Question
What is the difference between C3 and C4 plants? Explain with reference to the first stable product of carbon fixation and the role of photorespiration.
Solution — Step by Step
In all plants, CO₂ fixation happens in the Calvin cycle — but the first stable product differs. In C3 plants, CO₂ combines with RuBP (a 5-carbon compound) via the enzyme RuBisCO to give 3-phosphoglycerate (3-PGA), which has 3 carbons. That’s why they’re called C3 plants.
C4 plants evolved an extra CO₂-concentrating step before the Calvin cycle. CO₂ first combines with PEP (phosphoenolpyruvate) in mesophyll cells via PEP carboxylase, producing oxaloacetate (OAA) — a 4-carbon compound. This is the first stable product in C4 plants. PEP carboxylase has a much higher affinity for CO₂ than RuBisCO does, and critically, it doesn’t react with O₂.
C4 plants have a specialised leaf structure called Kranz anatomy — bundle sheath cells surround the vascular bundles like a wreath (Kranz = wreath in German). OAA is transported from mesophyll cells to bundle sheath cells, where it’s decarboxylated, releasing a concentrated burst of CO₂ right next to RuBisCO. The Calvin cycle then runs normally here.
At high temperatures, O₂ concentration inside leaves rises relative to CO₂. RuBisCO in C3 plants starts reacting with O₂ instead of CO₂ — this wasteful process is photorespiration. It consumes ATP and releases CO₂ without producing any sugar. C4 plants essentially eliminate photorespiration because CO₂ is pumped in concentrated form to RuBisCO, so it never “mistakes” O₂ for its substrate.
C4 plants fix more carbon per unit time at high temperatures and high light intensities. This is why sugarcane and maize (C4) grow so aggressively in tropical Indian summers, while wheat and rice (C3) are relatively less efficient in the same conditions.
Why This Works
The entire C4 system is essentially a CO₂ pump. Mesophyll cells act as CO₂ collectors, and bundle sheath cells act as the “reaction chamber.” By separating CO₂ collection from the Calvin cycle spatially, C4 plants hand RuBisCO an environment where CO₂ is always in excess — no confusion with O₂.
Photorespiration in C3 plants isn’t a “defect” exactly; RuBisCO evolved when atmospheric O₂ was very low. It’s an evolutionary relic that becomes costly only in today’s high-O₂, high-temperature environments. C4 plants simply evolved a workaround.
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First stable product | 3-PGA (3C) | OAA (4C) |
| CO₂ acceptor | RuBP | PEP |
| Carboxylating enzyme | RuBisCO | PEP carboxylase |
| Leaf anatomy | Mesophyll only | Kranz (mesophyll + bundle sheath) |
| Photorespiration | Yes (significant) | Negligible |
| Examples | Wheat, rice, potato | Maize, sugarcane, sorghum |
Alternative Method — Memory Trick for NEET
If the MCQ asks you to identify C3 or C4 plants from a list, remember: “Wheat and rice feed India, maize and cane earn India.” Wheat/rice/potato → C3. Maize/sugarcane/sorghum/bajra → C4.
NEET PYQs frequently ask: “In which plants does photorespiration NOT occur?” — answer is C4 plants. Also asked as: “Which enzyme fixes CO₂ in mesophyll cells of C4 plants?” — answer is PEP carboxylase (not RuBisCO — that’s only in bundle sheath cells of C4 plants).
For short-answer board questions, structure your answer as: first stable product → anatomy → photorespiration status → example. Four points, full marks.
Common Mistake
Students write “RuBisCO fixes CO₂ in mesophyll cells of C4 plants.” Wrong. RuBisCO is located only in the bundle sheath cells of C4 plants. Mesophyll cells use PEP carboxylase. This distinction is a favourite NEET trap — in C3 plants, RuBisCO is in all mesophyll cells; in C4, it’s compartmentalised to bundle sheath only.
A related error: students say C4 plants “don’t have the Calvin cycle.” They absolutely do — it runs in bundle sheath cells. The C4 pathway is an addition to the Calvin cycle, not a replacement.