Calvin Cycle — Revision Notes

⚡ 30-Second Revision

Location: — Stroma of chloroplasts
Phases: — Carboxylation, Reduction, Regeneration
CO2 Acceptor: — Ribulose-1,5-bisphosphate (RuBP, 5C)
Key Enzyme: — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
First Stable Product (C3): — 3-Phosphoglycerate (3-PGA, 3C)
Direct Sugar Product: — Glyceraldehyde-3-phosphate (G3P, 3C)
Energy Input per $CO_2$: — 3 ATP, 2 NADPH
Energy Input per Glucose ($C_6H_{12}O_6$): — 18 ATP, 12 NADPH
Overall Equation (for 1 G3P): — $3CO_2 + 9ATP + 6NADPH \rightarrow 1G3P + 9ADP + 8P_i + 6NADP^+$

2-Minute Revision

The Calvin Cycle, or C3 cycle, is the 'sugar factory' of the plant, occurring in the chloroplast stroma. It's the light-independent phase of photosynthesis, meaning it doesn't directly use sunlight but relies entirely on the ATP and NADPH generated by the light reactions.

The cycle begins with Carboxylation, where the enzyme RuBisCO fixes atmospheric $CO_2$ onto a 5-carbon sugar, RuBP, forming two molecules of 3-PGA (the first stable product). Next is Reduction, where 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP for phosphorylation and NADPH for reduction.

G3P is the direct sugar product; some leaves the cycle to form glucose, while the rest enters the final phase. In Regeneration, the remaining G3P molecules are rearranged, consuming more ATP, to reform RuBP, ensuring the cycle continues.

For every $CO_2$ fixed, 3 ATP and 2 NADPH are consumed. To make one glucose molecule, 6 $CO_2$ are fixed, requiring 18 ATP and 12 NADPH. Remember, RuBisCO can also bind $O_2$, leading to wasteful photorespiration, which C4 plants avoid through spatial separation.

5-Minute Revision

The Calvin Cycle is the central metabolic pathway for converting inorganic carbon dioxide into organic sugars, forming the biosynthetic core of photosynthesis. It operates in the stroma of chloroplasts and is critically dependent on the ATP (energy) and NADPH (reducing power) supplied by the light-dependent reactions. The cycle is divided into three main phases:

1
Carboxylation: — This is the initial carbon fixation step. One molecule of $CO_2$ combines with one molecule of the five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO. The resulting unstable six-carbon compound immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound, hence the name C3 cycle. This step effectively 'fixes' atmospheric carbon into an organic form.

1
Reduction: — In this phase, the 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a sugar. Each 3-PGA molecule undergoes two steps: first, it's phosphorylated by ATP to become 1,3-bisphosphoglycerate (consuming 1 ATP per 3-PGA). Second, 1,3-bisphosphoglycerate is reduced by NADPH to form G3P (consuming 1 NADPH per 1,3-bisphosphoglycerate). For every three $CO_2$ molecules fixed, six G3P molecules are produced. Crucially, only one of these six G3P molecules exits the cycle to be used for synthesizing glucose and other carbohydrates. The remaining five G3P molecules proceed to the next phase.

1
Regeneration: — The five remaining G3P molecules (totaling 15 carbons) are rearranged through a series of complex enzymatic reactions to regenerate three molecules of RuBP (totaling 15 carbons). This regeneration step is vital for the continuous operation of the cycle, as it replenishes the $CO_2$ acceptor. This phase consumes additional ATP (specifically, 3 ATP molecules for every 3 RuBP regenerated).

Stoichiometry: To produce one molecule of glucose ($C_6H_{12}O_6$), the cycle must fix six $CO_2$ molecules. This requires 18 ATP molecules (12 for reduction, 6 for regeneration) and 12 NADPH molecules (all for reduction).

A key point for NEET is understanding RuBisCO's dual nature: it can also bind $O_2$ instead of $CO_2$, leading to photorespiration, a wasteful process that reduces photosynthetic efficiency, especially in hot, dry conditions.

C4 and CAM plants have evolved mechanisms to mitigate this.

Prelims Revision Notes

The Calvin Cycle, also known as the C3 cycle or light-independent reactions, is the biosynthetic phase of photosynthesis. It occurs in the stroma of chloroplasts.

Key Components & Steps:

CO2 Acceptor: — Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar.
Enzyme for Fixation: — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme is bifunctional; it can also bind $O_2$, leading to photorespiration.
First Stable Product: — 3-Phosphoglycerate (3-PGA), a 3-carbon compound. This gives the cycle its C3 designation.
Direct Sugar Product: — Glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar phosphate.

Phases of the Cycle:

1
Carboxylation: — $CO_2$ combines with RuBP, catalyzed by RuBisCO, forming an unstable 6-carbon intermediate that immediately splits into two 3-PGA molecules.
2
Reduction: — 3-PGA is converted to G3P. This phase requires both ATP (for phosphorylation of 3-PGA to 1,3-bisphosphoglycerate) and NADPH (for reduction of 1,3-bisphosphoglycerate to G3P).
3
Regeneration: — Five out of six G3P molecules are used to regenerate three RuBP molecules, ensuring the cycle continues. This phase consumes ATP.

Energy Requirements:

For every 1 $CO_2$ fixed: 3 ATP and 2 NADPH are consumed.
To synthesize 1 molecule of Glucose ($C_6H_{12}O_6$): 6 $CO_2$ must be fixed. This requires a total of 18 ATP and 12 NADPH.

Relationship with Light Reactions: The Calvin Cycle is indirectly dependent on light because it uses the ATP and NADPH produced during the light-dependent reactions. Without these, the cycle cannot proceed.

Photorespiration: A wasteful process where RuBisCO binds $O_2$ instead of $CO_2$, especially in hot, dry conditions. This reduces photosynthetic efficiency. C4 and CAM plants have evolved mechanisms to minimize photorespiration.

Vyyuha Quick Recall

To remember the phases and key inputs: Carbon Really Requires All Nutrients.

Carboxylation: $CO_2$ fixation by RuBisCO
Reduction: 3-PGA to G3P
Regeneration: RuBP from G3P
ATP & NADPH are the 'nutrients' (energy/reducing power) required.