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

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.

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.