Section 11 — Board Previous Year Questions (Class 12 Biology · Chapter 8)
Chapter 8 (Microbes in Human Welfare) is a short but reliable scoring chapter in the Class 12 Biology Board paper. As a broad guide, about 5–7 marks of a paper tend to come from here — usually a 1-mark or 2-mark question on a household or industrial microbe, a 2/3-mark question on sewage treatment or biogas, and quite often a 3- or 5-mark question on biocontrol or biofertilisers.
This section curates 24 high-yield Board questions in the format and weighting examiners typically use:
| Marks | Number of Qs | Typical paper allocation |
|---|---|---|
| 1 mark | 7 questions (Q1–Q7) | 2–3 / paper |
| 2 marks | 6 questions (Q8–Q13) | 1–2 / paper |
| 3 marks | 6 questions (Q14–Q19) | 1 / paper |
| 5 marks | 5 questions (Q20–Q24) | 1 / paper (high probability) |
| Total | 24 questions | ~24 marks worth |
How to use: Cover the answer, attempt it yourself scaled to the marks, then check against the model answer and its keyword notes.
No quiz at the end: Pair this set with the Solved Examples for the chapter.
What the Board Keeps Asking
Most questions on Chapter 8 fall into six recurring themes:
1. Household products (1- or 2-mark) — lactic acid bacteria and curd, dough fermentation and CO2, baker's yeast, cheese and its microbes.
2. Industrial products (2- or 3-mark) — fermented beverages and distillation, penicillin and other antibiotics, organic acids, enzymes and bioactive molecules such as statins, cyclosporin A and streptokinase.
3. Sewage treatment (2-, 3- or 5-mark) — primary vs secondary treatment, flocs, BOD and its meaning, activated sludge, anaerobic digesters.
4. Biogas (2- or 3-mark) — methanogens, the role of the rumen, gobar gas and the working of a biogas plant.
5. Biocontrol agents (2-, 3- or 5-mark) — Bacillus thuringiensis, ladybird and dragonfly, Trichoderma, baculoviruses and integrated pest management.
6. Biofertilisers (2-, 3- or 5-mark) — Rhizobium, free-living nitrogen fixers, mycorrhiza and cyanobacteria.
1-Mark Questions (Q1–Q7)
Q1. [1 mark] Name the group of bacteria that convert milk into curd, and one nutritional benefit they add.
Answer: Lactic acid bacteria (LAB), e.g. Lactobacillus; they increase the vitamin B12 content of the curd.
Keyword note: LAB and vitamin B12 both carry the mark.
Q2. [1 mark] Which gas gives the dough of idli and dosa its puffed-up appearance?
Answer: Carbon dioxide () produced by the fermenting bacteria.
Q3. [1 mark] Name the microbe used as brewer's yeast for producing beverages.
Answer: Saccharomyces cerevisiae.
Q4. [1 mark] Expand BOD and state what a high BOD value indicates.
Answer: Biochemical Oxygen Demand; a high BOD means more organic matter and greater polluting potential of the water.
Q5. [1 mark · Assertion–Reason]
Assertion (A): Methanogens are found in the rumen of cattle. Reason (R): Methanogens help break down the cellulosic material in the cattle's food.
Options: (a) Both A and R true and R explains A. (b) Both true but R doesn't explain A. (c) A true, R false. (d) A false, R true.
Answer: (a) — methanogens do live in the rumen, and the reason they thrive there is that they act on the abundant cellulose in the food, so R correctly explains A.
Q6. [1 mark] Name the bacterium whose toxin gene makes Bt-cotton resistant to insect pests.
Answer: Bacillus thuringiensis.
Q7. [1 mark] From which fungus is the immunosuppressive drug cyclosporin A obtained?
Answer: Trichoderma polysporum.
2-Mark Questions (Q8–Q13)
Q8. [2 marks] How do lactic acid bacteria convert milk into curd, and how is a starter used?
Answer: LAB growing in milk produce acids that coagulate and partially digest the milk proteins, turning it into curd. A small amount of curd added to fresh milk acts as an inoculum or starter, since it already contains millions of LAB that multiply at a suitable temperature and set the whole milk.
Keyword note: acid coagulates protein; starter = inoculum rich in LAB.
Q9. [2 marks] Distinguish, with one example each, between beverages produced with and without distillation.
Answer: Without distillation — the fermented broth is used as such, giving drinks of lower alcohol content, e.g. wine and beer. With distillation — the fermented broth is distilled to raise the alcohol content, e.g. whisky, brandy and rum.
Q10. [2 marks] Name the microbial source and the industrial use of (i) streptokinase and (ii) statins.
Answer: (i) Streptokinase is produced by the bacterium Streptococcus (genetically modified) and is used as a 'clot buster' to dissolve blood clots in heart-attack patients. (ii) Statins are produced by the yeast Monascus purpureus and are used to lower blood cholesterol by inhibiting the enzyme that makes cholesterol.
Q11. [2 marks] What is the key difference between primary and secondary treatment of sewage?
