Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

Which part of the leaf primarily contains air spaces for gas exchange?

1. Palisade mesophyll

2. Spongy mesophyll

3. Stomata

4. Chloroplasts

The correct answer is: Spongy mesophyll

The spongy mesophyll is primarily responsible for gas exchange within the leaf due to its structure and arrangement. This layer, located beneath the palisade mesophyll, consists of loosely packed cells with large intercellular air spaces. These spaces facilitate the movement of gases, such as carbon dioxide and oxygen, to and from the leaf surfaces. The presence of these air spaces allows for efficient diffusion, which is crucial for photosynthesis, respiration, and overall plant health. The spongy mesophyll also provides a significant surface area for gas exchange while minimizing water loss, making it an essential component in the leaf's anatomy. Although the palisade mesophyll is important for capturing light energy due to its high concentration of chloroplasts, it does not have the same level of air spaces for gas exchange as the spongy mesophyll. Stomata, while critical for regulating gas exchange, are not part of the mesophyll layer itself but are openings on the leaf surface. Chloroplasts are the organelles where photosynthesis occurs but do not function in gas exchange directly. Thus, the spongy mesophyll plays the most significant role in facilitating the necessary gas exchange processes in the leaf.