Question
In C4 plants, CO₂ is first fixed in mesophyll cells to form a 4-carbon compound (oxaloacetate). This compound is then transported to bundle sheath cells where CO₂ is released and enters the Calvin cycle. Why does this elaborate two-step process make C4 plants more efficient than C3 plants, especially in hot, dry conditions?
Solution — Step by Step
In C3 plants, RuBisCO fixes CO₂ directly in mesophyll cells. The catch: RuBisCO is a dual-function enzyme — it can bind both CO₂ (carboxylation) and O₂ (oxygenation). When O₂ wins, we get photorespiration, which wastes energy and releases CO₂ instead of fixing it.
C4 plants run a CO₂ concentrating mechanism. Mesophyll cells use PEP carboxylase (PEPC) to fix CO₂ into oxaloacetate (4C), then malate. This malate travels to bundle sheath cells, where it’s decarboxylated — releasing a concentrated burst of CO₂ right where RuBisCO sits.
PEPC has no oxygenase activity — it only fixes CO₂, never O₂. So photorespiration is completely bypassed at the first step. By the time CO₂ reaches RuBisCO in the bundle sheath, its concentration is so high that O₂ doesn’t stand a chance.
Bundle sheath cells in C4 plants are thick-walled and have abundant chloroplasts — this is Kranz anatomy (German for “wreath”, describing the ring of bundle sheath cells around the vascular bundle). The thick walls physically prevent CO₂ from leaking back out, maintaining that high concentration.
C4 plants show zero or negligible photorespiration. They fix CO₂ efficiently even at low atmospheric CO₂ concentrations, high temperatures, and high light intensities. This is why sugarcane, maize, and sorghum thrive in tropical conditions where C3 plants struggle.
Why This Works
The whole C4 advantage comes down to one thing: keeping RuBisCO’s environment CO₂-rich and O₂-poor. At high temperatures, O₂ solubility in water decreases less than CO₂ solubility does — so the CO₂:O₂ ratio drops, making photorespiration worse in C3 plants. C4 plants are immune to this problem.
Think of it as a two-compartment system. Mesophyll = CO₂ capture and transport. Bundle sheath = actual carbon fixation and Calvin cycle. The mesophyll acts as a pump that pre-concentrates CO₂ before handing it to RuBisCO.
This is why C4 plants have a higher CO₂ fixation rate per unit of water lost — their water use efficiency (WUE) is superior. Stomata can stay partially closed (reducing water loss) and the plants still fix CO₂ efficiently, because PEPC in the mesophyll catches even low CO₂ concentrations.
Alternative Method — Comparing via the CO₂ Compensation Point
For NEET, you can also approach this from the CO₂ compensation point angle.
The CO₂ compensation point is the CO₂ concentration at which photosynthesis rate equals respiration rate (net gas exchange = zero).
| Plant Type | CO₂ Compensation Point |
|---|---|
| C3 plants | 25–100 ppm |
| C4 plants | 0–10 ppm |
C4 plants can keep fixing CO₂ even when atmospheric CO₂ drops very low — because PEPC is far more efficient at capturing dilute CO₂ than RuBisCO is. This directly demonstrates their superior efficiency without needing to explain the full biochemistry.
NEET often asks which plants have a lower CO₂ compensation point or which plants show no photorespiration. The answer is always C4 plants. Bundle sheath chloroplasts in C4 plants are agranal (lack grana) — this appears as a separate 1-mark question frequently.
Common Mistake
Most students write “C4 plants don’t have photorespiration because they have Kranz anatomy.” This is backwards — Kranz anatomy is the structural feature that enables CO₂ concentration, but the actual suppression of photorespiration happens because RuBisCO is exposed to very high CO₂. The anatomy supports the biochemistry, not the other way around. Write it in the correct causal order in NEET answers.
A second trap: confusing the two carboxylation steps. PEP carboxylase fixes CO₂ in mesophyll cells (first step). RuBisCO fixes CO₂ in bundle sheath cells (second step, in the Calvin cycle). Many students swap these and lose marks on assertion-reason questions.
Key facts for NEET revision:
- First CO₂ acceptor in C4: PEP (phosphoenolpyruvate), 3C compound
- First stable product in C4: OAA (oxaloacetate), 4C compound
- First CO₂ acceptor in C3: RuBP (ribulose-1,5-bisphosphate), 5C compound
- First stable product in C3: 3-PGA (3-phosphoglyceric acid), 3C compound
- C4 examples: maize, sugarcane, sorghum, Amaranthus
- C3 examples: wheat, rice, potato, most trees