GATE BT 2026 • Q24 MCQ • 1 Mark

Section 4: Bioprocess Engineering and Process Biotechnology

Mass transfer in bioprocesses (kLa)

Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)

Question Statement:

The correct sequence of the four steps involved in anaerobic production of biogas from complex organics is:

Options:

  • (A) Hydrolysis  Acidogenesis  Acetogenesis  Methanogenesis
  • (B) Methanogenesis  Acetogenesis  Acidogenesis  Hydrolysis
  • (C) Acidogenesis  Hydrolysis  Methanogenesis  Acetogenesis
  • (D) Hydrolysis  Methanogenesis  Acidogenesis  Acetogenesis

Official Answer Key:

A

Step-by-Step Solution & Explanation

### **Official Solution & Concept Analysis (GATE BT 2026 Q.24)** **1. Governing Concept**: - **Topic**: Environmental Biotechnology & Anaerobic Digestion Pathway - Anaerobic biological digestion of complex organic matter to biogas ($\text{CH}_4 + \text{CO}_2$) proceeds through four strictly sequential microbiological stages: 1. **Hydrolysis**: Extracellular enzymes (cellulases, proteases, lipases) hydrolyze insoluble complex polymers (polysaccharides, proteins, lipids) into soluble monomers (sugars, amino acids, fatty acids). 2. **Acidogenesis (Fermentation)**: Acidogenic fermentative bacteria convert monomers into volatile fatty acids (propionate, butyrate), alcohols, lactic acid, $\text{CO}_2$, and $\text{H}_2$. 3. **Acetogenesis**: Acetogenic bacteria convert volatile fatty acids into **acetic acid**, $\text{CO}_2$, and $\text{H}_2$. 4. **Methanogenesis**: Strictly anaerobic methanogenic Archaea convert acetate (acetotrophic methanogenesis) and $\text{H}_2 + \text{CO}_2$ (hydrogenotrophic methanogenesis) into **methane ($\text{CH}_4$)** and water/carbon dioxide. **2. Option Analysis**: - **Option (A) [Hydrolysis → Acidogenesis → Acetogenesis → Methanogenesis] is CORRECT**: The universally established chronological biochemical cascade of biogas production. - **Options (B), (C), and (D) are INCORRECT**: Invert the chronological order (e.g. placing methanogenesis before acetogenesis or acidogenesis before hydrolysis). **3. Final Answer**: - **Correct Choice**: **A**

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