Calvin Cycle Stages
கால்வின் சுழற்சியின் நிலைகள்
Most students fail to realize that the Calvin cycle is not 'dark'—it is light-dependent. It relies entirely on the ATP and NADPH generated in the light reactions. If the light reaction stalls, the Calvin cycle dies. Do not fall for the 'Dark Reaction' misnomer.
The idea
The Calvin cycle proceeds in three stages: Carboxylation, Reduction, and Regeneration. Carboxylation involves the fixation of CO2 into RuBP, catalyzed by the enzyme RuBisCO. Reduction utilizes ATP and NADPH to convert 3-PGA into triose phosphate. Finally, Regeneration restores the CO2 acceptor, RuBP, requiring one additional ATP.
Why it matters
This cycle is the primary mechanism for carbon fixation in all photosynthetic plants to synthesize sugars.
NEET lens
NEET focuses on the stoichiometry: how many ATP and NADPH are consumed per CO2 molecule fixed. Memorize the ratio: 3 ATP and 2 NADPH per CO2. Examiners frequently trap students by asking for the requirements to synthesize one molecule of glucose, which requires 6 turns of the cycle. Always calculate for 6 CO2 molecules.
Clinical significance
RuBisCO is the most abundant protein on Earth, yet it is notoriously inefficient, often binding O2 instead of CO2, leading to photorespiration. In humans, metabolic efficiency is governed by enzyme kinetics; if our enzymes were as 'indecisive' as RuBisCO, cellular respiration would fail, leading to rapid metabolic acidosis.
Introduction
The Calvin cycle (C3 cycle/dark reactions/light-independent reactions) is the set of biochemical reactions in the stroma that uses ATP and NADPH from light reactions to fix CO₂ into 3-carbon sugars (G3P).
Core concept
Three stages: (1) Carbon fixation: CO₂ + RuBP (5C) → 2 × 3-PGA (3C) via RuBisCO. (2) Reduction: 3-PGA → G3P (glyceraldehyde-3-phosphate) using ATP + NADPH. (3) Regeneration of RuBP: G3P → RuBP using ATP (via complex reactions including pentose phosphate pathway steps). For every 3 CO₂ fixed: 9 ATP + 6 NADPH consumed → 1 G3P net gain (5 of 6 G3P go back to regenerate RuBP).
Go deeper
Why 3 turns per one G3P? Each turn fixes 1 CO₂ → 2 G3P (6C total from 3C+3C), but only 1 G3P is net gain (5 recycle). To synthesize glucose (C6H12O6), you need 2 G3P → need 6 turns of Calvin cycle → 6 CO₂ + 18 ATP + 12 NADPH.
From Class 10
After the light reactions make ATP and NADPH, these are used to build sugar from CO₂. This is the Calvin cycle — the dark reactions of photosynthesis.