Calvin cycle C₃ pathway — carbon fixation, reduction, regeneration

Question

Describe the Calvin cycle (C₃ pathway) of photosynthesis. Explain the three stages — carbon fixation, reduction, and regeneration of RuBP — with the role of key enzymes and molecules at each stage. How many turns of the cycle are needed to produce one molecule of glucose?

Solution — Step by Step

The enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase) fixes one molecule of CO₂ by combining it with the 5-carbon compound RuBP (ribulose-1,5-bisphosphate). This produces an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-phosphoglyceric acid — the first stable product, hence “C₃ pathway”).

\text{CO}_2 + \text{RuBP (5C)} \xrightarrow{\text{RuBisCO}} 2 \times \text{3-PGA (3C)}

Each 3-PGA molecule is phosphorylated by ATP and then reduced by NADPH to form G3P (glyceraldehyde-3-phosphate), also called PGAL. This is the actual “reduction” step — carbon goes from an oxidised form (in 3-PGA) to a reduced form (in G3P).

For every 3 CO₂ fixed, we get 6 G3P molecules. Only one G3P (out of 6) exits the cycle as net gain. The other 5 are used for regeneration.

The remaining 5 molecules of G3P (total 15 carbons) are rearranged using ATP to regenerate 3 molecules of RuBP (total 15 carbons). This requires additional ATP (but not NADPH).

Without regeneration, the cycle would stop after one turn — no RuBP, no fixation.

flowchart TD
    A[3 CO₂ molecules enter] --> B[RuBisCO fixes CO₂ + RuBP]
    B --> C[6 molecules of 3-PGA]
    C --> D[6 ATP + 6 NADPH used]
    D --> E[6 G3P molecules]
    E --> F[1 G3P exits as net product]
    E --> G[5 G3P remain]
    G --> H[3 ATP used for regeneration]
    H --> I[3 RuBP regenerated]
    I --> B

The Numbers per Glucose

One glucose (6C) needs 6 CO₂. So the Calvin cycle turns 6 times per glucose molecule.

Per Glucose (6 turns)	Amount
CO₂ fixed	6
ATP consumed	18 (12 for reduction + 6 for regeneration)
NADPH consumed	12
G3P produced (net)	2 (which combine to form 1 glucose)

Why This Works

The Calvin cycle is essentially the reverse of oxidation — we take CO₂ (fully oxidised carbon) and reduce it to sugar (reduced carbon) using energy from ATP and reducing power from NADPH. Both come from the light reactions.

RuBisCO is the most abundant enzyme on Earth, and it is slow (about 3 reactions per second). Plants compensate by producing enormous amounts of it — up to 50% of total leaf protein.

Alternative Method

C₄ and CAM plants have evolved modifications to concentrate CO₂ around RuBisCO, reducing photorespiration. But the Calvin cycle itself is the same in all plants — C₃, C₄, and CAM. The difference is in how CO₂ reaches RuBisCO, not in the cycle itself.

Common Mistake

Students often say “6 G3P molecules form 1 glucose.” Actually, the cycle produces 6 G3P per 3 CO₂, but only 1 G3P is the net gain. You need 6 turns of the cycle (6 CO₂) to get 2 net G3P molecules, which combine to form 1 glucose. The ATP and NADPH count per glucose (18 ATP, 12 NADPH) is derived from this logic.

RuBisCO is both a carboxylase (fixes CO₂) and an oxygenase (fixes O₂ — leading to photorespiration). In C₃ plants, about 25% of fixed carbon is lost to photorespiration. This is why C₄ plants like maize and sugarcane are more efficient in hot climates.