Which of the following statements about ATP and NADPH produced during the light reactions is accurate?

• A. They are used to break down starch into glucose.
• B. They pass back to the thylakoid lumen to generate more protons.
• C. They are used by the Calvin cycle to synthesize carbohydrates.
• D. They are converted into carbon dioxide and water.

Answer: (C)

ATP and NADPH from the light reactions supply the energy and reducing power the Calvin cycle needs to build sugars. In the stroma, ATP provides the energy to drive the phosphorylation steps that convert fixed carbon, while NADPH donates electrons to reduce 3-phosphoglycerate to glyceraldehyde-3-phosphate. This triose phosphate can then be used to synthesize carbohydrates such as glucose, starch, or sucrose. They aren’t used to break down starch, nor do they travel back into the thylakoid lumen to pump more protons, and they aren’t converted into carbon dioxide and water. So using ATP and NADPH in the Calvin cycle to make carbohydrates is the correct description of their role.