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

Which part of the leaf has the most stomata for reducing water loss?

1. Upper surface

2. Lower surface

3. Inner layer

4. Margins of the leaf

The correct answer is: Lower surface

The lower surface of the leaf contains the most stomata, which are small openings that facilitate gas exchange while also playing a critical role in regulating water loss through transpiration. By having a greater number of stomata on the underside, the leaf minimizes direct exposure to sunlight and wind, both of which can increase the rate of water loss. This strategic positioning helps to reduce dehydration under environmental stress. In contrast, the upper surface typically has fewer stomata, as it is more exposed to sunlight and higher temperatures, which would otherwise accelerate water loss. Additionally, the inner layers of the leaf, such as the mesophyll, are involved in photosynthesis and gas exchange but do not have stomata. The margins of the leaf are not specialized for stomatal density either; their structure primarily contributes to the overall shape and function of the leaf. Therefore, the lower surface is optimized for maintaining water retention while allowing necessary gas exchange.