Question

In the $C_4$ pathway, the $C{O}_2$ fixation in mesophyll cells is carried out by the enzyme

• A. Pyruvate dehydrogenase.
• B. Pyruvate decarboxylase.
• C. PEP carboxylase.
• D. Rubisco.

Answer 1

Answer: (C)

PEP carboxylase.

The details of the $C_4$ cycle: after entering through stomata, $C{O}_2$ diffuses into a mesophyll cell. Being close to the leaf surface, these cells are exposed to high levels of $O_2$, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). Instead, the $C{O}_2$ is inserted into a 3-carbon compound ($C_3$) called phosphoenolpyruvic acid (PEP) forming the 4-carbon compound oxaloacetic acid ($C_4$). Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot diffuse easily to them; often have thylakoids with reduced photosystem II complexes (the one that produces $O_2$). Both of these features keep oxygen levels low. Here the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch. Pyruvic acid ($C_3$), is transported back to a mesophyll cell where it is converted back into PEP.