In C4 plants, initial CO2 fixation occurs in the mesophyll cells by which enzyme?

• A. Rubisco
• B. PEP carboxylase
• C. Pyruvate carboxylase
• D. Malic enzyme

Answer: (B)

In C4 photosynthesis, the first CO2 capture happens in the mesophyll cells using a carboxylase that has a high affinity for CO2 and does not react with O2. This enzyme is phosphoenolpyruvate carboxylase, which fixes CO2 to phosphoenolpyruvate to form oxaloacetate. Oxaloacetate is then converted to malate and transported to the neighboring bundle-sheath cells, where it is decarboxylated to release CO2 around Rubisco for the Calvin cycle. This arrangement concentrates CO2 and minimizes photorespiration, explaining why the initial fixation is not Rubisco. The other enzymes listed are involved in different steps or pathways: Rubisco acts later in the Calvin cycle after CO2 concentration, while pyruvate carboxylase and malic enzyme participate in other metabolic routes or decarboxylation steps, not the initial CO2 capture in mesophyll cells.