Biological Oxidation (ETC)

Biological Oxidation & Electron Transport Chain

The Electron Transport Chain (ETC) couples the oxidation of NADH and FADH₂ to the synthesis of ATP. It is located in the inner mitochondrial membrane and accounts for ~90% of cellular ATP production.

ETC Components (4 Complexes)

  • Complex I (NADH Dehydrogenase): NADH → NAD+ (electrons enter via FMN → Fe-S clusters → CoQ). Pumps 4 H+ across membrane. Inhibited by: Rotenone, Amytal (barbiturate), MPP+ (Parkinson's toxin).
  • Complex II (Succinate Dehydrogenase): FADH₂ → FAD (electrons enter via Fe-S → CoQ). Does NOT pump H+ — that is why FADH₂ yields less ATP than NADH. Inhibited by Malonate (competitive), Carboxin.
  • Coenzyme Q (Ubiquinone): Mobile electron carrier in the inner membrane. Accepts electrons from both Complex I and II → transfers to Complex III. Can accept 2e⁻ (fully reduced = QH₂, ubiquinol).
  • Complex III (Cytochrome bc1): QH₂ → Cyt c (Q cycle). Pumps 4 H+ per 2e⁻. Inhibited by: Antimycin A, Myxothiazol.
  • Cytochrome c: Mobile peripheral protein on outer face of inner membrane; carries electrons one at a time from Complex III → IV. Also released during apoptosis.
  • Complex IV (Cytochrome c Oxidase): 4 Cyt c → O₂ → 2H₂O (final electron acceptor). Pumps 4 H+ per 4e⁻. Inhibited by: Cyanide, Azide, CO (bind Fe of Cyt a3), H₂S. CO poisoning: cells cannot use O₂ despite adequate delivery.
  • Complex V (ATP Synthase, F₁F₀): Proton flow back through F₀ (driven by ΔΨ + ΔpH = proton-motive force) → rotates γ subunit → synthesizes ATP in F₁. Inhibited by Oligomycin (F₀ channel; increases proton-motive force). DCCD also inhibits.

Chemiosmotic Theory (Peter Mitchell, Nobel 1978)

ETC pumps H+ from matrix to intermembrane space → creates electrochemical gradient (proton-motive force: ΔΨ ~−180 mV + ΔpH). H+ flows back through ATP synthase → drives ATP synthesis. ~2.5 ATP per NADH; ~1.5 ATP per FADH₂ (P/O ratios).

Uncouplers

Dissipate proton gradient WITHOUT making ATP → energy released as heat.

  • Thermogenin (UCP-1): Brown adipose tissue; natural uncoupler; generates heat (non-shivering thermogenesis — newborns, cold adaptation)
  • DNP (2,4-dinitrophenol): Proton ionophore; was used as weight loss drug (fatal — no antidote if overdose)
  • Aspirin overdose: Metabolite salicylate can uncouple ETC → high fever, respiratory alkalosis then metabolic acidosis

ATP Yield — Complete Glucose Oxidation

  • Glycolysis: 2 ATP + 2 NADH → 5 ATP (from NADH via malate-aspartate shuttle)
  • Pyruvate → Acetyl-CoA: 2 NADH → 5 ATP
  • TCA (×2): 6 NADH + 2 FADH₂ + 2 GTP → 20 ATP
  • Total: ~30–32 ATP per glucose (modern estimates using actual P/O ratios)

Shuttle Systems

NADH cannot cross inner mitochondrial membrane. Electrons transferred via shuttles:

  • Malate-Aspartate Shuttle (Liver, Heart): Transfers cytosolic NADH → mitochondrial NADH → 2.5 ATP
  • Glycerol-3-Phosphate Shuttle (Brain, Muscle): Cytosolic NADH → mitochondrial FADH₂ → 1.5 ATP only (less efficient)

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

How can I quickly memorize the concepts of Quiz?

The most effective way is to create short, handwritten revision notes and continuously test your knowledge using our interactive Mock Tests. Spaced repetition and active recall are much better than passive reading.

What type of questions are most commonly asked from Quiz?

Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.