Q.85 Methane belched (eructation) out by cattle arises from the carbon dioxide produced
(A) during normal respiration
(B) oxidation of food stuff occurring in mitochondria
(C) lactic acid fermentation occurring in muscles
(D) bacterial fermentation occurring in the gut
The correct answer is (D) bacterial fermentation occurring in the gut. Methane belched by cattle (eructation) originates from rumen methanogenesis during microbial fermentation of plant material, not from host respiration or oxidation processes.
Ruminants produce ~70-120 liters CH₄/day via enteric fermentation, with 85-95% released by belching. This contributes ~30% of global anthropogenic methane emissions.
Option Analysis
(A) During normal respiration
Incorrect. Animal respiration produces CO₂ + H₂O from carbohydrate oxidation. No methane pathway exists in eukaryotic mitochondria or cytosol respiration.
(B) Oxidation of food stuff occurring in mitochondria
Wrong. Mitochondrial β-oxidation/TCA cycle fully oxidizes substrates to CO₂. CH₄ requires strict anaerobiosis and methanogenic archaea (Methanobrevibacter), absent in mitochondria.
(C) Lactic acid fermentation occurring in muscles
False. Muscle glycolysis → lactate under anaerobiosis (exercise), but no methanogens or H₂/CO₂ reduction to CH₄ occurs in animal tissues.
(D) Bacterial fermentation occurring in the gut
Correct. Rumen fermentation by bacteria (fibrolytics), protozoa, and fungi produces H₂ + CO₂. Methanogens consume H₂: 4H₂ + CO₂ → CH₄ + 2H₂O. Acetate/propionate/butyrate provide host energy.
Introduction
Methane belched eructation out by cattle contributes significantly to greenhouse gases via bacterial fermentation occurring in the gut. This GATE Life Sciences question tests rumen microbiology, distinguishing enteric methanogenesis from host metabolism.
Rumen Fermentation Pathway
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Feed breakdown: Cellulolytic bacteria hydrolyze cellulose → cellodextrins → glucose
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Mixed acid fermentation: Bacteria produce acetate (60%), propionate (25%), butyrate (10%) + H₂ + CO₂
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Methanogenesis: Archaea reduce CO₂ using H₂ (interspecies H₂ transfer prevents inhibition)
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Eructation: 90-95% CH₄ expelled via mouth/nostrils
Daily output: 200-500g CH₄/cow, higher with fibrous diets.
Source Comparison
| Process | Location | Gas Produced | Methanogens Present? |
|---|---|---|---|
| Bacterial gut fermentation | Rumen | CH₄ (90%) | Yes |
| Normal respiration | Lungs | CO₂ | No |
| Mitochondrial oxidation | Mitochondria | CO₂ | No |
| Muscle lactate fermentation | Muscles | Lactate | No |
Mitigation Strategies & Exam Relevance
Diet: High-starch reduces rumen pH → less methanogens (propionate favors H₂ sink).
Additives: 3-NOP (Bovaer®) inhibits methyl-coenzyme M reductase.
