What is the primary function of RuBisCO in the Calvin cycle?

What is the primary function of RuBisCO in the Calvin cycle?

• A. It fixes CO2 to RuBP to form 2 molecules of 3-PGA
• B. It fixes O2 to RuBP
• C. It splits water to supply electrons
• D. It transports ATP across membranes

Answer: (A)

RuBisCO’s job is to fix carbon dioxide in the Calvin cycle. It catalyzes the addition of CO2 to ribulose-1,5-bisphosphate (RuBP), forming a six-carbon intermediate that rapidly splits into two molecules of 3-phosphoglycerate (3-PGA). This carboxylation step is the starting point for building sugar molecules, with 3-PGA later being reduced and eventually converted into glyceraldehyde-3-phosphate for glucose synthesis. While RuBisCO can also react with O2, producing a less efficient pathway (photorespiration), that is not its primary function in the Calvin cycle. The other options describe processes from the light reactions or energy transport, not RuBisCO’s role in carbon fixation.

RuBisCO’s job is to fix carbon dioxide in the Calvin cycle. It catalyzes the addition of CO2 to ribulose-1,5-bisphosphate (RuBP), forming a six-carbon intermediate that rapidly splits into two molecules of 3-phosphoglycerate (3-PGA). This carboxylation step is the starting point for building sugar molecules, with 3-PGA later being reduced and eventually converted into glyceraldehyde-3-phosphate for glucose synthesis. While RuBisCO can also react with O2, producing a less efficient pathway (photorespiration), that is not its primary function in the Calvin cycle. The other options describe processes from the light reactions or energy transport, not RuBisCO’s role in carbon fixation.