Section 10 — Solved Examples
This is the dedicated problem set for Chapter 8 (Microbes in Human Welfare). The 30 worked examples below are arranged in three tiers — concept checks (microbe-product pairings, definitions, key terms), application & scenarios (case-style situations on fermentation, sewage, biogas and biocontrol), and analytical & multi-concept (reasoning problems that link several topics across the whole chapter).
Almost every question in this chapter turns on one habit: naming the exact microbe and the exact product, and knowing whether the microbe is a bacterium, a fungus or a yeast. Recall alone is rarely enough — you are usually asked why a process works, how a treatment step reduces pollution, or what advantage a biological method has over a chemical one.
How to use this section
- Concept Checks (Q1–Q10): Quick recall of microbes, products and definitions. If you stumble on more than one or two, revisit Sections 2–7.
- Application & Scenarios (Q11–Q20): The workhorse 2- and 3-mark questions — fermentation logic, sewage stages, BOD, biogas and biocontrol.
- Analytical & Multi-Concept (Q21–Q30): Multi-step reasoning that ties several topics together, the kind that separates a good answer from a full-mark answer.
Total target time: around an hour for a complete revision sweep.
Note: This section is for practice and revision only — there is no quiz at the end. Treat each answer as a model of how much detail an examiner expects.
Memory Capsule — Facts Worth Locking In
Before working through the problems, fix these ten high-yield facts firmly in mind:
| # | Fact | Where it is tested |
|---|---|---|
| 1 | Curd — lactic acid bacteria (LAB) such as Lactobacillus; they also raise vitamin B12 | Household products |
| 2 | Idli, dosa and bread dough puff up because microbes release CO2; bread uses Saccharomyces cerevisiae | Fermentation logic |
| 3 | Wine and beer — without distillation; whisky, brandy and rum — with distillation; all use brewer's yeast | Beverage questions |
| 4 | Penicillin — first antibiotic; Alexander Fleming from Penicillium notatum; Chain and Florey developed it | Antibiotic history |
| 5 | Citric acid — Aspergillus niger; acetic acid — Acetobacter aceti; butyric acid — Clostridium butylicum; lactic acid — Lactobacillus | Acid producers |
| 6 | Cyclosporin A — Trichoderma polysporum (immunosuppressant); statins — Monascus purpureus (lower cholesterol); streptokinase — Streptococcus (clot buster) | Bioactive molecules |
| 7 | BOD = biochemical oxygen demand; higher BOD means more organic matter and greater polluting potential | Sewage questions |
| 8 | Secondary treatment forms flocs and lowers BOD; leftover sludge goes to anaerobic digesters that yield biogas | Sewage stages |
| 9 | Methanogens such as Methanobacterium make methane from cellulose; they live in the rumen of cattle, so gobar gives gobar gas | Biogas |
| 10 | Biocontrol — Bt (Bacillus thuringiensis) kills caterpillars, Trichoderma fights plant pathogens, Baculovirus (Nucleopolyhedrovirus) is species-specific | Biocontrol and IPM |
Pro tip: For almost every question here, two things earn marks — the correct microbe (with its type) and the product or role. Add one line on how it works and you have a full answer.
Concept Checks (Q1–Q10)
Q1. Which group of bacteria converts milk into curd, and name one common example.
Answer: Milk is converted into curd by lactic acid bacteria (LAB), a common example being Lactobacillus. As they grow, the LAB produce acids that coagulate and partially digest the milk proteins. A little curd added to fresh milk acts as the starter, since it already contains millions of these bacteria. Curd also improves the nutritional quality of milk by increasing its vitamin B12 content.
Q2. The dough for idli and dosa puffs up before cooking. Which gas causes this, and where does it come from?
Answer: The puffed-up appearance is due to carbon dioxide (CO2) produced by the fermentation of the dough by bacteria. The gas is released as the microbes break down sugars in the dough, and being trapped inside, it makes the dough rise and gives the cooked food its soft, spongy texture.
Q3. Which microbe is used to ferment dough for making bread, and what is its common name?
Answer: Bread dough is fermented using Saccharomyces cerevisiae, commonly called baker's yeast. The yeast produces CO2, which makes the dough rise. The very same species, when used for producing alcoholic drinks, is called brewer's yeast.
