Final answer:
During a turn of the Calvin cycle, carbon is fixed before RuBP is regenerated.
Step-by-step explanation:
During a turn of the Calvin cycle, carbon is fixed before RuBP is regenerated. In the Calvin cycle, carbon fixation occurs when CO2 reacts with RuBP (ribulose bisphosphate) to form 3-PGA (3-phosphoglycerate).
This is the first step of the Calvin cycle and is catalyzed by the enzyme Rubisco. After carbon fixation, the remaining molecules of 3-PGA are then reduced and used to regenerate RuBP, which allows the cycle to continue. ATP is used in the regeneration of RuBP.