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

Why does oxygen diffuse out of a leaf after photosynthesis?

1. There is a higher concentration of oxygen inside the leaf

2. There is less sunlight available

3. Oxygen is used up by the plant immediately

4. Oxygen is removed by stomata

The correct answer is: There is a higher concentration of oxygen inside the leaf

Oxygen diffuses out of a leaf after photosynthesis primarily because there is a higher concentration of oxygen inside the leaf compared to the surrounding atmosphere. During photosynthesis, plants convert carbon dioxide and water into glucose and oxygen, producing a substantial amount of oxygen that accumulates within the leaf. As a result, when the concentration of oxygen inside the leaf becomes higher than that outside, diffusion occurs, allowing oxygen to move out into the environment. The other options do not accurately describe the main reason for the diffusion of oxygen. Less sunlight would not influence the concentration gradient directly after photosynthesis has taken place. Although oxygen is used by the plant for respiration, this process typically occurs at a different rate and location, and does not account for the immediate diffusion of excess oxygen. Lastly, while stomata play a role in the release of gases including oxygen, they are not responsible for removing oxygen but rather serve as openings that facilitate the movement of gases in and out of the leaf when there is a concentration gradient.