Final answer:
The carbon atoms incorporated into sugar molecules in the Calvin cycle come from carbon dioxide (CO2).
Step-by-step explanation:
The carbon atoms that are incorporated into sugar molecules in the Calvin cycle come from carbon dioxide (CO2). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas into sugar molecules like glucose (C6H12O6). The enzyme RuBisCO incorporates the carbon dioxide into an organic molecule called 3-PGA, which is then reduced and converted into glucose, the final product of the Calvin cycle.