The carbon atom from carbon dioxide is used to change the five-carbon sugar RuBP (ribulose-1,5-bisphosphate) into two molecules of 3-phosphoglycerate (3-PGA).

This process happens during the first step of the Calvin cycle, which is the light-independent stage of photosynthesis:

1. Carbon Fixation: The enzyme RuBisCo adds a carbon dioxide molecule to RuBP. This unstable six-carbon compound immediately splits into two molecules of 3-PGA.

This reaction is crucial because it incorporates inorganic carbon (CO2) into an organic molecule, effectively "fixing" carbon into a form usable by the plant.