ATP and NADPH Required Per CO2 and Per Glucose in the Calvin Cycle
Biology · Photosynthesis in Higher Plants · NEET
Per CO2 fixed, the Calvin cycle uses 3 ATP and 2 NADPH. To make one glucose you fix 6 CO2 (6 turns), so you need 18 ATP and 12 NADPH. Memory hook: "3 and 2 per CO2; multiply by 6 for glucose = 18 and 12." NEET repeats these exact numbers almost every year.
Per CO2 the Calvin cycle spends 3 ATP and 2 NADPH; multiply by 6 turns (6 CO2 = 1 glucose) to get 18 ATP and 12 NADPH. ATP stays higher because of the extra ATP used to regenerate RuBP.
Your doubts, answered
How many ATP and NADPH are needed per CO2 fixed?
3 ATP and 2 NADPH per CO2. NCERT splits it clearly: the reduction phase uses 2 ATP + 2 NADPH, and the regeneration phase uses 1 more ATP. Add them: 3 ATP + 2 NADPH for each CO2 molecule that enters the cycle.
Why 18 ATP and 12 NADPH for one glucose?
One glucose has 6 carbons, so 6 CO2 must be fixed, meaning 6 turns of the Calvin cycle. Multiply the per-CO2 cost: 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH. This matches the NCERT 'In and Out' table exactly.
Why is ATP 3 but NADPH only 2 per CO2?
NADPH is only used for reduction (2 per CO2). ATP is used in both reduction (2) and regeneration of RuBP (1 extra). So ATP has one extra step, giving 3 ATP but 2 NADPH. This is why ATP and NADPH numbers are never equal.
How many turns of the Calvin cycle make one glucose?
6 turns. Each turn fixes exactly one CO2, and glucose needs 6 carbons. So 6 CO2 = 6 turns = one glucose. For three glucose molecules it would be 6 x 3 = 18 turns.
Is it written NADPH or NADPH2 in NCERT?
NEET question papers use both 'NADPH' and 'NADPH2' to mean the same reduced carrier. Do not get confused by the '2'. The number that matters is still 2 per CO2 and 12 per glucose, whichever spelling the paper uses.
Does this ATP/NADPH come from the light or dark reaction?
The ATP and NADPH are made in the light reaction. The Calvin cycle (dark/biosynthetic reaction) only spends them. So these numbers tell you how much light-reaction product is needed to build sugar.
⚠️ The NEET trap ✗ Students write 12 ATP and 12 NADPH (equal numbers) or swap them to 12 ATP and 18 NADPH. ✓ It is 18 ATP and 12 NADPH per glucose. ATP is always the larger number because of the extra 1 ATP used in regeneration each turn. 🧠 ATP > NADPH, always. Bigger number (18) goes to ATP, smaller (12) to NADPH.
Real NEET questions
NEET 2024
How many molecules of ATP and NADPH are required for every molecule of CO2 fixed in the Calvin cycle?
A · 2 molecules of ATP and 2 molecules of NADPH
B · 3 molecules of ATP and 3 molecules of NADPH
C · 3 molecules of ATP and 2 molecules of NADPH ✓
D · 2 molecules of ATP and 3 molecules of NADPH
Solution: Per CO2 fixed: reduction uses 2 ATP + 2 NADPH and regeneration uses 1 more ATP. Total = 3 ATP and 2 NADPH. NCERT Ch 11, p.145: 'for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.'
NEET 2023 Phase 1
How many ATP and NADPH2 are required for the synthesis of one molecule of Glucose during Calvin cycle?
A · 12 ATP and 12 NADPH2
B · 18 ATP and 12 NADPH2 ✓
C · 12 ATP and 16 NADPH2
D · 18 ATP and 16 NADPH2
Solution: One glucose needs 6 CO2 fixed (6 turns). Cost per turn is 3 ATP + 2 NADPH, so 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH. The NCERT In/Out table lists exactly 18 ATP and 12 NADPH per glucose.
ReNEET 2026
How many turns of the Calvin cycle are required for the formation of three molecules of glucose?
A · 6
B · 3
C · 1
D · 18 ✓
Solution: One glucose needs 6 turns (each turn fixes one CO2; 6 CO2 to one hexose). So three glucose molecules require 6 x 3 = 18 turns.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The ratio is 3:2 per CO2 (3 ATP to 2 NADPH), which stays 3:2 for glucose too (18:12 simplifies to 3:2). ATP is always higher than NADPH.
How many CO2 molecules are fixed to make one glucose?
Six CO2 molecules, because glucose (C6H12O6) has six carbon atoms and each turn of the cycle fixes only one CO2.
Where does the extra ATP get used?
In the regeneration phase, where the CO2 acceptor RuBP is remade. Each turn spends 2 ATP in reduction plus 1 ATP in regeneration, giving 3 ATP per CO2.
Does the Calvin cycle need light directly?
No. It is called the dark or biosynthetic reaction and runs on ATP, NADPH and CO2 supplied by the light reaction, not on light itself.
