Respiration in Plants 🌬️ | Class 11 Notes | NEET | NCERT

Introduction

• Respiration = Biochemical process of releasing energy (ATP) from organic compounds (mainly glucose).
• Occurs in all living cells.
• Opposite to photosynthesis.
• Energy is released in a stepwise, controlled manner (not explosive).
• ATP is the universal energy currency.

Equation (Aerobic respiration):

$$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+38ATP$$

Types of Respiration

1. Aerobic Respiration

• Requires oxygen.
• Complete oxidation of glucose → CO₂ + H₂O.
• Occurs in mitochondria.
• Produces maximum ATP (36–38 ATP per glucose).

Major steps:

1. Glycolysis (cytoplasm).
2. Pyruvate oxidation (link reaction).
3. Krebs cycle (mitochondrial matrix).
4. Electron Transport System (inner mitochondrial membrane).

2. Anaerobic Respiration (Fermentation)

• No oxygen required.
• Incomplete breakdown of glucose.
• Occurs in cytoplasm.
• Produces 2 ATP only per glucose.

Pathways:

Alcoholic fermentation (yeast, some bacteria):
$$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2+2ATP$$

Lactic acid fermentation (muscles, certain microbes):
$$C_6{H}_{12}{O}_6\to 2C{H}_{3}CHOHCOOH+2ATP$$

Glycolysis (Embden–Meyerhof Pathway)

• First step of both aerobic and anaerobic respiration.
• Occurs in cytoplasm.
• Glucose (6C) → 2 Pyruvate (3C).
• Net products: 2 ATP + 2 NADH.

Pyruvate Fate

• Aerobic: → Acetyl CoA (enters Krebs cycle).
• Anaerobic: → Lactic acid or ethanol.

Krebs Cycle (Citric Acid Cycle / TCA Cycle)

• Discovered by Hans Krebs.
• Occurs in mitochondrial matrix.
• Starts with Acetyl CoA (2C) + Oxaloacetate (4C) → Citrate (6C).

Steps (simplified):

1. Acetyl CoA + OAA → Citrate (6C).
2. Citrate → Isocitrate.
3. Isocitrate → α-ketoglutarate (5C) + CO₂ + NADH.
4. α-ketoglutarate → Succinyl CoA (4C) + CO₂ + NADH.
5. Succinyl CoA → Succinate + GTP (→ ATP).
6. Succinate → Fumarate + FADH₂.
7. Fumarate → Malate.
8. Malate → Oxaloacetate + NADH.

Net gain (per Acetyl CoA):

• 3 NADH
• 1 FADH₂
• 1 GTP (≈1 ATP)
• 2 CO₂

Since 1 glucose → 2 Acetyl CoA, total yield = 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂.

Electron Transport System (ETS) and Oxidative Phosphorylation

• Occurs in inner mitochondrial membrane (cristae).
• Involves a series of electron carriers:

NADH → FMN → Fe-S proteins → CoQ → Cyt b → Cyt c1 → Cyt c → Cyt a → Cyt a3 → O₂.

Key points:

• NADH donates electrons at Complex I (produces 3 ATP).
• FADH₂ donates at Complex II (produces 2 ATP).
• O₂ is the final electron acceptor → forms H₂O.
• Chemiosmosis: Protons pumped into intermembrane space; flow back via ATP synthase, generating ATP.

Energy Yield Summary (Aerobic Respiration of 1 Glucose)

Step	ATP/NADH/FADH₂	ATP Equivalent
Glycolysis	2 ATP + 2 NADH	2 + 6 = 8 ATP
Pyruvate oxidation (Link)	2 NADH	6 ATP
Krebs Cycle	2 ATP + 6 NADH + 2 FADH₂	2 + 18 + 4 = 24 ATP
Total	–	38 ATP (max)

(Some books mention 36 ATP due to transport losses.)

Anaerobic vs Aerobic Respiration – Quick Comparison

Feature	Aerobic	Anaerobic
O₂ requirement	Yes	No
Site	Cytoplasm + Mitochondria	Cytoplasm only
End products	CO₂ + H₂O	Ethanol + CO₂ / Lactic acid
ATP yield	36–38	2
Efficiency	High	Low

Respiratory Quotient (RQ)

$$RQ=\frac{C{O}_{2}released}{O_{2}consume\mathrm{d}}$$

• Carbohydrates = 1
• Fats < 1 (≈0.7)
• Proteins ≈ 0.8
• Anaerobic respiration = ∞ (since O₂ = 0)

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