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

What effect does increased humidity have on transpiration?

1. Enhances transpiration rate

2. Reduces transpiration rate

3. No direct effect on transpiration

4. Only affects transpiration in the dark

The correct answer is: Reduces transpiration rate

Increased humidity reduces the transpiration rate in plants. Transpiration is the process where water vapor is lost from the leaves to the atmosphere. When humidity is high, the concentration gradient between the inside of the leaf and the surrounding air decreases. This reduced gradient makes it less favorable for water molecules to move out of the leaf, slowing down the rate at which transpiration occurs. As humidity increases, the air becomes saturated with moisture, diminishing the driving force that causes water to evaporate from the plant's surface. Consequently, plants may lose less water under humid conditions, helping them conserve moisture during times when the air is already rich in water vapor. The other options do not align with the physiological processes involved in transpiration. For example, enhancing transpiration would suggest that more water is lost, contradicting the established effects of high humidity. Similarly, stating that there is no direct effect or that transpiration is only affected in the dark overlooks the key role humidity plays in influencing water vapor movement in plants.