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

• A. It provides reducing power by donating high-energy electrons (and H) to reduce carbon intermediates, such as converting 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P).
• B. It directly splits water to generate $O_2$ and protons during the light reactions.
• C. It supplies energy for carbon fixation by hydrolyzing to release phosphate groups used to phosphorylate intermediates.
• D. It acts as the enzyme that catalyzes the fixation of $C_{0}2$ to ribulose-1,5-bisphosphate (RuBP).

Answer: (A)

Verified step by step guidance

1. Understand that NADPH is a key molecule in the Calvin cycle, which is the set of light-independent reactions in photosynthesis responsible for synthesizing sugars.
2. Recognize that NADPH functions primarily as a reducing agent, meaning it donates electrons and hydrogen ions (H+) to other molecules.
3. Identify the specific step in the Calvin cycle where NADPH donates these high-energy electrons: the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P).
4. Note that this reduction step is crucial because it converts a phosphorylated intermediate into a sugar molecule that can be used to form glucose and other carbohydrates.
5. Conclude that NADPH's role is not to split water, supply energy by phosphate hydrolysis, or act as an enzyme, but to provide the reducing power necessary for carbon intermediate reduction in the Calvin cycle.