Respiration in Plants Class 11 Notes | CBSE Biology Chapter 12

Table of Contents

• Key Concepts - Cellular respiration, glycolysis, fermentation, aerobic respiration, Krebs cycle, ETS, ATP yield, amphibolic pathway, RQ
• Weightage in Board & Entrance Exams
• Important Definitions
• Solved Examples
• Important Questions for Board Exams
• Quick Revision Points

Key Concepts

1. Cellular Respiration - The Basics

Cellular respiration is the enzyme-controlled, step-wise oxidation of food (mainly glucose) inside the cell to release energy stored in C–C bonds, trapping it as ATP. Unlike burning, it happens in many small steps so energy is released gradually and safely.

The substrate that is oxidised is called the respiratory substrate - usually carbohydrates, but fats and proteins can also be used. The molecule that supplies energy on demand is ATP (adenosine triphosphate), often called the “energy currency” of the cell.

Overall Equation (aerobic)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat)

2. Glycolysis (EMP Pathway)

Glycolysis (“splitting of sugar”) is the breakdown of one glucose molecule into two molecules of pyruvic acid. It was given by Embden, Meyerhof and Parnas, so it is called the EMP pathway.

• Site: cytoplasm (cytosol) - does NOT need oxygen.
• It is common to both aerobic and anaerobic respiration.
• Net products per glucose: 2 pyruvate + 2 ATP (net) + 2 NADH + 2H⁺.

[DIAGRAM: Glucose → glucose-6-phosphate → fructose-1,6-bisphosphate → splits into 2 triose phosphate (PGAL) → 2 pyruvic acid; 2 ATP used in the investment phase, 4 ATP made in the payoff phase, net 2 ATP.]

Key idea: 2 ATP are spent early (priming) and 4 ATP are produced later, giving a net gain of 2 ATP per glucose.

3. Fate of Pyruvate

The fate of pyruvic acid depends on whether oxygen is available and on the type of cell.

• Aerobic respiration: pyruvate enters the mitochondrion and is completely oxidised to CO₂ and H₂O.
• Fermentation (anaerobic): pyruvate is incompletely broken down in the cytoplasm without oxygen.

4. Fermentation (Anaerobic Respiration)

Fermentation is the incomplete oxidation of glucose under anaerobic conditions. Only the 2 ATP of glycolysis are gained - most energy stays locked in the products.

Two Common Types

• Alcoholic fermentation: pyruvate → ethanol + CO₂ (by yeast). Enzymes: pyruvate decarboxylase and alcohol dehydrogenase.
• Lactic acid fermentation: pyruvate → lactic acid (in some bacteria and in our muscles during heavy exercise). Enzyme: lactate dehydrogenase.

Important: In both, NADH is reoxidised to NAD⁺ so glycolysis can continue. Fermentation yields less than 7% of the energy of glucose and can be dangerous (alcohol/acid accumulation).

5. Aerobic Respiration - Overview

Aerobic respiration is the complete oxidation of pyruvate to CO₂ and H₂O in the presence of oxygen, occurring in the mitochondria. It has two major steps after glycolysis: the Krebs cycle and the electron transport system.

First, pyruvate is converted in the mitochondrial matrix by pyruvate dehydrogenase:

Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺

6. Krebs Cycle (TCA / Citric Acid Cycle)

The Krebs cycle, also called the tricarboxylic acid (TCA) cycle or citric acid cycle, was discovered by Hans Krebs. It is the complete oxidation of acetyl CoA.

• Site: mitochondrial matrix.
• Acetyl CoA (2C) combines with oxaloacetic acid (4C) to form citric acid (6C).
• The cycle regenerates oxaloacetic acid, so it turns continuously.

[DIAGRAM: Acetyl CoA + OAA → citrate → isocitrate → α-ketoglutarate → succinyl CoA → succinate → fumarate → malate → OAA; CO₂ released twice, NADH formed thrice, FADH₂ once, GTP/ATP once.]

Yield per turn (per acetyl CoA)

• 3 NADH + 1 FADH₂ + 1 GTP (≈ 1 ATP) + 2 CO₂.
• Since one glucose gives 2 acetyl CoA, the cycle turns twice per glucose.

7. Electron Transport System (ETS) and Oxidative Phosphorylation

The electron transport system is a chain of carriers on the inner mitochondrial membrane that passes electrons from NADH and FADH₂ to oxygen, the final electron acceptor.

• Electrons flow through Complex I → ubiquinone → Complex III → cytochrome c → Complex IV → O₂.
• Oxygen accepts electrons and protons to form water (H₂O) - this is why O₂ is vital.

Oxidative phosphorylation is the synthesis of ATP using the energy of this electron flow. As electrons move, protons are pumped into the intermembrane space, creating a gradient. Protons flow back through ATP synthase (F₀–F₁ particle), driving ATP formation - the chemiosmotic hypothesis (Peter Mitchell).

• 1 NADH → 3 ATP
• 1 FADH₂ → 2 ATP

8. The Respiratory Balance Sheet (ATP Yield)

Adding up every step gives the total ATP from one glucose under ideal aerobic conditions.

Net = 4 + 34 = 38 ATP per glucose. In eukaryotic cells, glycolytic NADH must be shuttled into the mitochondrion, costing energy, so the practical yield is often quoted as 36 ATP.

Note: These assumptions are theoretical (one substrate metabolised at a time, perfect functioning). Real values vary, so always state the assumptions in answers.

9. Amphibolic Pathway

The respiratory pathway is described as amphibolic because it works in both directions - it is catabolic (breaking molecules down) and anabolic (building molecules up).

• Intermediates like acetyl CoA and α-ketoglutarate are withdrawn to synthesise fatty acids and amino acids.
• Fats are broken into glycerol and fatty acids; proteins into amino acids - these feed into respiration at various points.

Key idea: Because the pathway both breaks down and builds up substrates, it is amphibolic, not purely catabolic.

10. Respiratory Quotient (RQ)

The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.

RQ = Volume of CO₂ released / Volume of O₂ consumed

Note: In anaerobic respiration of carbohydrates (no O₂ consumed), RQ is infinite (∞).