Q.54 In Calvin–Benson cycle, to produce 1 molecule of glyceraldehyde 3–phosphate by
fixing 3 molecules of carbon dioxide, 9 molecules of ATP and ________ molecules
(in integer) of NADPH are typically utilized.

6 molecules of NADPH are utilized.

Calvin-Benson Cycle Overview

The Calvin-Benson cycle, also known as the Calvin cycle, fixes carbon dioxide into organic sugars during photosynthesis. To produce one glyceraldehyde 3-phosphate (G3P) molecule by fixing three CO₂ molecules, the cycle consumes 9 ATP and 6 NADPH molecules.​

Stoichiometry Breakdown

Fixation incorporates 3 CO₂ into 6 molecules of 3-phosphoglycerate (3-PGA) using ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Reduction converts these 6 3-PGA to 6 G3P, requiring 6 ATP (for phosphorylation to 1,3-bisphosphoglycerate) and 6 NADPH (for reduction). Regeneration recycles 5 G3P back to 3 ribulose-1,5-bisphosphate (RuBP), using 3 additional ATP, yielding net 1 G3P.​

Overall equation:
3CO₂ + 9ATP + 6NADPH → G3P + 9ADP + 8P_i + 6NADP⁺ + 3H₂O

Question Solution

The query states 9 ATP for 1 G3P from 3 CO₂, matching standard stoichiometry. NADPH totals 6: 2 per CO₂ fixed (one per 3-PGA reduced). No options provided, but common distractors include 12 (for glucose, requiring 2 G3P) or 9 (confusing with ATP).​

In the Calvin-Benson cycle 9 ATP 6 NADPH 1 G3P 3 CO2 process, plants convert atmospheric CO₂ into sugars essential for growth. This dark reaction of photosynthesis occurs in chloroplast stroma, powered by light reaction products.​

Carbon Fixation Phase

RuBisCO catalyzes 3 CO₂ + 3 RuBP → 6 3-PGA. No ATP/NADPH here.​

Reduction Phase

6 ATP phosphorylate 6 3-PGA to 1,3-bisphosphoglycerate; 6 NADPH reduce them to 6 G3P.​

Regeneration Phase

5 G3P rearrange to 3 RuBP using 3 ATP, netting 1 G3P exportable for glucose synthesis.​

• Total: 9 ATP (6 reduction + 3 regeneration), 6 NADPH (all reduction).​

• For glucose (2 G3P): Double to 18 ATP, 12 NADPH.​