Question
Describe the Calvin cycle (C pathway). Explain the three stages — carbon fixation, reduction, and regeneration of RuBP. What is the net input and output per cycle?
(NEET, CBSE Class 11 — Photosynthesis in Higher Plants)
Solution — Step by Step
The enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase) catalyses the fixation of CO onto a 5-carbon molecule RuBP (ribulose-1,5-bisphosphate). The unstable 6C intermediate immediately splits into two molecules of 3-PGA (3-phosphoglyceric acid, a 3C molecule). This is why it is called the C pathway.
Each 3-PGA is phosphorylated by ATP and then reduced by NADPH (both from the light reactions) to form G3P (glyceraldehyde-3-phosphate). For every 3 CO fixed: 6 ATP and 6 NADPH are consumed in this step to form 6 G3P.
Of the 6 G3P molecules formed, 5 G3P are used to regenerate 3 RuBP (the CO acceptor), consuming 3 more ATP. The remaining 1 G3P is the net gain — this exits the cycle and is used to synthesise glucose, sucrose, starch, and other organic molecules.
graph TD
A["3 CO₂"] -->|"RuBisCO"| B["3 RuBP (5C) + 3 CO₂"]
B --> C["6 × 3-PGA (3C)"]
C -->|"6 ATP + 6 NADPH"| D["6 × G3P (3C)"]
D -->|"5 G3P"| E["Regeneration<br/>3 ATP used"]
E --> F["3 RuBP (recycled)"]
D -->|"1 G3P (net output)"| G["→ Glucose, Sucrose, Starch"]Why This Works
The Calvin cycle is essentially a carbon-fixing machine: it takes inorganic CO and converts it to organic carbon (G3P) using the ATP and NADPH generated by the light reactions. Three turns of the cycle fix 3 CO and produce 1 net G3P (3C). Six turns produce 2 G3P, which can combine to form 1 glucose (6C).
Per glucose (6 CO fixed): 18 ATP + 12 NADPH are consumed.
RuBisCO is the most abundant enzyme on Earth — and one of the slowest. It fixes only about 3 CO molecules per second. Plants compensate by producing massive amounts of it.
Alternative Method — Tracking Carbon Atoms
3 CO (3C total) + 3 RuBP (15C total) = 6 x 3-PGA (18C total). After reduction: 6 G3P (18C). Five G3P (15C) regenerate 3 RuBP. One G3P (3C) is the net product. Carbon in = Carbon out.
For NEET: the first stable product of C pathway is 3-PGA (not G3P). The CO acceptor is RuBP. The enzyme is RuBisCO. These three facts appear in NEET almost every year. Also, the Calvin cycle occurs in the stroma of chloroplasts.
Common Mistake
Students confuse G3P with 3-PGA. 3-PGA is the first stable product of carbon fixation. G3P is the reduced form (after ATP and NADPH act on 3-PGA). The sequence is: CO + RuBP to 3-PGA to G3P. Getting these two confused is a common NEET error.