Final answer:
In the Calvin cycle, carbon is fixed when carbon dioxide is added to ribulose bisphosphate (RuBP). This process is catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO), forming a 6-carbon intermediate that quickly splits into two 3-carbon molecules.
Step-by-step explanation:
In the Calvin cycle, carbon is fixed when carbon dioxide is added to ribulose bisphosphate (RuBP).
This process is catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO), which incorporates carbon dioxide into RuBP, forming a 6-carbon intermediate. This intermediate quickly splits into two 3-carbon molecules, which are then used to produce glucose.
So, the addition of carbon dioxide to RuBP is the first step in the process of carbon fixation in the Calvin cycle.