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

When do stomata typically open and close?

1. Open during the night and closed during the day

2. Open during the day and closed at night

3. Always open

4. Open randomly

The correct answer is: Open during the day and closed at night

Stomata are small openings on the surface of leaves that facilitate gas exchange, allowing carbon dioxide to enter for photosynthesis and enabling the release of oxygen and water vapor. They typically open during the day when sunlight is available, which aligns with the period when photosynthesis occurs. This daytime opening is also influenced by environmental factors such as light intensity and humidity. At night, stomata generally close to minimize water loss, as photosynthesis is not taking place, and the plant's need for carbon dioxide decreases. By closing at night, plants conserve water and reduce the risk of desiccation. The option indicating that stomata open during the day and close at night accurately reflects the common behavior of most plants, although there are some exceptions in specific plant species known as CAM plants that operate differently.