Q4. Name the microbe responsible for the large holes in Swiss cheese and explain what forms them.
Answer: The large holes in Swiss cheese are formed by the bacterium Propionibacterium sharmanii, which produces a large amount of carbon dioxide during ripening. The trapped gas creates the characteristic holes. In contrast, Roquefort cheese gets its special flavour from a specific fungus grown on it.
Q5. Which yeast is used to produce alcoholic beverages, and what is it commonly called in this role?
Answer: Alcoholic beverages are produced using Saccharomyces cerevisiae, called brewer's yeast in this context. It ferments malted cereals and fruit juices to produce ethanol. Depending on the raw material and whether distillation is used, different drinks are obtained.
Q6. Who discovered penicillin, and from which mould was it obtained?
Answer: Penicillin, the first antibiotic to be discovered, was found by Alexander Fleming. He obtained it from the mould Penicillium notatum, having noticed that Staphylococci could not grow around the mould on one of his culture plates. Its full potential was later established by Ernest Chain and Howard Florey, and the three shared the Nobel Prize in 1945.
Q7. Match each acid with the microbe that produces it: (a) citric acid, (b) acetic acid.
Answer: (a) Citric acid is produced by the fungus Aspergillus niger. (b) Acetic acid is produced by the bacterium Acetobacter aceti. For completeness, butyric acid comes from the bacterium Clostridium butylicum and lactic acid from the bacterium Lactobacillus.
Q8. What is meant by BOD, and what does a high BOD value indicate about water?
Answer: 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. It is measured by the rate at which micro-organisms take up oxygen from a water sample. The greater the BOD, the more organic matter the water contains and the greater its polluting potential.
Q9. Name the group of bacteria that produce methane and give one common example.
Answer: Bacteria that grow anaerobically on cellulosic material and produce large amounts of methane are collectively called methanogens. A common example is Methanobacterium. They are found in the anaerobic sludge during sewage treatment and also in the rumen of cattle.
Q10. Name the bacterium used as a biocontrol agent against butterfly caterpillars, and its short name.
Answer: The bacterium is Bacillus thuringiensis, usually written as Bt. It is sold as dried spores in sachets, which are mixed with water and sprayed on vulnerable crops. When the insect larvae eat the sprayed plants, the toxin is released in their gut and kills them, while leaving other insects unharmed.
Application & Scenarios (Q11–Q20)
Q11. A student says curd is simply spoiled milk. Correct this and explain how curd forms and why it is more nutritious.
Answer: Curd is not spoiled milk; it is the product of a controlled fermentation. Lactic acid bacteria added as a starter multiply at a suitable temperature and produce acids that coagulate the milk proteins, setting the milk into curd. Far from being spoiled, curd is more nutritious than the milk it came from, because the bacteria increase its vitamin B12 content. These same bacteria also help check disease-causing microbes in our stomach.
Q12. Wine, beer, whisky and rum are all made by fermenting sugars with yeast, yet they differ. On what single processing step does the difference chiefly depend?
Answer: The key difference lies in distillation. Wine and beer are produced without distillation, so they have a lower alcohol content. Whisky, brandy and rum are produced by distilling the fermented broth, which concentrates the alcohol and gives a stronger drink. The raw material used also affects the final product, but distillation is the decisive processing step separating the two groups.
Q13. A patient recovering from a heart attack is given a drug to dissolve blood clots. Name the bioactive molecule, its microbial source, and how it was improved.
Answer: The drug is streptokinase, produced by the bacterium Streptococcus and modified by genetic engineering. It acts as a clot buster, removing clots from the blood vessels of patients who have suffered a myocardial infarction. Improving it through genetic engineering made it more suitable for medical use.
Q14. An organ-transplant patient is prescribed a drug to prevent rejection. Name this bioactive molecule and the fungus that produces it.
Answer: The drug is cyclosporin A, an immunosuppressive agent given to organ-transplant patients to stop the body from rejecting the new organ. It is produced by the fungus Trichoderma polysporum. By suppressing the immune response, it allows the transplanted organ to be accepted.
Q15. Bottled fruit juices from the market are clearer than juice made at home. Which enzymes are responsible, and where do they come from?
Answer: Bottled juices are clarified using the enzymes pectinases and proteases, which are obtained from microbes. These enzymes break down the substances that make home-made juice cloudy, leaving the bottled juice clearer. In the same way, microbial lipases are added to detergents to remove oily stains from laundry.
