After 3-phosphoglycerate (3-PGA) is phosphorylated during the Calvin cycle, it is reduced by which of the following molecules?

• A. ATP
• B. NADPH
• C. FADH_2
• D. NADH

Answer: (B)

Verified step by step guidance

1. Understand the context: The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts during photosynthesis. It is responsible for converting carbon dioxide into glucose using energy from ATP and reducing power from NADPH.
2. Identify the step in question: After 3-phosphoglycerate (3-PGA) is phosphorylated by ATP, it is converted into 1,3-bisphosphoglycerate. The next step involves the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P).
3. Determine the reducing agent: Reduction reactions involve the addition of electrons. In the Calvin cycle, the molecule that donates electrons (and hydrogen) to reduce 1,3-bisphosphoglycerate is NADPH.
4. Clarify the role of NADPH: NADPH is a high-energy electron carrier generated during the light-dependent reactions of photosynthesis. It provides the reducing power necessary for the Calvin cycle to synthesize carbohydrates.
5. Eliminate other options: ATP provides energy but does not act as a reducing agent. FADH₂ and NADH are electron carriers used in cellular respiration, not photosynthesis. Therefore, NADPH is the correct molecule responsible for reducing 1,3-bisphosphoglycerate.