Important Questions Class 12 Biology Chapter 8 Microbes in Human Welfare With Answers

Microbes are microscopic organisms and biological agents that include bacteria, fungi, protozoa, viruses, viroids, and prions. Curd, bread, antibiotics, sewage treatment, biogas, and biofertilisers show useful roles of microbes.

Microbes are often remembered for disease, but Class 12 Biology Chapter 8 shows their useful side in food, medicine, farming, waste treatment, and energy production. Important Questions Class 12 Biology Chapter 8 help students revise Microbes in Human Welfare for CBSE 2026-27 board exams, school tests, and pre-board papers. NCERT Chapter 8 includes lactic acid bacteria, fermentation, antibiotics, industrial enzymes, sewage treatment, BOD, activated sludge, biogas, methanogens, biocontrol agents, and biofertilisers.

Key Takeaways

• Lactic Acid Bacteria: LAB convert milk into curd and increase vitamin B12 content.
• BOD: Higher biochemical oxygen demand shows higher organic pollution in waste water.
• Methanogens: Methanobacterium produces methane-rich biogas from cattle dung and sewage sludge.
• Biofertilisers: Rhizobium, Azospirillum, Azotobacter, Glomus, Anabaena, Nostoc, and Oscillatoria improve soil fertility.

Important Questions Class 12 Biology Chapter 8 Structure 2026-27

Important Questions Class 12 Biology Chapter 8 for CBSE 2026-27

CBSE questions from this chapter often ask where a microbe is used and why that use matters. Students should revise names of microbes with their exact products.

1. What are microbes?

Microbes are microscopic organisms and biological agents present in air, water, soil, plants, animals, and extreme habitats. They include bacteria, fungi, protozoa, viruses, viroids, and prions.

1. Some microbes cause diseases.
2. Many microbes support human welfare.
3. Useful microbes help in food, medicine, agriculture, and waste treatment.

Final answer:
Microbes are tiny life forms or infectious agents with major ecological and industrial roles.

2. Where are microbes found?

Microbes are found almost everywhere, including soil, water, air, living bodies, thermal vents, snow layers, and acidic habitats. They can survive where many larger organisms cannot.

1. They live inside humans, animals, and plants.
2. They occur deep inside soil.
3. They survive in hot geysers and acidic conditions.
4. They grow on nutritive media as visible colonies.

Final fact:
Microbial diversity includes bacteria, fungi, protozoa, viruses, viroids, and prions.

3. Why are all microbes not harmful?

All microbes are not harmful because many of them produce useful products and support essential processes. They help in curd formation, fermentation, antibiotics, sewage treatment, and soil fertility.

Useful roles include:

1. Production of curd and cheese.
2. Production of antibiotics.
3. Treatment of sewage.
4. Production of biogas.
5. Improvement of soil fertility.

Final fact:
Microbes play a major role in human welfare.

Microbes in Human Welfare Class 12 Important Questions on Household Products

Indian students often meet this chapter first through familiar foods like curd, idli, dosa, bread, toddy, and cheese. These examples make household microbial use easy to remember for CBSE 2026-27.

4. How is curd formed from milk?

Curd forms when lactic acid bacteria grow in milk and convert it into curd. These bacteria produce acids that coagulate and partially digest milk proteins.

Steps:

1. A small amount of curd acts as starter or inoculum.
2. It adds millions of LAB to fresh milk.
3. LAB multiply at suitable temperature.
4. They produce lactic acid.
5. Milk proteins coagulate to form curd.

Final fact:
Lactobacillus is a common lactic acid bacterium involved in curd formation.

5. What is the role of LAB in curd formation?

LAB convert milk into curd and improve its nutritional quality. They increase vitamin B12 content in curd.

1. LAB stands for lactic acid bacteria.
2. LAB produce acids during growth.
3. These acids coagulate milk proteins.
4. LAB also check disease-causing microbes in the stomach.

Final answer:
LAB help in curd formation, nutrition improvement, and gut protection.

6. Why is a small amount of curd added to fresh milk?

A small amount of curd is added because it contains millions of lactic acid bacteria. It acts as a starter culture for fresh milk.

