NADH Oxidation: Fermentation vs Aerobic Respiration
Fermentation (Anaerobic)
Final electron acceptor: Organic molecules (e.g., pyruvate)
Oxidation of NADH is slow
Less energy released (ΔG less negative)
~0 ATP produced directly from NADH
Occurs in cytoplasm
Aerobic Respiration
Final electron acceptor: Oxygen (O₂)
Oxidation of NADH is vigorous and fast
Large energy release (ΔG highly negative)
Yields ~2.5 to 3 ATP per NADH
Occurs in mitochondria via ETC
❓ Why is NADH oxidation slower in Fermentation?
• Organic molecules are weaker oxidizing agents than oxygen.
• ΔG (free energy change) is lower, so the reaction proceeds less vigorously.
• Enzymes like lactate or alcohol dehydrogenase are less efficient.
• No membrane potential or proton gradient to enhance the process.
➡ Result: Slower and less efficient NAD⁺ regeneration.
🌬️ Why is NADH oxidation vigorous in Aerobic Respiration?
• Oxygen is a strong oxidizing agent (very electronegative).
• Large redox potential (ΔE°′ ≈ +1.14 V).
• High free energy release (ΔG°′ ≈ -220 kJ/mol).
• Fast NAD⁺ regeneration and ATP generation (~3 ATP/NADH).
NADH → Complex I → CoQ → Complex III → Cytochrome C → Complex IV → O₂ → H₂O
| Feature | Fermentation | Aerobic Respiration |
|---|---|---|
| Final Electron Acceptor | Organic molecules | Oxygen (O₂) |
| Oxidation Speed of NADH | Slow | Fast/Vigorous |
| Efficiency of NAD⁺ Regeneration | Low | High |
| Energy Yield per NADH | ~0 ATP | 2.5–3 ATP |
| Type of Reaction | Substrate-level, less exergonic | Oxidative phosphorylation, highly exergonic |
| Location | Cytoplasm | Mitochondria |