Q.66 Which ONE of the following components is NOT an electron acceptor during anaerobic respiration?
(A) Lactate (B) Carbonate (C) Nitrate (D) Sulphate
Carbonate is NOT an electron acceptor during anaerobic respiration.
Question Breakdown
Anaerobic respiration uses alternative terminal electron acceptors (instead of O₂) in the electron transport chain to generate proton motive force/ATP. Common acceptors ranked by reduction potential: NO₃⁻ > SO₄²⁻ > CO₂ > fumarate.
Option Analysis
(A) Lactate
Used in fermentation (not respiration). Pyruvate accepts electrons from NADH → lactate (homofermentative bacteria like Lactobacillus). No ETC involved.
(B) Carbonate
NOT an electron acceptor. CO₂ (not carbonate CO₃²⁻) accepts electrons in methanogenesis (acetogens/methanogens: CO₂ → CH₄). Carbonate lacks suitable redox potential.
(C) Nitrate
Correct acceptor. Denitrification: NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ (Pseudomonas, Paracoccus). High energy yield.
(D) Sulphate
Correct acceptor. Sulfate reduction: SO₄²⁻ → SO₃²⁻ → S²⁻ (Desulfovibrio). Common in sediments.
Correct Choice
(B) Carbonate
Electron acceptor anaerobic respiration excludes carbonate; standard acceptors are nitrate (denitrification), sulphate (sulfate reduction), not CO₃²⁻.
Anaerobic Electron Transport Chain
NADH/FADH₂ → Quinone → bc₁ → NO₃⁻ reductase → N₂ [Nitrate]
→ SO₄²⁻ reductase → H₂S [Sulfate]
→ Fumarate reductase → Succinate
Fermentation (no ETC): Pyruvate → Lactate/Ethanol
Acceptor Comparison
| Option | Used in Anaerobic Respiration? | Process | Microbes |
|---|---|---|---|
| (A) Lactate | No | Fermentation product | Lactobacillus |
| (B) Carbonate | No | CO₂ in methanogenesis | Methanogens |
| (C) Nitrate | Yes | Denitrification | Pseudomonas |
| (D) Sulphate | Yes | Sulfate reduction | Desulfovibrio |
GATE Exam Relevance
Microbiology PYQ tests respiration vs fermentation distinction. Respiration = ETC + external acceptor (NO₃⁻, SO₄²⁻). Fermentation = substrate-level phosphorylation (lactate, ethanol). CO₂ (not carbonate) for methanogens.
