CBSE Class 11 Biology – Respiration in Plants | Complete Study Module & Revision Notes (NCERT Based)

Course & Examination Details

Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 14 – Respiration in Plants
Prescribed Textbook: NCERT
Board: CBSE

CBSE Board Examination Context

• Respiration in Plants is a core metabolic chapter of Plant Physiology
• Frequently assessed through:
  • MCQs and Assertion–Reason questions
  • Short and Long Answer Questions
  • Case-based / competency-based questions
• Forms the conceptual foundation for photosynthesis–respiration relationship, energy metabolism, and growth
• Highly relevant for Class 11 annual exams and Class 12 Biology (cellular respiration)

Introduction to Respiration in Plants

Respiration is a vital physiological and biochemical process in which organic compounds such as glucose are oxidised to release energy required for various life activities. Unlike photosynthesis, respiration occurs continuously, day and night, in all living plant cells.

In plants, respiration:

• Supplies ATP for growth, repair, and maintenance
• Provides intermediates for biosynthetic pathways
• Maintains cellular organisation and metabolism

The overall reaction of aerobic respiration is:

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

Respiration is a stepwise enzyme-controlled process, ensuring efficient energy transfer and minimal loss as heat.

Types of Respiration in Plants

Based on oxygen requirement, respiration is of two main types:

1. Aerobic Respiration

• Occurs in presence of oxygen
• Complete oxidation of glucose
• Produces maximum energy (ATP)
• Includes glycolysis, Krebs cycle, and electron transport system

2. Anaerobic Respiration (Fermentation)

• Occurs in absence of oxygen
• Incomplete oxidation of glucose
• Produces less energy
• End products include ethanol or lactic acid

1. Glycolysis (Embden–Meyerhof–Parnas Pathway)

1.1 Definition

Glycolysis is the first stage of respiration in which one molecule of glucose (6-carbon) is partially oxidised into two molecules of pyruvic acid (3-carbon).

1.2 Site of Glycolysis

• Occurs in the cytoplasm of the cell
• Does not require oxygen

1.3 Phases of Glycolysis

A. Preparatory Phase

• Glucose is phosphorylated using ATP
• Formation of fructose-1,6-bisphosphate
• Energy investment phase

B. Pay-off Phase

• Splitting of fructose-1,6-bisphosphate
• Formation of pyruvic acid
• Production of ATP and NADH

1.4 Energy Yield of Glycolysis

1.5 Fate of Pyruvic Acid

Depending on oxygen availability, pyruvic acid may:

• Enter mitochondria for aerobic respiration
• Undergo fermentation under anaerobic conditions

1.6 Significance of Glycolysis

• Universal pathway in living organisms
• Supplies intermediates for other metabolic processes
• Links aerobic and anaerobic respiration

2. Krebs Cycle (Citric Acid Cycle / TCA Cycle)

2.1 Definition

The Krebs cycle is a cyclic series of reactions in which acetyl-CoA is completely oxidised to carbon dioxide with the release of energy.

2.2 Site

• Occurs in the mitochondrial matrix

2.3 Formation of Acetyl-CoA (Link Reaction)

• Pyruvic acid enters mitochondria
• Converted into acetyl-CoA
• CO₂ is released
• NAD⁺ is reduced to NADH

2.4 Steps of Krebs Cycle

• Acetyl-CoA (2C) combines with oxaloacetic acid (4C)
• Forms citric acid (6C)
• Through a series of reactions:
  • CO₂ is released
  • Hydrogen atoms are removed
  • NADH and FADH₂ are produced

2.5 Energy Yield per Acetyl-CoA

2.6 Significance of Krebs Cycle

• Central metabolic pathway
• Provides energy-rich molecules
• Supplies intermediates for amino acid and lipid synthesis

3. Electron Transport System (ETS) and Oxidative Phosphorylation

3.1 Definition

The Electron Transport System is a series of electron carriers through which electrons from NADH and FADH₂ are transferred to oxygen.

3.2 Site

• Located on the inner mitochondrial membrane (cristae)

3.3 Components of ETS

• NADH dehydrogenase
• Cytochromes
• Coenzyme Q (ubiquinone)
• FAD-linked dehydrogenases

3.4 Mechanism

• Electrons pass through carriers
• Energy released pumps protons into intermembrane space
• Proton gradient develops
• ATP is synthesized by ATP synthase (chemiosmosis)

3.5 Role of Oxygen

• Oxygen acts as final electron acceptor
• Combines with electrons and protons to form water

3.6 ATP Yield

• From one glucose molecule:
  • NADH and FADH₂ generate ATP
  • Total ATP yield: ~36 ATP in plants

3.7 Significance of ETS

• Major ATP-producing stage
• Efficient energy conversion
• Maintains redox balance

4. Fermentation (Anaerobic Respiration)

4.1 Definition

Fermentation is the anaerobic breakdown of glucose producing energy in absence of oxygen.

4.2 Site

• Occurs in the cytoplasm

4.3 Types of Fermentation

A. Alcoholic Fermentation

• Occurs in yeast and plant tissues
• Pyruvic acid → Ethanol + CO₂
• Enzymes involved: pyruvate decarboxylase and alcohol dehydrogenase

B. Lactic Acid Fermentation

• Occurs in muscles and some microbes
• Pyruvic acid → Lactic acid

4.4 Energy Yield

• Net gain: 2 ATP per glucose
• Much lower compared to aerobic respiration

4.5 Significance of Fermentation

• Allows survival under anaerobic conditions
• Regenerates NAD⁺ for glycolysis
• Used industrially in brewing and baking

Respiratory Quotient (RQ)

Definition

Respiratory Quotient is the ratio of volume of CO₂ evolved to volume of O₂ consumed.

RQ = CO₂ released / O₂ consumed

Values of RQ

• Carbohydrates: 1
• Fats: <1
• Proteins: <1
• Organic acids: >1

Amphibolic Nature of Respiration

Respiration is amphibolic, meaning it functions both as:

• Catabolic pathway (breakdown of molecules)
• Anabolic pathway (providing intermediates for synthesis)

Examples:

• Krebs cycle intermediates used in biosynthesis
• Glycolysis intermediates form amino acids and fats

Comparison: Aerobic vs Anaerobic Respiration

Revision Notes (Quick Recall)

• Glycolysis occurs in cytoplasm
• Krebs cycle in mitochondrial matrix
• ETS on inner mitochondrial membrane
• Oxygen is final electron acceptor
• Fermentation produces only 2 ATP
• Respiration is amphibolic
• RQ indicates respiratory substrate

Important Exam-Oriented Points

• Energy accounting of respiration
• Differences between aerobic and anaerobic respiration
• Role of ETS and ATP synthase
• Significance of fermentation
• Amphibolic nature of respiration

Common Student Mistakes to Avoid

• Confusing glycolysis with Krebs cycle
• Forgetting site of ETS
• Writing incorrect ATP count
• Ignoring role of oxygen
• Mixing fermentation with aerobic respiration

Chapter Summary

Respiration in plants is a multistep process involving glycolysis, Krebs cycle, and electron transport system, leading to efficient energy production. In absence of oxygen, plants can respire anaerobically through fermentation. Respiration not only releases energy but also provides metabolic intermediates essential for growth and maintenance, making it a central life process.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based school assessments
• Conceptual revision and foundation for Class 12 Biology