Read the Passage carefully and answer the questions.
Microbes in human welfare
Microbes are a very important component of life on earth. We use microbes and microbially derived products almost every day, like LAB to make curd, yeast to make bread, adding flavor and texture to cheese, producing antibiotics etc. The addition of LAB, improves the milk quality by increasing vit. B12. Cheese is one of the oldest food items in which microbes are used. For example, large holes in Swiss cheese are due to CO2 produced by the bacterium Propionibacterium sharmanii. The BOD test measures the rate of uptake of oxygen by microorganisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, the more is its polluting potential. In anaerobic sludge digesters, bacteria produce a mixture of gases like methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as a source of energy. Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. You have studied about the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as a nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter).
In anaerobic sludge digesters, the biogas produced by the bacteria, is composed of -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → CH4, H2S and CO2
In sewage treatment, most of the sludge is pumped into large tanks called anaerobic sludge digesters. There, bacteria that grow without oxygen digest the bacteria and fungi of the sludge. The gases they give off form biogas, which burns and is used as a source of energy.
Option (1) CH4, H2 and CO2 — Incorrect. Methanogens growing on cellulose give methane with CO2 and H2. But for the gas of the sludge digester, the book names hydrogen sulphide (H2S), not hydrogen (H2).
Option (2) CH4, H2S and H2 — Incorrect. Methane and hydrogen sulphide are right. But carbon dioxide is missing, and hydrogen stands in its place.
Option (3) CH4, H2S and CO2 — Correct. The digester gives a mixture of methane, hydrogen sulphide and carbon dioxide. Methane is the gas that burns, so it makes the mixture useful as fuel.
Option (4) CH4, H2 and H2 — Incorrect. This names hydrogen twice. It leaves out both hydrogen sulphide and carbon dioxide.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.