Calvin cycle — C3 pathway steps with carbon fixation by RuBisCO

Question

Describe the three stages of the Calvin cycle. How many molecules of ATP and NADPH are consumed to fix 6 molecules of $\text{CO}_2$ into one molecule of glucose? What is the role of RuBisCO?

(NCERT Class 11 — high-weightage for NEET)

Solution — Step by Step

The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the first reaction:

\text{RuBP (C}_5\text{)} + \text{CO}_2 \xrightarrow{\text{RuBisCO}} 2 \times \text{3-PGA (C}_3\text{)}

One 5-carbon molecule (RuBP) combines with $\text{CO}_2$ to form an unstable 6-carbon intermediate, which immediately splits into two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA).

This is why it’s called the C3 pathway — the first stable product is a 3-carbon compound.

3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH from the light reactions:

\text{3-PGA} \xrightarrow{\text{ATP, NADPH}} \text{G3P}

For each $\text{CO}_2$ fixed: 2 ATP and 2 NADPH are consumed in this stage.

Out of every 6 molecules of G3P produced (from 3 $\text{CO}_2$ ), only 1 G3P exits the cycle to contribute to glucose synthesis. The other 5 are recycled.

The 5 remaining G3P molecules are rearranged to regenerate 3 molecules of RuBP (the $\text{CO}_2$ acceptor). This step requires 3 ATP (for 3 $\text{CO}_2$ fixed).

Without regeneration, the cycle would stop — no RuBP means no carbon fixation.

For 1 $\text{CO}_2$ : 3 ATP + 2 NADPH.

For 6 $\text{CO}_2$ (to make 1 glucose):

6 \times 3 = \mathbf{18 \text{ ATP}} \quad \text{and} \quad 6 \times 2 = \mathbf{12 \text{ NADPH}}

The overall equation:

6\text{CO}_2 + 18\text{ATP} + 12\text{NADPH} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 18\text{ADP} + 12\text{NADP}^+ + 18\text{P}_i

Why This Works

The Calvin cycle is essentially a carbon-recycling machine. RuBisCO grabs $\text{CO}_2$ from the atmosphere and attaches it to a pre-existing organic molecule (RuBP). The energy from ATP and NADPH (produced in the light reactions) is then used to convert this into a useful sugar.

The reason 5 out of 6 G3P molecules must be recycled is mathematical necessity — you need to keep the cycle running. If all G3P molecules were siphoned off, there would be no RuBP left to accept new $\text{CO}_2$ .

RuBisCO is the most abundant enzyme on Earth precisely because it’s slow — it processes only about 3 $\text{CO}_2$ molecules per second. Plants compensate by making enormous quantities of it.

Alternative Method — The Numbers Shortcut

For NEET, remember these numbers for one turn of the Calvin cycle (1 $\text{CO}_2$ fixed):

3 ATP consumed (2 in reduction + 1 in regeneration)
2 NADPH consumed (in reduction)

Multiply by 6 for one glucose. That’s it — 18 ATP, 12 NADPH.

NEET often asks: “The Calvin cycle is light-independent. Does it occur in the dark?” The answer is no — it occurs in the light but doesn’t directly use light energy. It depends on ATP and NADPH from the light reactions. In the dark, ATP and NADPH run out and the cycle stops. Call it “light-independent,” not “dark reaction.”

Common Mistake

Students confuse 3-PGA (the first stable product of C3 cycle) with G3P (the reduced product). 3-PGA is a 3-carbon acid. G3P is a 3-carbon sugar phosphate. The reduction step converts 3-PGA to G3P using ATP and NADPH. Mixing up these two names in NEET will cost you marks because the question might specifically ask “what is the first stable product of the Calvin cycle?” — the answer is 3-PGA, not G3P.

Question

Solution — Step by Step

Why This Works

Alternative Method — The Numbers Shortcut

Common Mistake

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