Which enzyme catalyzes the fixation of CO2 in the Calvin cycle?

• A. PEP carboxylase
• B. Rubisco
• C. DNA polymerase
• D. Ligase

Answer: (B)

Rubisco is the enzyme that fixes CO2 in the Calvin cycle. It catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) with carbon dioxide to form two molecules of 3-phosphoglycerate, producing the first stable organic products that drive the cycle forward toward sugar creation. This step is the entry point for inorganic carbon into the photosynthetic pathway, setting up the series of reductions and regenerations that ultimately synthesize glucose.

In contrast, PEP carboxylase fixes CO2 in C4 and CAM plants to help concentrate CO2 before it enters the Calvin cycle, but that enzyme’s action is outside the Calvin cycle proper. DNA polymerase and ligase are enzymes involved in DNA replication and repair, not carbon fixation.