What is the Calvin cycle's main function in photosynthesis?

• A. Synthesize glucose by splitting water.
• B. Uses ATP and NADPH to fix CO2 into sugar.
• C. Produces oxygen.
• D. Replicates DNA.

Answer: (B)

The Calvin cycle’s main function is to convert inorganic carbon dioxide into organic sugar using the energy carriers ATP and NADPH produced by the light reactions. In the stroma, CO2 is fixed by the enzyme rubisco to form a three-carbon compound, which is then reduced to glyceraldehyde-3-phosphate with ATP and NADPH. Some of this trio-phosphate is eventually used to synthesize glucose, while others are recycled to regenerate the CO2 acceptor, RuBP, so the cycle can continue. This process builds carbohydrate from CO2, rather than releasing oxygen or performing unrelated tasks. Splitting water and oxygen production happen during the light reactions, and DNA replication is not part of photosynthesis.