1. The starter introduces LAB into milk.
2. LAB multiply at suitable temperature.
3. They produce acid.
4. Milk changes into curd.

Final fact:
Curd works as an inoculum for curd formation.

7. Why does dosa and idli batter become puffed up?

Dosa and idli batter becomes puffed up because microbes release carbon dioxide during fermentation. The gas increases the volume of the dough.

1. Bacteria ferment the batter.
2. Fermentation produces CO2.
3. CO2 gets trapped in the batter.
4. The batter becomes light and spongy.

Final answer:
The puffed appearance comes from carbon dioxide release.

8. Which microbe is used in bread-making?

Saccharomyces cerevisiae is used in bread-making. It is commonly called baker’s yeast.

1. Yeast ferments the dough.
2. Fermentation releases CO2.
3. The dough rises due to trapped gas.
4. Baking gives bread its final texture.

Final fact:
Saccharomyces cerevisiae is used for both bread and alcoholic fermentation.

9. How are traditional drinks and fermented foods made using microbes?

Traditional drinks and foods are made when microbes ferment plant or food materials. Fermentation changes flavour, texture, and digestibility.

Examples:

1. Toddy forms by fermenting palm sap.
2. Fish can be fermented by microbes.
3. Soyabean can be fermented by microbes.
4. Bamboo shoots can be fermented by microbes.
5. Idli and dosa batter undergo bacterial fermentation.

Final fact:
Fermentation is a common microbial process in Indian food traditions.

10. Why does Swiss cheese have large holes?

Swiss cheese has large holes because Propionibacterium sharmanii produces a large amount of carbon dioxide. The gas creates holes during cheese formation.

1. The bacterium grows during cheese making.
2. It releases CO2.
3. CO2 forms spaces inside the cheese.
4. These spaces appear as large holes.

Final answer:
The holes in Swiss cheese come from CO2 produced by Propionibacterium sharmanii.

11. Which microbe gives Roquefort cheese its flavour?

Roquefort cheese gets its flavour from a specific fungus grown on it during ripening. The fungus gives the cheese its characteristic taste and aroma.

1. Cheese ripening uses selected microbes.
2. The fungus changes flavour.
3. It also affects texture.

Final fact:
Different cheeses have different microbes behind their texture and flavour.

Class 12 Biology Chapter 8 Important Questions on Industrial Products

Industrial microbiology uses microbes in large fermentors to produce beverages, antibiotics, chemicals, enzymes, and bioactive molecules. Exact microbe-product pairs are common in board questions.

12. What are fermentors?

Fermentors are large vessels used to grow microbes on an industrial scale. They support large-scale production of microbial products.

1. Microbes grow under controlled conditions.
2. Industries use fermentors for antibiotics, beverages, enzymes, and organic acids.
3. Fermentors allow mass production.

Final fact:
Industrial production needs controlled microbial growth in large vessels.

13. Which microbe is used to produce alcoholic beverages?

Saccharomyces cerevisiae is used to produce alcoholic beverages. It is also called brewer’s yeast.

1. It ferments malted cereals.
2. It ferments fruit juices.
3. It produces ethanol.
4. It is used in wine, beer, whisky, brandy, and rum production.

Final fact:
The same yeast is also used in bread-making.

14. What is the difference between wine, beer, whisky, brandy, and rum production?

Wine and beer are produced without distillation, while whisky, brandy, and rum are produced by distillation of fermented broth.

1. Wine comes from fermented fruit juice.
2. Beer comes from fermented cereals.
3. Whisky, brandy, and rum need distillation after fermentation.

Final answer:
Distillation separates whisky, brandy, and rum from wine and beer.

15. What are antibiotics?

Antibiotics are chemical substances produced by some microbes that kill or slow the growth of disease-causing microbes. They are useful against bacterial diseases.

1. “Anti” means against.
2. “Bio” means life.
3. In medicine, antibiotics act against harmful microbes.
4. For humans, antibiotics are life-saving.

Final fact:
Antibiotics greatly improved treatment of infectious diseases.

16. How was penicillin discovered?

Alexander Fleming discovered penicillin by chance while working on Staphylococci bacteria. He noticed that bacteria could not grow around a mould.

