Previous year questions: Microbes in Human Welfare
Microbes in Human Welfare25 min readLesson 6 of 6
Board pattern for this chapter
The chapter turns up in board papers as short answers on everyday products, one-line identifications of microbes, and structured questions on sewage treatment and biogas. All ten questions below are rewritten from actual CBSE board papers between 2022 and 2026, so the year label tells you which paper to open if you want the original. Marks are printed in each heading; use them to decide how long the answer should be. The set covers five board years, which is useful practice: the same section has been asked as a definition, as a process and as a labelled diagram over the years, so the wording changes while the content does not.
Q1 (CBSE Board 2022, 2 marks)
Lactic acid bacteria set milk into curd and also play a beneficial role for human health. Give two reasons.
Answer: The bacteria grow in milk and produce acids that coagulate the milk proteins and partly digest them, which turns the milk into curd. The same growth raises the vitamin B12 content of the curd, so its nutritional quality improves. A third benefit comes when these bacteria live in the human stomach: they check the growth of disease-causing microbes and support digestion.
Q2 (CBSE Board 2022, 2 marks)
A certain bacterium is common both in the anaerobic sludge of a sewage plant and in the rumen of cattle. Name it and state how it is commercially useful.
Answer: The bacterium is Methanobacterium, a methanogen. It grows anaerobically on cellulosic material and produces methane along with carbon dioxide and hydrogen. The methane is the flammable part of biogas, so the bacterium is the reason cattle dung and sewage sludge can be turned into gobar gas for cooking and lighting.
Q3 (CBSE Board 2022, 2 marks)
Name the biocontrol agents of several plant pathogens that belong to the group of viruses. How do they support the environment?
Answer: Baculoviruses, mostly of the genus Nucleopolyhedrovirus, are the viral biocontrol agents. They attack insects and other arthropods and are specific to a narrow range of species. They have no negative effect on plants, mammals, birds, fish or non-target insects, so they cut the need for chemical pesticides and fit integrated pest management, which is why they support the environment.
Q4 (CBSE Board 2022, 2 marks)
Different species of Trichoderma are useful to humans as well as to plants. Justify with one instance for each.
Answer: For plants, Trichoderma species are free-living fungi common in root ecosystems and act as effective biocontrol agents against several plant pathogens. For humans, Trichoderma polysporum produces cyclosporin A, an immunosuppressive agent used in organ transplant patients so that the new organ is not rejected.
Q5 (CBSE Board 2023, 2 marks)
Name the two institutes that developed the technology of biogas production in India. Explain the main principle used in this technology.
Answer: The technology was developed mainly by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC). The principle is anaerobic digestion of cellulosic waste by methanogens. Dung slurry is kept in a concrete tank without oxygen, the bacteria break down the organic matter, and a mixture of gases rich in methane collects under the floating cover. The spent slurry is used as fertiliser.
Q6 (CBSE Board 2024, 1 mark)
During biological treatment of sewage, the masses of bacteria held together by fungal filaments to form mesh-like structures are called
(A) primary sludge (B) activated sludge (C) flocs (D) anaerobic sludge
Answer: Option (C), flocs. The flocs form in the aeration tank during secondary treatment, and the microbes inside them consume the organic matter of the effluent, which lowers its BOD. Activated sludge is the sediment that settles later in the settling tank.
Q7 (CBSE Board 2024, 2 marks)
A biogas plant has a concrete tank with a floating cover, an inlet for dung slurry and an outlet for the spent slurry. Answer the questions based on this plant. (a) Name the gas that collects under the floating cover and state its main flammable component. (b) Why is cattle dung preferred for the production of biogas?
Answer: (a) The gas collected is biogas, and its main flammable component is methane. The mixture also contains carbon dioxide and hydrogen sulphide. (b) Cattle dung is preferred because it is rich in methanogens such as Methanobacterium. These bacteria live in the rumen of cattle and help break down the cellulose of their food, so the dung carries a ready population that can act on the cellulosic slurry inside the biogas plant.
Q8 (CBSE Board 2025, 3 marks)
Explain the biological treatment of the primary effluent when it is passed into the large aeration tanks of a sewage treatment plant.
Answer: The primary effluent enters big aeration tanks, where mechanical stirring keeps it moving and air is bubbled through. This supports rapid growth of aerobic microbes, which gather into flocs: bacteria bound with fungal filaments into a mesh. As they grow, the microbes take up most of the organic matter, so the BOD falls sharply. The effluent then passes to a settling tank where the flocs settle as activated sludge.
Q9 (CBSE Board 2025, 3 marks)
Explain the biological treatment given to the major part of the sludge transferred from the aeration tank into the anaerobic sludge digesters before its final release into natural water bodies.
Answer: The major part of the activated sludge is pumped into large anaerobic sludge digesters. There, anaerobic bacteria break down the bacteria and fungi left in the sludge. Digestion releases gases, among them methane, hydrogen sulphide and carbon dioxide. This mixture is biogas, which is inflammable and is used as a source of energy. The digested sludge can then be disposed of, and the effluent released to water bodies has a much lower BOD.
Q10 (CBSE Board 2026, 1 mark)
Microbes commonly used in a kitchen are
(A) Lactobacillus and yeast (B) Penicillium and yeast (C) Lactobacillus and Penicillium (D) Aspergillus and yeast
Answer: Option (A). Lactobacillus turns milk into curd, and yeast (Saccharomyces cerevisiae) ferments dough for bread and batters for idli and dosa. Penicillium is a mould used for penicillin and for ripening some cheeses, and Aspergillus niger is an industrial citric acid producer; neither is a kitchen microbe in the everyday sense the question means.
What the board rewards
These questions reward named examples and ordered processes. A two-mark answer needs one name and one reason. A three-mark answer needs the steps in order, with terms such as flocs, activated sludge, BOD, methanogens and sludge digester used correctly. Never open with a general paragraph about microbes being useful; the marks sit inside the specific process. When a question gives a diagram, name the labelled parts before explaining them, because the label carries its own mark. Read the marks before you start writing. One mark wants a name or a single fact, two marks want a name plus a reason, and three marks want a process in order. If the answer is running long for a two-mark question, cut the introduction and keep the named example.
Traps to avoid in this chapter's questions
Four traps repeat across years. The first is calling every microbe harmful; the chapter is about useful microbes and the board expects that opening line. The second is mixing the waste water stages: filtration and sedimentation belong to primary treatment, aeration and floc formation to secondary treatment, and the sludge digester to the handling of activated sludge. The third is treating BOD as a pollution measure that rises when water is cleaned; a falling BOD is the sign of successful treatment. The fourth is swapping the biocontrol pairs, for example giving Trichoderma for insect larvae or Bt for root pathogens. Keep the pairs in one line each and revise them together. For the products table, the safe method is to say the product out loud with the microbe attached, since board questions often award one mark per correct pair and nothing for a vague answer.
A last check before the exam
Write the six steps of recombinant DNA technology from memory in under a minute. Then write the three features of a vector and the three tools. If any of those lists stalls, revise that topic page rather than attempting more questions. Most lost marks in this chapter come from missing a step in a sequence or naming the wrong tool, not from a lack of understanding.