What is the stoichiometry of ATP and NADPH required per CO2 fixed in the Calvin cycle?

• A. 2 ATP and 3 NADPH
• B. 1 ATP and 1 NADPH
• C. 3 ATP and 2 NADPH
• D. 4 ATP and 1 NADPH

Answer: (C)

In the Calvin cycle, turning each molecule of carbon dioxide into carbohydrate requires both energy and reducing power from the light reactions. Specifically, ATP provides the energy to drive phosphorylation steps and to regenerate the RuBP acceptor, while NADPH supplies the electrons needed to reduce the fixed carbon toward glyceraldehyde-3-phosphate.

When you fix one CO2, the overall balance comes out to three molecules of ATP and two molecules of NADPH being consumed. This pairing reflects the combined needs of regenerating RuBP and reducing the fixed carbon through the steps that form carbohydrate. If you look at three CO2, you’d multiply those amounts and get nine ATP and six NADPH per triose phosphate produced, which is consistent with the known energy demands of synthesizing one G3P molecule.

So, the per-CO2 requirement of three ATP and two NADPH best fits the Calvin cycle’s accounting for energy and reducing power.