GATE 2026 BT Q.24: solution and answer

Official answer: ASource: Official answer key, GATE 2026 BT (IIT Guwahati)

1. Anaerobic digestion converts complex organic matter to biogas (${\text{CH}}_4$ and ${\text{CO}}_2$) in four consecutive stages, each feeding the next.

2. Stage 1, hydrolysis: extracellular enzymes break polymers (polysaccharides, proteins, lipids) into soluble monomers such as sugars, amino acids and fatty acids. This must come first because later microbes can only take up small soluble molecules.

3. Stage 2, acidogenesis: fermentative bacteria convert these monomers into volatile fatty acids, alcohols, ${\text{H}}_2$ and ${\text{CO}}_2$.

4. Stage 3, acetogenesis: acetogenic bacteria oxidise the volatile fatty acids and alcohols to acetate, ${\text{H}}_2$ and ${\text{CO}}_2$.

5. Stage 4, methanogenesis: methanogenic archaea make methane from acetate (acetoclastic route) and from ${\text{H}}_2$ plus ${\text{CO}}_2$ (hydrogenotrophic route). It is last because it needs acetate and ${\text{H}}_2$ as substrates.

6. Option A gives exactly this order. B reverses it, so methane would be made before any substrate exists. C puts acidogenesis before hydrolysis, which is wrong because polymers must be broken down first. D puts methanogenesis before the acid and acetate stages, so it has no substrate.