Answer: Primary treatment is mainly physical — floating debris and grit are removed by filtration and sedimentation, leaving the primary sludge and the effluent. Secondary (biological) treatment uses aerobic microbes grown in aeration tanks that consume the organic matter and greatly reduce the BOD of the effluent.
Q12. [2 marks] What are methanogens? State their role in the production of biogas.
Answer: Methanogens are bacteria (e.g. Methanobacterium) that grow anaerobically on cellulosic material and produce methane along with and . In biogas plants and in anaerobic sludge, they digest organic waste and cattle dung to yield biogas (gobar gas), an inflammable fuel.
Q13. [2 marks] How do Rhizobium and mycorrhiza act as biofertilisers?
Answer: Rhizobium lives in the root nodules of leguminous plants and fixes atmospheric nitrogen into forms the plant can use. Mycorrhiza — a symbiosis of fungi such as Glomus with roots — helps the plant by absorbing phosphorus from the soil and passing it on, besides giving resistance to root pathogens and drought.
3-Mark Questions (Q14–Q19)
Q14. [3 marks] Explain how microbes are used to make three different household products, naming the microbe in each case.
Answer: (1) Curd — lactic acid bacteria such as Lactobacillus grow in milk and coagulate it into curd. (2) Bread — baker's yeast Saccharomyces cerevisiae ferments the dough, and the it releases makes the bread rise. (3) Cheese — specific microbes give each variety its texture and flavour; the large holes of Swiss cheese come from produced by Propionibacterium sharmanii, and Roquefort cheese is ripened by growing a specific fungus on it.
Keyword note: one product + its microbe = one mark each.
Q15. [3 marks] Who discovered penicillin and how? Why are antibiotics important to human welfare?
Answer: Alexander Fleming discovered penicillin by chance while working on Staphylococci; he noticed a mould, Penicillium notatum, on an unwashed plate around which the bacteria could not grow, and traced this to a chemical the mould produced. Its full potential was later established by Ernest Chain and Howard Florey (the three shared the Nobel Prize in 1945). Antibiotics are chemicals that kill or retard disease-causing microbes, and have let us treat once-deadly diseases such as plague, whooping cough, diphtheria and leprosy.
Q16. [3 marks] Describe the secondary treatment of sewage and explain what BOD measures.
Answer: In secondary treatment the primary effluent is passed into large aeration tanks where it is agitated and air is pumped in. This lets aerobic microbes grow vigorously as flocs (masses of bacteria with fungal filaments), which consume most of the organic matter and so reduce the BOD. The treated water is then sent to a settling tank where the flocs sediment as activated sludge. BOD (Biochemical Oxygen Demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria — an indirect measure of the organic load, and hence of the polluting potential, of the water.
Keyword note: aeration → flocs → BOD reduced → activated sludge; BOD definition.
Q17. [3 marks] Explain the working of a typical biogas plant with the fate of its two outlets.
Answer: A biogas plant is a concrete tank into which a slurry of cattle dung (gobar) is fed. A floating cover rests on the slurry and keeps rising as microbial activity produces biogas in the tank. One outlet carries the biogas through a pipe to nearby houses, where it is used for cooking and lighting. The spent slurry is drawn off through a second outlet and can be used as manure/fertiliser in the fields.
Q18. [3 marks] Name any three microbial biocontrol agents and state the pest or use of each.
Answer: (1) Bacillus thuringiensis (Bt) — sprayed as dried spores; its toxin, released in the gut of insect larvae/butterfly caterpillars, kills them while leaving other insects unharmed. (2) Trichoderma — free-living root fungi that are effective biocontrol agents against several plant pathogens. (3) Baculoviruses (genus Nucleopolyhedrovirus) — species-specific, narrow-spectrum pathogens of insects and arthropods, safe for plants, birds and non-target insects, useful in integrated pest management.
Q19. [3 marks] Name three types of biofertilisers with one example of each, and state how each enriches the soil.
Answer: (1) Bacteria — Rhizobium fixes atmospheric nitrogen in the root nodules of legumes, while free-living Azospirillum and Azotobacter fix nitrogen in the soil. (2) Fungi — Glomus forms mycorrhiza and supplies the plant with phosphorus absorbed from the soil. (3) Cyanobacteria — Anabaena, Nostoc, Oscillatoria fix atmospheric nitrogen and add organic matter, acting as important biofertilisers in paddy fields.
5-Mark Questions (Q20–Q24)
Q20. [5 marks] Describe the role of microbes in industrial products under the heads: fermented beverages, antibiotics, organic acids, enzymes and other bioactive molecules.
Answer:
Fermented beverages: yeast Saccharomyces cerevisiae (brewer's yeast) ferments malted cereals and fruit juices to ethanol; wine and beer are made without distillation, whisky, brandy and rum with distillation.
Antibiotics: chemicals from microbes that kill or retard disease-causing microbes, e.g. penicillin from Penicillium notatum; they have helped control diseases such as diphtheria, whooping cough and leprosy.
Organic acids: Aspergillus niger yields citric acid, Acetobacter aceti acetic acid, Clostridium butylicum butyric acid and Lactobacillus lactic acid.
