In the Calvin cycle, what molecule is fixed to begin carbon fixation, and which enzyme catalyzes this step?

• A. O2 fixed by Rubisco to RuBP
• B. CO2 fixed by PEP carboxylase to oxaloacetate
• C. CO2 fixed by Rubisco to RuBP, forming 3-PGA
• D. Nitrogen fixed by nitrogenase to ammonium

Answer: (C)

In the Calvin cycle, carbon fixation starts when carbon dioxide is combined with RuBP, a five-carbon sugar. The enzyme that does this is Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), which catalyzes the addition of CO2 to RuBP to form an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). This initial carboxylation step is the entry point of carbon into the cycle, setting up the subsequent steps that use ATP and NADPH to convert 3-PGA into sugars. The other options mix up the substrate, enzyme, or pathway: oxygen is not the molecule being fixed in this step (though Rubisco can catalyze oxygenation in photorespiration), PEP carboxylase fixes CO2 in C4 and CAM pathways rather than in the Calvin cycle, and nitrogenase fixes nitrogen, not carbon.