Final answer:
The carbon from CO2 initially hooks on to a 5-carbon ribulose bisphosphate molecule (RuBP) in the Calvin cycle.
Step-by-step explanation:
The carbon from CO2 initially hooks on to a 5-carbon ribulose bisphosphate molecule (RuBP), catalyzed by the enzyme RuBP carboxylase-oxygenase, or Rubisco.
This process is part of the Calvin cycle, which is the biochemical pathway plants use to convert carbon dioxide into glucose during photosynthesis.