Q16. Describe what happens during the primary treatment of sewage and name the two products it yields.
Answer: Primary treatment involves the physical removal of particles from the sewage by filtration and sedimentation. Floating debris is first removed by sequential filtration, then grit such as soil and small pebbles settles out by sedimentation. The solids that settle form the primary sludge, and the liquid left above, the effluent, is passed on for secondary treatment. No microbial digestion happens yet at this stage.
Q17. Explain how secondary treatment lowers the BOD of sewage, and name the microbial structures involved.
Answer: In secondary treatment the primary effluent is passed into large aeration tanks, where it is agitated and air is pumped in. This encourages the vigorous growth of aerobic microbes into flocs — masses of bacteria held together with fungal filaments in a mesh-like structure. As the flocs grow, the microbes consume most of the organic matter in the effluent, which is what reduces its BOD and makes the water far less polluting.
Q18. After the BOD of the effluent falls, what happens to the bacterial flocs, and how is biogas produced in the process?
Answer: Once the BOD is low enough, the effluent goes into a settling tank where the flocs sediment as activated sludge. A small part of this is pumped back into the aeration tank as inoculum. The rest is sent to anaerobic sludge digesters, where anaerobic bacteria digest the microbes in the sludge and, in doing so, produce a mixture of gases — methane, hydrogen sulphide and carbon dioxide — which together form biogas that can be used as fuel.
Q19. Cattle dung is a good raw material for biogas. Explain why, linking it to the biology of the cow.
Answer: Cattle dung, called gobar, is rich in methanogens because these bacteria live in the rumen of cattle, where they help digest the cellulose in the animal's food. Since the rumen is full of these methane-producing bacteria, the excreta of cattle carries them out too. Fed as a slurry into a biogas plant, the dung supplies both the microbes and the cellulosic material needed, so the plant produces gobar gas for cooking and lighting.
Q20. A farmer wants to control caterpillars on his brassica crop without spraying toxic chemicals. Suggest a microbial method and explain how it works selectively.
Answer: The farmer can use Bacillus thuringiensis (Bt), sold as dried spores that are mixed with water and sprayed on the plants. When the caterpillars feed on the sprayed leaves, the toxin is released in their gut and kills the caterpillars while leaving other insects unharmed. This selectivity is the advantage of biocontrol: harmful pests are removed, but beneficial predatory and parasitic insects survive.
Analytical & Multi-Concept (Q21–Q30)
Q21. In three separate processes — making idli dough, ripening Swiss cheese and brewing wine — a gas is produced by microbes. Identify the gas in each and explain why wine differs from the other two.
Answer: In all three, the microbes release carbon dioxide: it makes idli dough rise, it forms the large holes in Swiss cheese, and it bubbles off during wine fermentation. The difference is that in dough and cheese the CO2 is the useful outcome, giving texture and holes. In wine the valued product is not the gas but the ethanol made by the yeast alongside the CO2. So the same metabolic release serves different ends depending on what we want from the process.
Q22. Both statins and cyclosporin A are microbial products used in medicine, yet they act very differently. Name each microbe and contrast their medical roles.
Answer: Statins are produced by the yeast Monascus purpureus and are used as blood-cholesterol lowering agents; they work by competitively inhibiting the enzyme responsible for cholesterol synthesis. Cyclosporin A is produced by the fungus Trichoderma polysporum and is used as an immunosuppressant in organ-transplant patients. One controls a metabolic pathway; the other dampens the immune system, showing how varied microbial bioactive molecules can be.
Q23. Three water samples — clean river water, untreated sewage and secondary effluent — give BOD values of 20 mg/L, 8 mg/L and 400 mg/L in some order. Assign each value and identify the most polluted sample.
Answer: BOD rises with organic content, so the highest value is the dirtiest water. Untreated sewage has the highest BOD, 400 mg/L, and is the most polluted. Clean river water has the lowest, 8 mg/L. The secondary effluent, at 20 mg/L, sits in between — higher than clean river water but far below raw sewage, showing that treatment has removed most of the organic load though not quite all of it.
Q24. Explain why no man-made technology has replaced microbes for sewage treatment, tracing the organic matter from raw sewage right through to the gas that leaves the plant.
