RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Microbes have a bad reputation, but the overwhelming majority of them work quietly in our favour — curdling milk, raising bread, cleaning sewage, fixing nitrogen in soil, and supplying most of our antibiotics. This chapter names the specific organism behind each of these uses, so the surest way to score here is to learn the organism and its product as a pair. It also covers sewage treatment and biogas, where microbial metabolism is put to work on an industrial scale.
Which microorganism is used both to leaven bread and to ferment fruit juice into wine?
Answer
Saccharomyces cerevisiae is correct — this yeast ferments sugars, and in dough the CO₂ released makes the bread rise, while in fruit juice the ethanol produced makes wine. Lactobacillus is used for curd, Aspergillus niger for citric acid, and Propionibacterium sharmanii for the holes in Swiss cheese.
A patient who has just suffered a myocardial infarction is given an enzyme obtained from Streptococcus to dissolve the clot. This enzyme is:
Answer
Streptokinase is correct — produced by Streptococcus and modified by genetic engineering, it is used as a clot buster to remove clots from the blood vessels of patients who have had a heart attack. Lipase and pectinase are industrial enzymes used in detergents and juice clarification, and cyclosporin A is an immunosuppressant, not an enzyme.
Two water samples are tested and sample X is found to have a much higher biochemical oxygen demand than sample Y. What does this indicate?
Answer
Sample X contains more organic matter and is more polluted is correct — BOD measures the oxygen that bacteria would consume in oxidising the organic matter in a litre of water, so a high BOD means a heavy load of biodegradable organic material. It therefore indicates greater pollution and, in practice, less dissolved oxygen available for aquatic life, which is why lowering BOD is the goal of secondary treatment.
Which of the following is a biofertiliser that lives symbiotically in the root nodules of leguminous plants and fixes atmospheric nitrogen?
Answer
Rhizobium is correct — it forms a symbiotic association inside the root nodules of legumes and converts atmospheric nitrogen into a form the plant can use. Azotobacter fixes nitrogen but lives free in the soil rather than in nodules, Glomus is a mycorrhizal fungus that helps absorb phosphorus, and Trichoderma is a biocontrol agent against root pathogens.
Assertion (A): Biogas plants at dairy farms are supplied with cattle dung as the raw material. Reason (R): The rumen of cattle harbours methanogenic bacteria, so the dung is already rich in the microbes that generate methane.
Answer
Both A and R are true and R is the correct explanation of A — methanogens such as Methanobacterium live in the rumen where they act on cellulose-rich food, so cattle dung leaves the animal already carrying a dense population of these bacteria. Once the dung is slurried into an anaerobic digester, they immediately break down the organic matter and release biogas, which is why gobar is the standard feedstock for village biogas plants.
Name any two microbial products used as medicines and state the therapeutic use of each.
Answer
Cyclosporin A is obtained from the fungus Trichoderma polysporum and is used as an immunosuppressant in organ transplant patients, where it suppresses the recipient's immune response so that the transplanted organ is not rejected. Statins are produced by the yeast Monascus purpureus and are used to lower blood cholesterol. They act by competitively inhibiting the enzyme responsible for the synthesis of cholesterol in the body. Penicillin from Penicillium notatum, used as an antibiotic against bacterial infections, and streptokinase from Streptococcus, used to dissolve blood clots, are two further examples.
Describe the secondary treatment of sewage and explain how it reduces the BOD of the effluent.
Answer
The primary effluent, from which solids have already been physically removed, is passed into large aeration tanks and vigorously agitated while air is pumped through it. This constant supply of oxygen allows aerobic microbes to grow rapidly, and they form mesh-like masses called flocs, consisting of bacteria associated with fungal filaments. These flocs consume most of the organic matter dissolved in the effluent, using up a great deal of oxygen in the process. As the organic load falls, so does the amount of oxygen that would be needed to oxidise the remaining matter — that is, the BOD of the effluent drops sharply. Once the BOD has been reduced significantly, the effluent is passed into a settling tank where the flocs sediment out as activated sludge. A small part of this sludge is put back into the aeration tank as inoculum, and the rest is pumped into anaerobic sludge digesters. The treated effluent is then released into a natural water body.
How does Bacillus thuringiensis act as a biological control agent, and why is it preferred over a chemical insecticide?
Answer
Dried spores of Bacillus thuringiensis are mixed with water and sprayed on crops. When a caterpillar of a butterfly or moth eats the treated leaves, the alkaline conditions of its gut dissolve the crystals, releasing a toxin that kills the insect larva. It is preferred because it is specific — it kills only the target group of larvae and leaves other insects, including pollinators and natural predators, unharmed. A broad-spectrum chemical insecticide kills useful and harmful insects alike, persists in the environment, leaves residues on food, and drives the evolution of resistance, so the biological agent is safer for the ecosystem and for consumers.
Explain how a biogas plant works, naming the microbes involved and stating the composition and uses of the gas produced.
