61. The C₄ carbon cycle is a CO₂ concentrating mechanism evolved to reduce photorespiration. The following are stated as important features of the C₄ pathway:
A. The leaves of C₄ plants have Kranz anatomy that distinguishes mesophyll and bundle sheath cells.
B. In the peripheral mesophyll cells, atmospheric CO₂ is fixed by phosphoenol pyruvate carboxylase yielding a four-carbon acid.
C. In the inner layer of mesophyll, NAD-malic enzyme decarboxylates four-carbon add and releases CO₂
D. CO₂ is again re-fixed through Calvin cycle in the bundle sheath cells.
Which one of the following combinations is correct?
(1) B, C and D
(2) A, B and C
(3) A, B and D
(4) A, C and D

Introduction

The C4 carbon cycle is a specialized CO2 concentrating mechanism evolved in certain plants (C4 plants) to reduce photorespiration and increase photosynthetic efficiency, especially under hot and dry conditions. Key anatomical and biochemical features distinguish the C4 pathway and help optimize carbon fixation.

Explanation of the statements

A. The leaves of C4 plants have Kranz anatomy that distinguishes mesophyll and bundle sheath cells.

• Correct. C4 plants exhibit Kranz anatomy, where mesophyll cells surround the bundle sheath cells in a wreath-like arrangement, allowing spatial separation of initial CO2 fixation and the Calvin cycle.

B. In the peripheral mesophyll cells, atmospheric CO2 is fixed by phosphoenolpyruvate carboxylase yielding a four-carbon acid.

• Correct. The enzyme PEP carboxylase in mesophyll cells fixes CO2 to phosphoenolpyruvate (PEP) forming oxaloacetate, a four-carbon molecule, which reduces oxygenase activity of RuBisCO.

C. In the inner layer of mesophyll, NAD-malic enzyme decarboxylates four-carbon acid and releases CO2.

• Incorrect. The decarboxylation step occurs mainly in the bundle sheath cells, not the “inner layer of mesophyll.” NAD-malic enzyme (NAD-ME) is one type of decarboxylase found in bundle sheath cells in some C4 species. The mesophyll cells are the outer layer, not the inner layer.

D. CO2 is again re-fixed through Calvin cycle in the bundle sheath cells.

• Correct. The CO2 released by decarboxylation in bundle sheath cells enters the Calvin cycle, where RuBisCO fixes it efficiently under high CO2 concentrations.

Why option (3) A, B and D is correct

• Statements A, B, and D accurately describe major structural and biochemical features of the C4 pathway—Kranz anatomy, initial CO2 fixation in mesophyll with PEP carboxylase, and final CO2 refixation in bundle sheath cells by Calvin cycle enzymes.

• Statement C incorrectly locates NAD-malic enzyme decarboxylation in mesophyll cells instead of bundle sheath cells, so it is false.

Hence, the correct combination is option (3): A, B, and D.

This explanation aligns with biochemistry and plant physiology principles used in competitive exams and academic curricula.