Which five-carbon sugar is regenerated to enable continued CO2 fixation in the Calvin cycle?

• A. RuBP
• B. Ribose-5-phosphate
• C. Ribulose-5-phosphate
• D. Glucose-6-phosphate

Answer: (A)

In the Calvin cycle, the molecule that must be regenerated to keep fixing CO2 is RuBP, ribulose-1,5-bisphosphate. This five-carbon sugar phosphate is the CO2 acceptor that combines with CO2 via the enzyme RuBisCO to form an unstable six-carbon intermediate that splits into two molecules of 3-phosphoglycerate. After these are reduced and used to build sugars, most of the carbon is then reconstructed into RuBP through a series of steps powered by ATP, enabling the cycle to continue. The other five-carbon sugars listed aren’t the molecule that restarts CO2 fixation: ribose-5-phosphate is mainly used for nucleotide synthesis, ribulose-5-phosphate is an intermediate but not the final CO2 acceptor, and glucose-6-phosphate is a six-carbon sugar used in other pathways, not the Calvin cycle’s regeneration target.