Question

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

• A. Hydrolysis \(\rightarrow\) Acidogenesis \(\rightarrow\) Acetogenesis \(\rightarrow\) Methanogenesis
• B. Methanogenesis \(\rightarrow\) Acetogenesis \(\rightarrow\) Acidogenesis \(\rightarrow\) Hydrolysis
• C. Acidogenesis \(\rightarrow\) Hydrolysis \(\rightarrow\) Methanogenesis \(\rightarrow\) Acetogenesis
• D. Hydrolysis \(\rightarrow\) Methanogenesis \(\rightarrow\) Acidogenesis \(\rightarrow\) Acetogenesis

Hint:

Trace which molecule feeds which stage: monomers must exist before fermentation, and acetate or hydrogen must exist before methane can form.

Answer 1

The Correct Option is A

Step 1: Recall what each stage of anaerobic digestion does.
Complex organics (proteins, lipids, and polysaccharides) cannot be used directly by methane-forming microbes. They first need to be broken into small, soluble units. Hydrolysis is the first stage: extracellular enzymes secreted by fermentative bacteria break down proteins into amino acids, lipids into fatty acids and glycerol, and polysaccharides into simple sugars.

Step 2: Follow the small molecules through the next stages.
Acidogenesis comes next: acidogenic (fermentative) bacteria take up these sugars, amino acids, and fatty acids and convert them into short-chain volatile fatty acids (such as propionic and butyric acid), along with some hydrogen, carbon dioxide, and ammonia. Acetogenesis follows: acetogenic bacteria convert these longer volatile fatty acids further into acetate, hydrogen, and carbon dioxide, the direct precursors methanogens can use.

Step 3: Identify the final stage.
Methanogenesis is the last stage: methanogenic archaea use acetate (via the acetoclastic pathway) or hydrogen and carbon dioxide (via the hydrogenotrophic pathway) to produce methane, the main fuel component of biogas.

Step 4: Put the stages in order and check the options.
The correct order is Hydrolysis, then Acidogenesis, then Acetogenesis, then Methanogenesis, since each stage's product feeds the next. This matches option (A). Options (B), (C), and (D) all place Methanogenesis or Hydrolysis out of order, which breaks the feed-forward logic of the process; for instance option (C) needs acidogenesis to act before hydrolysis has even produced the small molecules it depends on.

Final Answer:
Hydrolysis \(\rightarrow\) Acidogenesis \(\rightarrow\) Acetogenesis \(\rightarrow\) Methanogenesis. \[ \boxed{\text{Hydrolysis} \rightarrow \text{Acidogenesis} \rightarrow \text{Acetogenesis} \rightarrow \text{Methanogenesis}} \]