies start to dry up

Stomata, which are small openings on the surface of leaves, play a crucial role in gas exchange and transpiration in plants. They open to allow carbon dioxide to enter for photosynthesis and close to prevent excessive water loss. The closing of stomata is an automatic response to environmental stress, particularly when water supplies begin to diminish. When plants detect that they are losing more water than they can take up from the soil, they respond by closing their stomata. This action helps to conserve water and maintain hydration, ensuring the plant can survive in conditions where water is limited. In essence, the closure of stomata during periods of drought or low water availability is a key adaptation that helps plants manage their internal water balance and avoid wilting or dehydration. In contrast, during high humidity, stomata may remain open due to reduced water loss. When photosynthesis is active, stomata are typically open to allow for carbon dioxide influx. During the daytime, many plants keep their stomata open to maximize photosynthesis, as this is when light is available for this process. Thus, the correct option reflects a physiological response to water stress, reinforcing the importance of stomatal regulation in plant survival.