🌱 C₄ and CAM Pathways – NEET 2025 Notes by Botany Sir Himansu

🌿 Introduction: Why Study C₄ & CAM Plants?

Every NEET exam asks conceptual, application-based questions on: ✅ C₃ vs C₄

✅ Kranz Anatomy

✅ Photorespiration

✅ PEP Carboxylase vs RuBisCO

✅ CAM pathway adaptations

These topics help students understand how plants survive high temperature, low CO₂, and arid conditions.

Let’s decode the topic simply and perfectly for NEET Aspirants.

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🌾 Section 1: Why Do C₄ and CAM Pathways Exist?

Plants face two major challenges:

✅ 1. High Temperature

Leads to increased photorespiration in C₃ plants.

✅ 2. Low CO₂ Availability

Occurs in:

Hot tropical regions

Saline habitats

Desert areas

To overcome this, plants evolved two special photosynthetic pathways:

🌾 C₄ pathway (Hatch–Slack Pathway)

🌵 CAM pathway (Crassulacean Acid Metabolism)

These adaptations help minimize photorespiration and water loss.

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🌱 Section 2: Understanding Photorespiration (C₂ Cycle)

Photorespiration happens due to the dual nature of RuBisCO:

Carboxylase (uses CO₂) ✅

Oxygenase (uses O₂) ❌

At high temperatures or low CO₂:

👉 RuBisCO binds with O₂ → produces phosphoglycolate → enters the photorespiratory pathway.

Key Points for NEET:

Photorespiration occurs in chloroplast → peroxisome → mitochondria

Wastes ATP

No sugars produced

Common in C₃ plants (Rice, Wheat, Potato)

Absent/minimal in C₄ plants

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🌿 Section 3: C₃ vs C₄ vs CAM — Quick Comparison Table

Feature C₃ Plants C₄ Plants CAM Plants

First stable product 3-PGA OAA (4-C) Malic acid

Photorespiration High Very low Very low

Anatomy Normal Kranz Succulent leaves

CO₂ fixation enzyme RuBisCO PEP Carboxylase PEP Carboxylase (night)

Habitat Cool Hot Desert

Examples Wheat, Rice Maize, Sugarcane Cactus, Pineapple

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🌾 Section 4: C₄ Pathway (Hatch & Slack Pathway)

C₄ pathway evolved to reduce photorespiration by increasing CO₂ concentration around RuBisCO.

✅ Key Features

1. Kranz Anatomy

Bundle sheath cells have large chloroplasts

Mesophyll cells have small chloroplasts

Thick-walled bundle sheath cells (no intercellular spaces)

2. Spatial Separation

C₄ has two cells involved:

Mesophyll cells → PEP Carboxylase fixes CO₂

Bundle sheath cells → Calvin Cycle happens

✅ Steps of C₄ Pathway (NEET must-learn)

(A) Carboxylation (Mesophyll)

CO₂ + PEP → OAA

Enzyme: PEP Carboxylase (no oxygenase activity)

(B) Reduction

OAA → Malate / Aspartate

(C) Transport

Malate transported to bundle sheath cells.

(D) Decarboxylation

Malate → CO₂ + Pyruvate

This CO₂ enriches bundle sheath, eliminating photorespiration.

(E) Regeneration

Pyruvate → PEP (requires ATP)

✅ Energy Cost

C₄ plants require 30 ATP molecules per glucose

(C₃ plants need 18 ATP)

But they compensate by higher productivity.

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🌿 Section 5: Advantages of C₄ Plants

✅ Very high photosynthetic rate

✅ Minimal photorespiration

✅ High water-use efficiency

✅ Thrive in tropical climates

✅ Outperform C₃ plants at high temperature

NEET Fact:

C₄ plants do not show photorespiration due to Kranz anatomy + PEP Carboxylase.

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🌵 Section 6: CAM Pathway (Desert Adaptation)

CAM = Crassulacean Acid Metabolism

Specially adapted for desert/low water conditions.

✅ Night (Dark Phase)

Stomata open (for saving water)

CO₂ + PEP → Oxaloacetic Acid → Malic Acid

Stored in vacuoles.

✅ Day (Light Phase)

Stomata close

Malic acid → CO₂ + Pyruvate

CO₂ enters Calvin Cycle.

✅ Why CAM is unique?

Temporal separation (night vs day)

High water conservation

Used by succulent plants

✅ Examples:

Cactus 🌵

Opuntia

Pineapple 🍍

Bryophyllum

Agave

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🌱 Section 7: C₃, C₄, CAM – Best NEET Trick

“C₄ = Spatial separation; CAM = Temporal separation.”

C₄ → Two cells

CAM → Night & Day mechanism

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🌸 Section 8: NEET PYQs (High Yield)

✅ NEET 2023

Kranz anatomy is characteristic of:

✅ C₄ Plants

✅ NEET 2022

Which enzyme in C₄ plants has no oxygenase activity?

✅ PEP Carboxylase

✅ NEET 2021

CAM plants open stomata during:

✅ Night

✅ NEET 2020

Photorespiration does not occur in:

✅ C₄ Plants

✅ NEET 2019

First stable product of C₄ cycle:

✅ Oxaloacetic Acid (OAA)

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🌳 Section 9: Flow Chart – Perfect for Revision

C₃ → High photorespiration → Low productivity

C₄ → Spatial separation → High productivity

CAM → Night CO₂ fixation → Water conservation

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🌿 Section 10: Conclusion

C₄ and CAM pathways are brilliant evolutionary adaptations that allow plants to survive extreme climates.

For NEET: ✅ Memorize enzymes

✅ Understand Kranz anatomy

✅ Compare pathways

✅ Revise PYQs

✅ Remember examples

Mastering this chapter guarantees full marks in related questions.