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

Why does carbon dioxide enter a leaf?

1. It is actively transported into the leaf

2. It diffuses due to a higher concentration outside the leaf

3. Leaves absorb CO2 through stomata only at night

4. Photosynthesis releases CO2, pulling it into the leaf

The correct answer is: It diffuses due to a higher concentration outside the leaf

The correct response highlights that carbon dioxide enters a leaf primarily through diffusion. This process occurs because the concentration of carbon dioxide in the atmosphere is typically higher than that inside the leaf. As a result, carbon dioxide naturally moves from an area of higher concentration (outside the leaf) to an area of lower concentration (inside the leaf). This movement occurs through small openings on the leaf surface known as stomata, which facilitate gas exchange. The diffusion of carbon dioxide is crucial for the process of photosynthesis, where plants convert light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen. This passive movement of gases allows plants to efficiently gather the necessary carbon dioxide without expending energy, distinguishing it from processes that require active transport, which is not the case here.