Key Equations

Overall Calvin Cycle:

$6CO_{2}$ + 18ATP + 12NADPH + $12H_{2}O$ → $C_{6}H_{12}O_{6}$ + 18ADP + 18Pi + 12NA$DP^{+}$ + $6H_{2}O$

Stage 1 — Carboxylation (per turn):

$CO_{2}$ + RuBP (5C) → [6C unstable] → 2 × 3-PGA (3C each) | Enzyme: RuBisCO | ATP/NADPH: 0

Stage 2 — Reduction (per PGA → G3P):

3-PGA + ATP → 1,3-BPG + ADP | Enzyme: Phosphoglycerate kinase 1,3-BPG + NADPH → G3P + Pi + NA$DP^{+}$ | Enzyme: G3P dehydrogenase Cost per turn: 2 ATP + 2 NADPH (for 2 PGA → 2 G3P)

Stage 3 — Regeneration (per set of 3 turns):

5 G3P (3C each = 15C) → 3 RuBP (5C each = 15C) | Cost: 3 ATP per 3 RuBP Cost per turn: 1 ATP (averaged)

Stoichiometry Summary

Critical NEET Distractor: "12 ATP + 12 NADPH" is WRONG. Correct = 18 ATP + 12 NADPH.