What is the energy cost to fix one CO2 into a sugar precursor in the Calvin cycle?

• A. About 3 ATP and 2 NADPH per CO2.
• B. 2 ATP and 1 NADPH per CO2.
• C. 1 ATP and 1 NADPH per CO2.
• D. 6 ATP and 6 NADPH per CO2.

Answer: (A)

In the Calvin cycle, turning one carbon dioxide into a sugar precursor uses energy from ATP and reducing power from NADPH. CO2 is fixed with RuBP to form two molecules of 3-PGA. To move toward a sugar precursor, each 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P). Since there are two 3-PGA per CO2, that intermediate step uses 2 ATP and 2 NADPH. Additionally, regenerating the five‑carbon RuBP acceptor to keep the cycle running requires extra ATP, bringing the total to about 3 ATP and 2 NADPH per CO2 fixed. The sugar precursor produced is glyceraldehyde-3-phosphate, which serves as the building block for larger carbohydrates. The other options either underestimate or overstate the energy needs for this step.