During the reduction phase of the Calvin cycle, which statement correctly describes the fate of 3-PGA?

• A. 3-PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).
• B. 3-PGA is oxidized to phosphoglycerate without NADPH.
• C. NADH provides the reducing equivalents for this step.
• D. G3P is formed directly from CO2 without phosphorylation.

Answer: (D)

During the reduction phase of the Calvin cycle, 3-PGA is prepared for conversion into sugar by first being phosphorylated with ATP and then reduced with NADPH. The phosphorylation adds a high-energy phosphate group, and the NADPH donates electrons to reduce the molecule to glyceraldehyde-3-phosphate (G3P). This step is a reduction, not oxidation, and it relies on NADPH (not NADH) as the reducing power. CO2 is not turned directly into G3P at this stage; CO2 is fixed into 3-PGA first, and only after phosphorylation and reduction does it become G3P, which can then be used to build sugars or to regenerate the carbon acceptor.