Q.55 Given below are two statements:
Statement-I: In C4 plants, carboxylation of PEP occurs in bundle sheath.
Statement-II: Decarboxylation of C4 acids occurs in mesophyll cells of C4 plants.
In the light of the above statements, choose the most appropriate answer
from the options given below:
(A) Both Statement-I and Statement-II are correct
(B) Both Statement-I and Statement-II are incorrect
(C) Statement-I is correct but Statement-II is incorrect
(D) Statement-I is incorrect but Statement-II is correct
C4 plants perform PEP carboxylation in mesophyll cells, not bundle sheath, while decarboxylation of C4 acids occurs in bundle sheath. Both statements are incorrect, making option (B) correct.
Introduction
C4 plants carboxylation location follows Kranz anatomy where mesophyll and bundle sheath cells specialize distinctly. Statement-I claims PEP carboxylation in bundle sheath (incorrect), while Statement-II places C4 acid decarboxylation in mesophyll (also incorrect). Understanding this spatial separation is crucial for photosynthesis efficiency in crops like maize, sugarcane.
C4 Pathway Locations
Mesophyll cells (outer layer):
-
PEP + CO₂ → Oxaloacetate (4C) via PEP carboxylase
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Oxaloacetate → Malate/Aspartate (transported to bundle sheath)
Bundle sheath cells (inner layer):
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C4 acids decarboxylated → CO₂ + pyruvate
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CO₂ + RuBP → 3PGA via RuBisCO (Calvin cycle)
This CO₂ pump minimizes photorespiration by concentrating CO₂ around RuBisCO.
Statement Analysis
| Statement | Claim | Reality | Correct? |
|---|---|---|---|
| I | PEP carboxylation in bundle sheath | Occurs in mesophyll chloroplasts/cytosol | Incorrect |
| II | C4 acid decarboxylation in mesophyll | Occurs in bundle sheath chloroplasts | Incorrect |
Option Breakdown
-
(A) Both correct: Wrong—both locations reversed.
-
(B) Both incorrect: Correct—matches C4 pathway biochemistry.
-
(C) I correct, II incorrect: Wrong—I is false.
-
(D) I incorrect, II correct: Wrong—II is also false.
Answer: (B) – Standard C4 photosynthesis textbook classification.
Key Visual Summary
Mesophyll: PEP → C4 acid (PEP carboxylase)
↓ Transport
Bundle sheath: C4 acid → CO₂ → Calvin cycle (RuBisCO)
Common Exam Trap: Students confuse initial fixation (mesophyll) with final CO₂ release (bundle sheath). The CO₂-concentrating mechanism demands this precise compartmentation.