Enzymes: lipases are used in detergents to remove oily stains; pectinases and proteases clarify bottled fruit juices.
Other bioactive molecules: streptokinase (from Streptococcus) is a clot buster; cyclosporin A (from Trichoderma polysporum) is an immunosuppressant for transplant patients; statins (from Monascus purpureus) lower blood cholesterol.
Keyword note: one correct microbe + product under each head carries the marks.
Q21. [5 marks] Trace the treatment of municipal waste water from raw sewage to the final effluent, naming each stage and the microbes involved.
Answer:
Sewage is municipal waste water rich in organic matter and microbes, many of them pathogenic, and cannot be released directly into rivers.
Primary treatment physically removes particles — floating debris by sequential filtration, then grit by sedimentation; settled solids form the primary sludge and the supernatant forms the effluent.
Secondary (biological) treatment passes this effluent into aeration tanks, where aerobic microbes grow as flocs and consume most of the organic matter, sharply reducing the BOD.
The treated water goes to a settling tank, where the flocs sediment as activated sludge; a small part is returned to the aeration tank as inoculum.
The rest of the sludge is sent to anaerobic sludge digesters, where anaerobic bacteria digest it and release biogas (methane, , ). The clear effluent from secondary treatment is finally released into rivers and streams. Under the Ganga and Yamuna Action Plans, more such plants are being built so that only treated sewage reaches the rivers.
Q22. [5 marks] What is biocontrol? Explain, with examples, how microbes and other organisms are used as biocontrol agents, and state its advantage over chemical pesticides.
Answer:
Biocontrol is the use of biological methods (natural predation and pathogens) to control plant diseases and pests, in place of toxic chemicals.
Predators: the ladybird beetle controls aphids and dragonflies control mosquitoes.
Bacterial agent: Bacillus thuringiensis (Bt), sprayed as dried spores, releases its toxin in the gut of insect larvae/caterpillars and kills them while sparing other insects; its toxin genes have also been engineered into plants such as Bt-cotton.
Fungal agent: Trichoderma, a free-living root fungus, controls several plant pathogens.
Viral agent: baculoviruses (Nucleopolyhedrovirus) are species-specific insect pathogens with no harm to plants, mammals, birds or non-target insects, ideal for integrated pest management.
Advantage: biocontrol is species-specific and eco-friendly, so it greatly reduces our dependence on toxic pesticides that pollute soil, water and food and harm beneficial organisms.
Q23. [5 marks] What are biofertilisers? Describe the main sources of biofertilisers and explain how each enriches soil fertility.
Answer:
Biofertilisers are living organisms that enrich the nutrient quality of the soil, offered as a safer alternative to polluting chemical fertilisers. Their main sources are bacteria, fungi and cyanobacteria.
Bacteria: Rhizobium lives symbiotically in the root nodules of legumes and fixes atmospheric nitrogen into organic forms for the plant; free-living Azospirillum and Azotobacter fix nitrogen directly in the soil.
Fungi: many Glomus species form mycorrhiza with roots; the fungus absorbs phosphorus from the soil and passes it to the plant, which also gains resistance to root pathogens, tolerance to salinity and drought, and better growth.
Cyanobacteria: autotrophic microbes such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and add organic matter to the soil; in paddy fields they are a key biofertiliser.
By replenishing nitrogen, phosphorus and organic matter, biofertilisers cut the need for chemical fertilisers and reduce environmental pollution.
Q24. [5 marks] Show how a single group of processes — microbial fermentation and digestion — is put to human use across the household, industry and energy production.
Answer:
- Curd and fermented foods: lactic acid bacteria convert milk to curd, and bacteria ferment the dough of idli and dosa, the giving it a puffed-up texture.
- Bread: baker's yeast Saccharomyces cerevisiae ferments dough, and its makes the bread rise.
- Beverages: brewer's yeast ferments malted cereals and fruit juices to ethanol for wine, beer, whisky and other drinks.
- Industrial chemicals: fermentation yields organic acids (citric acid from Aspergillus niger, acetic acid from Acetobacter aceti) and industrial ethanol.
- Energy — biogas: methanogens such as Methanobacterium anaerobically digest cattle dung and sludge to produce biogas (gobar gas), an inflammable fuel used for cooking and lighting.
Keyword note: the same core idea — microbes breaking down substrates and releasing useful products/gases — links the kitchen, the factory and the biogas plant.
End of Section 11
You have now worked through 24 high-yield Board questions spanning household and industrial microbes, sewage treatment and BOD, biogas, biocontrol and biofertilisers. If you can answer the five 5-markers (Q20–Q24) from memory with their key points, you have effectively secured this chapter's contribution to your paper.
Final tip: Learn the microbe–product pairs cold (LAB–curd, Saccharomyces–ethanol, Penicillium–penicillin, Monascus–statins, Trichoderma–cyclosporin A, Rhizobium–nitrogen, Glomus–phosphorus) — a large share of the marks in this chapter is simply naming the right organism for the right job. Draw and label the biogas plant and the sewage-treatment flow wherever a diagram is allowed.