Biogas is made by a group of anaerobic bacteria called methanogens (a common example is Methanobacterium). They grow without oxygen on cellulose-rich material like cattle dung and produce a large amount of methane (CH4) along with CO2 and H2. Memory hook: "Gobar gas = Methanogen gas" — no oxygen, dung goes in, methane comes out.
Methanogens (e.g., Methanobacterium) act anaerobically on cellulose-rich cattle dung to release biogas, which is mainly methane (CH4) along with CO2 and H2.
Your doubts, answered
Which microbes actually produce biogas?
Bacteria called methanogens produce biogas. They grow anaerobically (without oxygen) on cellulose-rich material and release large amounts of methane. The common example named in NCERT is Methanobacterium. Yeast and Lactobacillus are NOT involved in biogas — they make CO2 or lactic acid, not methane.
Are methanogens bacteria or archaebacteria? NCERT seems to say both.
Both statements are correct and NEET tests this. In Biological Classification (Class 11), methanogens are classed as archaebacteria (primitive prokaryotes with a special cell wall). In Microbes in Human Welfare (Class 12), they are simply called methanogenic bacteria. So for NEET: methanogens ARE archaebacteria, and biogas-producing bacteria = methanogens.
Why must the biogas process be anaerobic (no oxygen)?
Methanogens are strictly anaerobic. Oxygen actually kills or stops them. They break down cellulose without oxygen and this anaerobic digestion is what forms methane. This is why a biogas plant is a closed, sealed tank — to keep air out. If they grew aerobically, no methane would form.
Where are methanogens naturally found?
Three places NCERT names: (1) the rumen (part of the stomach) of cattle like cows and buffaloes, where they digest cellulose, (2) marshy areas, and (3) the anaerobic sludge produced during sewage treatment. Because they live in the cattle rumen, cattle dung (gobar) is naturally rich in methanogens — that is why dung is the raw material for biogas.
What gases are in biogas, and why is methane special?
Biogas is mostly methane (CH4) plus some CO2 and H2. Methane is the fuel — it burns and is used for cooking and lighting. Compare: dough fermentation, cheese making and beverages mainly release CO2 (not fuel). The gas produced depends on the microbe and the substrate; methanogens on cellulose give methane.
⚠️ The NEET trap ✗ Biogas is produced by yeast (Saccharomyces) or by Lactobacillus, like other fermentation products. ✓ Biogas is produced by methanogens (e.g., Methanobacterium), which act anaerobically on cellulose to release methane. Yeast and LAB give CO2 and lactic acid, not methane. 🧠 If the gas is METHANE and the food is DUNG/cellulose, the microbe is ALWAYS a methanogen — never yeast, never LAB.
Real NEET questions
NEET 2016 Phase 1
The primitive prokaryotes responsible for the production of biogas from the dung of ruminant animals, include the:
A · Halophiles
B · Thermoacidophiles
C · Methanogens ✓
D · Eubacteria
Solution: Biogas from cattle dung is produced by methanogens — anaerobic archaebacteria that grow on cellulose-rich material and release methane. They live in the rumen of cattle, so gobar is rich in them. Halophiles (salty areas) and thermoacidophiles (hot springs) are other archaebacteria not linked to biogas, and 'eubacteria' is too general.
NEET 2016 Phase 1 · NEET 2019 Odisha
Which of the following statements about methanogens is not correct?
A · They can be used to produce biogas.
B · They are found in the rumen of cattle and their excreta.
C · They grow aerobically and breakdown cellulose-rich food. ✓
D · They produce methane gas.
Solution: Methanogens are strictly ANAEROBIC — they grow without oxygen on cellulosic material, so 'grow aerobically' is the incorrect statement. The other three are all true: they make biogas (methane), live in the rumen and dung of cattle, and produce methane gas.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the common bacterium example for methanogens?
Methanobacterium is the common example named in NCERT for methanogens that produce biogas.
Is biogas the same as gobar gas?
Yes. Biogas made from cattle dung (gobar) is commonly called gobar gas. It is mostly methane and is used for cooking and lighting.
Do methanogens have any role besides biogas?
Yes. In the cattle rumen they help digest cellulose (important for cattle nutrition), and they are also found in the anaerobic sludge during sewage treatment, where they degrade organic waste.
Why is dung rich in methanogens?
Because methanogens live in the rumen of cattle, their excreta (dung/gobar) naturally carries these bacteria, making dung an ideal raw material for a biogas plant.
