Chapter Summary

Cellular respiration is a stepwise, enzyme-controlled catabolic process in which respiratory substrates (mainly glucose) are oxidised to release energy, which is conserved as ATP.

Glycolysis (EMP Pathway)

  • Occurs in the cytoplasm
  • Common to both aerobic and anaerobic respiration
  • One glucose (6C) → two pyruvic acid molecules (3C each)
  • Net yield: 2 ATP (substrate-level phosphorylation) + 2 NADH

Fermentation (Anaerobic Respiration)

  • Occurs in the absence of oxygen
  • Takes place in the cytoplasm
  • Main purpose: Regeneration of NAD⁺

Types:

  • Alcoholic fermentation: Pyruvate → Ethanol + CO₂ (yeast, plants)
  • Lactic acid fermentation: Pyruvate → Lactic acid (muscle cells)
  • Net ATP gain: 2 ATP (only from glycolysis)

Aerobic Respiration (Mitochondria)

Link Reaction (Matrix):

  • Pyruvic acid → Acetyl-CoA
  • Produces 2 NADH + 2 CO₂ per glucose

Krebs Cycle (Matrix):

  • Acetyl-CoA completely oxidised
  • Produces (per glucose):
  • 6 NADH
  • 2 FADH₂
  • 2 ATP/GTP
  • 4 CO₂

Electron Transport System (Inner Mitochondrial Membrane):

  • NADH and FADH₂ donate electrons
  • Proton gradient formed (chemiosmosis)
  • Oxygen is the final electron acceptor
  • ATP synthesised by ATP synthase

Respiratory Balance Sheet

  • Substrate-level ATP: 4
  • Oxidative phosphorylation: 32–34 ATP
  • Total ATP yield: 36–38 per glucose

Amphibolic Pathway

Respiration is amphibolic because its intermediates are used for both:

  • Catabolism: Energy production
  • Anabolism: Fatty acid and amino acid synthesis

Respiratory Quotient (RQ)

RQ = CO₂ evolved / O₂ consumed

  • Carbohydrates: 1.0
  • Fats: < 1 (≈ 0.7)
  • Proteins: < 1 (≈ 0.9)
  • Organic acids: > 1
  • Anaerobic respiration: ∞

Rapid Revision

LOCATIONS:

  • Glycolysis → Cytoplasm
  • Fermentation → Cytoplasm
  • Link Reaction → Mitochondrial matrix
  • Krebs Cycle → Mitochondrial matrix
  • ETS → Inner mitochondrial membrane

NUMBERS TO REMEMBER:

  • Glycolysis: 2 ATP + 2 NADH
  • Krebs (per glucose): 6 NADH + 2 FADH₂ + 2 ATP
  • Total NADH: 10 | Total FADH₂: 2
  • Final ATP: 36–38

ONE-LINE TRIGGERS:

  • NAD⁺ regeneration → Fermentation
  • Final electron acceptor → Oxygen
  • Proton gradient → Chemiosmosis
  • Central metabolic hub → Krebs cycle

Exam Tips (NEET / IAT / Boards)

1. Highest-Yield Areas

  • Locations of each stage
  • Net ATP, NADH, FADH₂ numbers
  • ETS mechanism & chemiosmosis
  • Respiratory Quotient values
  • 36 vs 38 ATP logic

2. 36 ATP vs 38 ATP – How to Remember

  • Cytoplasmic NADH uses shuttle systems
  • Malate–aspartate shuttle → 38 ATP
  • Glycerol phosphate shuttle → 36 ATP

3. Fermentation vs Anaerobic Respiration

  • Fermentation: Organic electron acceptor (pyruvate)
  • True anaerobic respiration: Inorganic acceptor (rare, bacteria)

4. Amphibolic Pathway Trick Think of respiration as a two-way road:

  • Molecules enter for breakdown
  • Intermediates exit for biosynthesis

5. Quick Identification in MCQs

  • CO₂ present → Alcoholic fermentation
  • No CO₂ → Lactic acid fermentation