What is the function of Rubisco in the Calvin cycle?

• A. It splits water to release oxygen.
• B. It fixes CO2 by carboxylating RuBP to produce 2 molecules of 3-PGA.
• C. It converts NADP+ to NADPH.
• D. It transports electrons in the electron transport chain.

Answer: (B)

Rubisco fixes carbon in the Calvin cycle by catalyzing the carboxylation of RuBP with CO2. This reaction forms an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA). This step is how carbon from CO2 enters the cycle, setting up the series of reactions that uses ATP and NADPH to convert 3‑PGA into sugars. The other processes described belong to the light reactions: splitting water to release oxygen, the production of NADPH, and electron transport, which occur before the Calvin cycle begins. Rubisco is a carboxylase, and while it can also catalyze an oxygenation reaction under certain conditions, its role in the Calvin cycle is to fix CO2 into RuBP to form 3‑PGA.