52. During photosynthetic carbon reduction cycle in green leaves, net production of one molecule of glyceraldehyde 3-phosphate requires one of the following combinations of energy equivalents:
(1) 9 NADPH and 6 ATP    (2) 3 NADPH and 9 ATP.
(3) 2 NADPH and 3 ATP    (4) 6 NADPH and 9 ATP.

Introduction

Understanding the ATP and NADPH requirement for producing one net glyceraldehyde 3-phosphate (G3P) in the Calvin cycle is a frequent concept question in NEET and other entrance exams. The key is to remember the overall stoichiometry of CO₂ fixation and the number of energy equivalents consumed to form one triose phosphate that actually leaves the cycle.​

Core concept and correct answer

The overall balanced equation for the photosynthetic carbon reduction (Calvin) cycle to produce one net molecule of glyceraldehyde 3-phosphate from 3 molecules of CO₂ is commonly written as:
3 CO₂ + 9 ATP + 6 NADPH + 6 H₂O → 1 G3P + 9 ADP + 8 Pi + 6 NADP⁺.​
This clearly shows that net production of one G3P requires 9 ATP and 6 NADPH, so among the given options, “6 NADPH and 9 ATP” is the only correct combination.​

Biochemically, ATP is used in both the reduction of 3‑phosphoglycerate to G3P and the regeneration of ribulose 1,5‑bisphosphate, while NADPH provides the reducing power in the reduction stage.​

Why option (4) is correct: 6 NADPH and 9 ATP

Option (4) states that one net G3P requires 6 NADPH and 9 ATP, which matches the standard Calvin cycle stoichiometry for fixation of 3 CO₂.​

• For each CO₂ molecule fixed in a C₃ plant, 3 ATP and 2 NADPH are required, so for 3 CO₂, the total becomes 9 ATP and 6 NADPH, giving one net G3P that can exit the cycle.​

• This G3P can subsequently be used to form glucose, sucrose, starch, and other carbohydrates, but the Calvin cycle energy requirement is defined at the level of net triose phosphate formation.​

Because the question explicitly asks for “net production of one molecule of glyceraldehyde 3-phosphate,” not just a single reduction step, the full cycle requirement (9 ATP, 6 NADPH) is the correct answer.​

Why option (3) looks familiar but is incomplete: 2 NADPH and 3 ATP

Option (3): “2 NADPH and 3 ATP” is actually the requirement to fix one molecule of CO₂, not to produce one net G3P leaving the cycle.​

• Per CO₂, the Calvin cycle uses 3 ATP and 2 NADPH; this comes from 2 molecules of 3‑phosphoglycerate formed and then reduced to G3P, plus regeneration reactions.​

• One net G3P requires 3 CO₂, so the per‑CO₂ values must be multiplied by 3, giving 9 ATP and 6 NADPH; using only 2 NADPH and 3 ATP would account for only one-third of the needed energy.​

Thus, option (3) is numerically correct per CO₂ but wrong for “net one G3P,” which is why it is not the right choice in this question.​

Why option (1) is wrong: 9 NADPH and 6 ATP

Option (1) reverses the correct ratio, claiming 9 NADPH and 6 ATP, which does not match the known Calvin cycle energy balance.​

• The Calvin cycle uses more ATP than NADPH; standard teaching emphasizes that 3 ATP are required for every 2 NADPH, so ATP:NADPH is 3:2, not the other way round.​

• If 9 NADPH and 6 ATP were required, this would imply more reducing power than phosphorylation energy, contradicting the experimentally determined stoichiometry and the coupling with the light reactions.​

Therefore, option (1) misrepresents both the absolute numbers and the ATP:NADPH ratio, making it incorrect.​

Why option (2) is wrong: 3 NADPH and 9 ATP

Option (2) suggests 3 NADPH and 9 ATP, which also fails to match the recognized energy requirement of the Calvin cycle.​

• Although 9 ATP is correct for one net G3P, the number of NADPH molecules must be 6, not 3; halving the NADPH would leave the reduction steps of 1,3‑bisphosphoglycerate to G3P underpowered.​

• The consistent ratio of 3 ATP to 2 NADPH would predict that if ATP is 9, NADPH must be 6; any other combination violates that fixed ratio and so cannot be correct for normal C₃ photosynthesis.​

Thus, option (2) mixes a correct ATP value with an incorrect NADPH value and must be rejected.​

In summary, the photosynthetic carbon reduction cycle in green leaves requires 6 NADPH and 9 ATP for the net production of one glyceraldehyde 3-phosphate, making option (4) the correct answer.