Given below are two statements: Statement I: The primary CO2 acceptor in C4 plants is phosphoenolpyruvate and is found in the mesophyll cells. Statement II: Mesophyll cells of C4 plants lack RuBisCo enzyme. In the light of the above statements, choose the correct answer from the options given below:

Both Statement I and Statement II are correct.

Statement I is correct but Statement II is incorrect.

Statement I is incorrect but Statement II is correct.

Both Statement I and Statement II are incorrect.

Hatch and Slack Pathway

ஹேட்ச் மற்றும் ஸ்லாக் வழிமுறை

NCERT Class 11 · Photosynthesis in Higher Plants · 4 NEET questions

Stop confusing C4 plants with C3 plants. C4 plants like sugarcane and maize have evolved to minimize photorespiration, a wasteful process that ruins efficiency in C3 plants. If you don't master the Hatch-Slack pathway, you are losing 4 marks on a guaranteed question.

The idea

The Hatch-Slack pathway occurs in mesophyll and bundle sheath cells. Phosphoenolpyruvate (PEP) is the primary CO2 acceptor in mesophyll cells, catalyzed by PEP carboxylase. This forms the 4-carbon compound Oxaloacetic acid (OAA). CO2 is then transported to bundle sheath cells, where it enters the Calvin cycle to produce sugars.

Why it matters

It allows plants to fix CO2 efficiently even at low concentrations and high temperatures by bypassing photorespiration.

NEET lens

NEET focuses on the enzyme PEP carboxylase and the location of the Calvin cycle. Remember: PEP carboxylase is in mesophyll, RuBisCO is in bundle sheath. Do not confuse the primary acceptor (PEP) with the final acceptor (RuBP). Expect a diagram-based question asking to label the cell types.

Clinical significance

While this is a plant mechanism, it mirrors the efficiency of metabolic compartmentalization. In humans, failure to compartmentalize metabolic pathways leads to toxic metabolite accumulation, similar to how photorespiration toxicity stunts plant growth. Efficiency in biology is always about spatial separation of incompatible reactions.

Introduction

The Hatch-Slack pathway (C4 cycle) is the set of reactions in C4 plants where CO₂ is first fixed into a 4-carbon compound (OAA) in mesophyll cells, then transferred to bundle sheath cells where CO₂ is released for the Calvin cycle.

Core concept

Step 1 (Mesophyll cells): CO₂ + PEP (3C) → OAA (4C) via PEP carboxylase. OAA reduced to malate (or transaminated to aspartate). Step 2 (Transport): Malate/aspartate transported to bundle sheath cells via plasmodesmata. Step 3 (Bundle sheath): Malate decarboxylated → pyruvate (3C) + CO₂. CO₂ enters Calvin cycle with RuBisCO. Step 4: Pyruvate returns to mesophyll → regenerated to PEP by pyruvate phosphate dikinase (PPDK) using ATP.

Go deeper

The C4 pump is essentially a CO₂-concentrating mechanism: CO₂ is 'pumped' from mesophyll (low concentration) to bundle sheath (high concentration) against a gradient, using ATP. This keeps CO₂ concentration around RuBisCO very high → no oxygenase activity → no photorespiration.

From Class 10

C4 plants like sugarcane are more efficient than most plants. The Hatch and Slack pathway explains how they fix carbon before the Calvin cycle.