Final answer:
One molecule of carbon is fixed in each round of the Calvin cycle, which must go through six rounds to produce a six-carbon glucose molecule.
Step-by-step explanation:
Each round of the Calvin cycle fixes one molecule of carbon, as the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.
The Calvin cycle incorporates carbon dioxide into the 5-carbon molecule RuBP with the help of the enzyme RuBisCO, leading to a 6-carbon intermediate, which quickly splits into two 3-carbon molecules. In the reduction stage of the cycle, energy from ATP and NADPH is used, and then RuBP is regenerated so the cycle can continue.