Answer: Microbes do the work at every stage cheaply and completely. In secondary treatment, aerobic microbes in flocs eat the bulk of the organic matter, sharply lowering the BOD. The settled activated sludge is then digested by anaerobic bacteria, which break down the microbial mass and give off biogas. So the organic pollution is not just removed but converted into clean water and usable fuel. This efficiency, sustained for over a century worldwide, is why microbial treatment has never been bettered by any man-made technology.
Q25. A rice farmer and a legume farmer each want to enrich their soil biologically. Recommend a suitable biofertiliser for each and explain how it enriches the soil.
Answer: For the legume farmer, Rhizobium is ideal: it lives in root nodules in symbiotic association with the leguminous plant and fixes atmospheric nitrogen into organic forms the plant can use. For the rice (paddy) farmer, cyanobacteria such as Anabaena, Nostoc and Oscillatoria work well: they fix atmospheric nitrogen and also add organic matter to the flooded paddy soil, raising its fertility. Both reduce the need for chemical fertilisers.
Q26. A plant forms a mycorrhizal association with a fungus of the genus Glomus. List the benefits the plant gains and state what the association represents.
Answer: In a mycorrhiza, the fungal partner absorbs phosphorus from the soil and passes it to the plant. Plants with this association gain several further benefits: resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth and development. Many members of the genus Glomus form such associations, which are an example of a mutually beneficial symbiosis between a fungus and a plant root.
Q27. Two nitrogen-fixing bacteria, Rhizobium and Azotobacter, both enrich soil nitrogen but differ in how they live. Explain the difference and why it matters to a farmer.
Answer: Rhizobium fixes atmospheric nitrogen only in symbiotic association, living inside the root nodules of leguminous plants; without a legume host it does not perform this role. Azotobacter (like Azospirillum) fixes nitrogen while free-living in the soil, needing no host. This matters because a farmer growing legumes benefits directly from Rhizobium, whereas Azotobacter can enrich the soil for non-leguminous crops too, since it does not depend on a partner plant.
Q28. Compare Bt and Baculovirus as biocontrol agents, and explain why a Baculovirus is especially valued in an integrated pest management programme.
Answer: Bacillus thuringiensis (Bt) is a bacterium that kills caterpillars when they eat sprayed plants, sparing other insects. Baculoviruses, mostly of the genus Nucleopolyhedrovirus, are viruses that attack insects and other arthropods. Their special value lies in being species-specific and narrow-spectrum, with no harmful effect on plants, mammals, birds, fish or even non-target insects. This makes them ideal in an integrated pest management (IPM) programme, where beneficial insects are being conserved or a sensitive area is being treated.
Q29. A region is switching from chemical pesticides and fertilisers to biological alternatives. Explain the two problems this addresses and give one microbial solution for each.
Answer: The two problems are chemical pollution and toxicity. Chemical insecticides and pesticides are toxic to humans and animals and pollute soil, groundwater and produce; the solution is biocontrol, for example using Trichoderma against plant pathogens or Bt against insect pests. The overuse of chemical fertilisers also causes serious pollution; the solution is biofertilisers such as Rhizobium, Azotobacter or cyanobacteria, which enrich the soil naturally. Together these greatly reduce dependence on harmful chemicals.
Q30. Rank curd, penicillin, biogas and citric acid by importance to human welfare and justify your ranking, naming the microbe behind each.
Answer: A defensible order, most important first, is penicillin, biogas, curd, citric acid. Penicillin (from Penicillium) ranks first because antibiotics have saved millions of lives from diseases such as diphtheria and whooping cough. Biogas (from methanogens like Methanobacterium) comes next as a clean, renewable fuel that also disposes of waste. Curd (from lactic acid bacteria) is a valuable daily food that improves nutrition. Citric acid (from Aspergillus niger) is useful in industry but least critical to survival. Any well-reasoned order is acceptable, provided the justification links each product to its microbe and its benefit.
End of Section 10
You have now worked through 30 examples spanning household products and fermentation, alcoholic beverages, antibiotics and their discovery, industrial acids, enzymes and bioactive molecules, sewage treatment and BOD, biogas and methanogens, biocontrol agents, and biofertilisers. For every question, hold to the same habit — name the exact microbe, name the product or role, and add one line on how it works — and you will rarely leave marks on the table.