What is the primary product formed when CO2 combines with RuBP in the Calvin cycle?

In the Calvin cycle, the primary product formed when carbon dioxide (CO2) combines with ribulose bisphosphate (RuBP) is two molecules of 3-phosphoglycerate (PGA), which are three-carbon compounds. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). When CO2 and RuBP react, they produce an unstable six-carbon intermediate that immediately splits into two molecules of PGA.

These molecules, derived from the initial combination of CO2 and RuBP, serve as the first stable product in the cycle and are crucial for subsequent steps in the synthesis of glucose and other carbohydrates. While glucose and PGAL (glyceraldehyde-3-phosphate, G3P) are ultimately formed later in the cycle, the immediate and primary outcome of the CO2 fixation step is the creation of PGA. Other options listed, such as oxaloacetate, are not produced during this stage of the Calvin cycle, further reinforcing the role of PGA as the main product at this point.