In C4 photosynthesis, CO2 is fixed in mesophyll cells by which enzyme and transported to bundle-sheath cells?

• A. RuBisCO fixes CO2 in mesophyll cells and forms 3-PGA.
• B. PEP carboxylase fixes CO2 in mesophyll cells and oxaloacetate is transported to bundle-sheath cells.
• C. Carbonic anhydrase fixes CO2 in bundle-sheath cells.
• D. NADP+ reductase fixes CO2 in the thylakoid.

Answer: (B)

In C4 photosynthesis, the fixation of CO2 in mesophyll cells is carried out by phosphoenolpyruvate carboxylase, which fixes CO2 onto phosphoenolpyruvate to form oxaloacetate. This oxaloacetate is then transported to bundle-sheath cells, where it is decarboxylated to release CO2 for the Calvin cycle. This CO2 concentration around RuBisCO helps minimize oxygenation and photorespiration.

Other options don’t fit because RuBisCO is the enzyme that fixes CO2 in the Calvin cycle, typically in bundle-sheath cells after the C4 cycle has delivered CO2; it is not the initial fixer in mesophyll cells in C4 plants. Carbonic anhydrase speeds the interconversion between CO2 and bicarbonate but does not perform the fixation step, and NADP+ reductase is involved in the light reactions, not CO2 fixation.