Answer
A typical biogas plant consists of a concrete tank several metres deep into which the slurry of raw material is fed, a floating cover that rises as gas accumulates beneath it and is fitted with an outlet pipe, and an outlet chamber from which the spent slurry is removed. The raw material is usually cattle dung, known as gobar, mixed with water to form a slurry. Cattle dung is chosen because the rumen of cattle already contains large numbers of methanogenic bacteria, chiefly Methanobacterium, that act on the cellulose-rich food the animal eats. These bacteria are therefore present in abundance in the dung itself. The tank is sealed so that conditions inside are anaerobic. Under these conditions the methanogens break down the organic matter of the slurry, and the gas they produce collects under the floating cover. As more gas is generated the cover rises, which both stores the gas and keeps it under slight pressure so it can be piped out. The gas is drawn off through the outlet pipe and supplied to nearby houses. Biogas is a mixture consisting mainly of methane, together with carbon dioxide and hydrogen sulphide. It is used as a fuel for cooking and lighting in villages, and the spent slurry that remains after digestion is removed through the outlet chamber and used as an excellent organic manure, since digestion does not remove the nitrogen and phosphorus content. The technology developed in India largely through the efforts of the Indian Agricultural Research Institute and the Khadi and Village Industries Commission, and it gives rural households a renewable fuel from a waste material that would otherwise be burnt inefficiently.
Discuss the role of microbes in household food production, in industry, and in agriculture, giving named examples in each case.
Answer
In household food production: Lactobacillus and related lactic acid bacteria convert milk into curd. A small amount of curd added as starter multiplies at a suitable temperature, and the acid produced coagulates and partially digests the milk proteins, while the vitamin B₁₂ content of the milk is improved. The dough used for idli and dosa is fermented by bacteria whose CO₂ makes it puff up, and toddy is made by fermenting sap from palms. Bread is leavened by baker's yeast, Saccharomyces cerevisiae, whose CO₂ raises the dough. Cheese is a microbial product too — the large holes in Swiss cheese are formed by CO₂ produced by Propionibacterium sharmanii, and Roquefort cheese owes its flavour to ripening by the fungus Penicillium roqueforti. In industry: Saccharomyces cerevisiae fermenting malted cereals and fruit juices gives beer and wine, which are not distilled, and whisky, brandy, and rum, which are. Antibiotics are the most important microbial products of all — penicillin was discovered by Alexander Fleming from the mould Penicillium notatum and developed as a drug by Ernest Chain and Howard Florey, and antibiotics have since transformed the treatment of bacterial disease. Other products include the immunosuppressant cyclosporin A from Trichoderma polysporum, statins from Monascus purpureus for lowering blood cholesterol, and industrial enzymes such as lipases used in detergents, pectinases and proteases used to clarify bottled fruit juices, and streptokinase used as a clot buster. In agriculture: microbes serve both as biofertilisers and as biocontrol agents. Rhizobium fixes atmospheric nitrogen symbiotically in the root nodules of legumes, while Azospirillum and Azotobacter fix nitrogen living free in the soil, and the cyanobacterium Anabaena within the water fern Azolla enriches paddy fields. Mycorrhizal fungi of the genus Glomus associate with plant roots and absorb phosphorus from the soil for the plant, also improving its tolerance of drought and root pathogens. As biocontrol agents, Bacillus thuringiensis kills butterfly caterpillars, the free-living fungus Trichoderma controls several plant root pathogens, and Baculoviruses of the genus Nucleopolyhedrovirus are species-specific insecticides that leave beneficial insects unharmed. All of these reduce dependence on chemical fertilisers and pesticides.
A cooperative dairy in a village decides to become self-sufficient. It sets up a digester fed with cattle dung from its sheds, pipes the gas produced to the kitchen of a nearby school, and spreads the leftover slurry on its fodder fields. Alongside, the dairy runs a small unit that adds a spoon of previous day's curd to warm milk to produce fresh curd for sale. Its fodder fields are sown with a legume crop and are not given any nitrogen fertiliser, yet the soil nitrogen improves each season. A visiting inspector notes that the wastewater from the milk-washing area has a high BOD. (a) Name the group of microbes responsible for the gas in the digester and the main gas produced. (b) Name the microbe added with the spoon of curd and state one nutritional benefit it brings. (c) Explain why the legume fodder crop improves soil nitrogen without fertiliser. (d) What does the high BOD of the wastewater indicate, and which stage of sewage treatment would reduce it?
Answer
(a) The microbes are methanogens, chiefly Methanobacterium, which are present in abundance in cattle dung because they inhabit the rumen. Under the anaerobic conditions of the sealed digester they break down the organic matter and produce biogas, whose main constituent is methane, along with carbon dioxide and hydrogen sulphide. (b) The spoon of curd acts as a starter or inoculum and introduces Lactobacillus. It produces acids that coagulate and partially digest the milk proteins to set the curd, and it improves the nutritional quality of the milk by increasing its vitamin B₁₂ content. (c) The legume roots form nodules inhabited by the symbiotic bacterium Rhizobium, which fixes atmospheric nitrogen into a form the plant can absorb. When the crop residues and nodules decay in the soil, this fixed nitrogen is released, so the field is enriched each season without any external nitrogen fertiliser. Rhizobium is therefore acting as a biofertiliser. (d) A high BOD indicates that the wastewater carries a heavy load of biodegradable organic matter — milk residues here — and is therefore heavily polluting, since bacteria oxidising it would strip oxygen from any water body it entered. Secondary or biological treatment would reduce it: the effluent is aerated in tanks so that aerobic microbial flocs consume the organic matter, after which the flocs settle out as activated sludge and the BOD of the effluent falls sharply.
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