In photosynthesis, what is the role of the Calvin cycle?

• A. It captures light energy to generate ATP and NADPH
• B. It splits water to release oxygen
• C. It transports sugars throughout the plant
• D. It uses ATP and NADPH to fix CO2 into sugars

Answer: (D)

The Calvin cycle uses the energy and reducing power from the light reactions to build sugars from carbon dioxide. In the stroma, CO2 is fixed by Rubisco and first turned into 3-phosphoglycerate, then energy from ATP and electrons from NADPH convert it into glyceraldehyde-3-phosphate. Some of this sugar phosphate goes on to form glucose and other carbohydrates, while most is recycled to regenerate the molecule that accepts CO2, so the cycle can run again. This process does not capture light or split water—that happens in the light-dependent reactions—nor is sugar transport the job of the Calvin cycle itself; the cycle’s job is to convert inorganic CO2 into organic sugars using ATP and NADPH produced earlier.