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

What structure on the lower surface of leaves allows gas exchange?

1. Chloroplasts

2. Stomata

3. Cuticle

4. Guard cells

The correct answer is: Stomata

The structure on the lower surface of leaves that facilitates gas exchange is stomata. Stomata are small openings that allow carbon dioxide to enter the leaf for photosynthesis and oxygen, a byproduct of this process, to exit. They play a crucial role in regulating gas exchange in plants, allowing them to breathe while minimizing water loss. While chloroplasts are the sites of photosynthesis within the leaf cells, they do not directly facilitate gas exchange. The cuticle, on the other hand, is a waxy layer that covers the leaf surface, serving primarily to reduce water loss and protect the leaf from environmental factors, but it does not function in gas exchange. Guard cells, which surround each stoma, regulate its opening and closing; however, they are not the structures that directly allow gas exchange themselves, they manage the stomata's function. Thus, stomata are the answer as they are specifically designed for gas exchange, making them essential for the plant's respiration and photosynthesis processes.