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

What environmental condition tends to promote stomatal opening?

1. High light intensity

2. Low temperature

3. High carbon dioxide concentration

4. Low humidity

The correct answer is: High light intensity

High light intensity promotes stomatal opening primarily because it indicates favorable conditions for photosynthesis. When light intensity increases, plants have more energy available to convert carbon dioxide and water into glucose and oxygen, a process that occurs in the chloroplasts. Stomata are small openings on the surface of leaves that allow for the exchange of gases; they need to open to facilitate the entry of carbon dioxide, which is essential for photosynthesis. When light levels are high, plants signal the guard cells that surround each stoma to take up water and swell, thereby opening the stomata. This process allows for an efficient uptake of carbon dioxide while also facilitating the release of oxygen as a byproduct. Other environmental conditions, such as low temperature or high carbon dioxide concentrations, do not promote the same level of stomatal opening because they do not signal the same need for gas exchange in the context of photosynthesis. Low humidity, in fact, might cause stomata to close to conserve water loss, highlighting the importance of light in driving the opening of stomata for healthy plant growth and photosynthesis.