1. A mould grew on an unwashed culture plate.
2. Staphylococci did not grow around it.
3. Fleming identified a chemical from the mould.
4. He named it penicillin after Penicillium notatum.

Final fact:
Ernest Chain and Howard Florey later established its full medical value.

17. Why was penicillin important during World War II?

Penicillin was important because it treated wounded American soldiers during World War II. It controlled bacterial infections effectively.

1. Fleming discovered penicillin.
2. Chain and Florey developed its use.
3. It became useful in wartime treatment.
4. Fleming, Chain, and Florey received the Nobel Prize in 1945.

Final fact:
Penicillin was the first antibiotic discovered.

18. Name microbes used for organic acid production.

Different microbes produce different organic acids in industry. NCERT gives specific microbe-product pairs.

Final fact:
Microbes produce commercially useful organic acids.

19. Which microbe is used for commercial ethanol production?

Saccharomyces cerevisiae is used for commercial production of ethanol. It ferments sugars anaerobically.

1. Yeast uses sugar as substrate.
2. Fermentation produces ethanol.
3. CO2 is also released.

Final answer:
Saccharomyces cerevisiae produces ethanol commercially.

20. What are lipases used for?

Lipases are used in detergent formulations to remove oily stains from clothes. They break down fats and oils.

1. Oily stains contain lipid material.
2. Lipases act on lipids.
3. Detergents with lipases clean oily stains better.

Final fact:
Lipases are industrially useful enzymes.

21. Why are bottled fruit juices clearer than homemade juices?

Bottled fruit juices are clearer because pectinases and proteases clarify them. These enzymes break down substances that make juice cloudy.

1. Pectinases break pectin.
2. Proteases break proteins.
3. Cloudiness decreases after enzyme action.

Final fact:
Commercial juice clarification uses microbial enzymes.

22. What is streptokinase used for?

Streptokinase is used as a clot buster in patients with myocardial infarction. It removes clots from blood vessels.

1. Streptococcus produces streptokinase.
2. Genetic engineering modifies it.
3. It dissolves blood clots.
4. It helps heart attack patients.

Final answer:
Streptokinase removes clots from blood vessels.

23. What are cyclosporin A and statins?

Cyclosporin A is an immunosuppressant, while statins lower blood cholesterol. Both are bioactive molecules from microbes.

1. Cyclosporin A comes from Trichoderma polysporum.
2. It helps organ transplant patients.
3. Statins come from Monascus purpureus.
4. Statins inhibit cholesterol synthesis.

Final fact:
Microbes produce important bioactive molecules.

Microbes in Sewage Treatment Class 12 Questions

Sewage treatment is one of the most searched parts of this chapter because it connects microbes, pollution, BOD, activated sludge, and biogas. It is also common in CBSE competency-based questions.

24. What is sewage?

Sewage is municipal waste water that contains human excreta, organic matter, and many microbes. Some microbes in sewage are pathogenic.

1. Cities and towns produce large amounts of sewage.
2. Sewage cannot enter rivers directly.
3. Untreated sewage pollutes water bodies.
4. It can spread water-borne diseases.

Final fact:
Sewage must be treated before disposal.

25. Why should sewage not be discharged directly into rivers?

Sewage should not enter rivers directly because it contains organic matter and pathogenic microbes. It increases pollution and disease risk.

1. Organic matter increases microbial growth.
2. Pathogens can cause water-borne diseases.
3. Polluted water becomes unsafe.
4. Oxygen levels may fall due to decomposition.

Final answer:
Untreated sewage damages rivers and public health.

26. What happens during primary sewage treatment?

Primary sewage treatment physically removes large and small particles through filtration and sedimentation. It does not mainly depend on microbial action.

Steps:

1. Sequential filtration removes floating debris.
2. Sedimentation removes grit, soil, and small pebbles.
3. Settled solids form primary sludge.
4. The supernatant forms primary effluent.

Final fact:
Primary effluent goes for secondary treatment.

27. What happens during secondary sewage treatment?

Secondary sewage treatment uses aerobic microbes to reduce organic matter in primary effluent. It is also called biological treatment.

Steps:

1. Primary effluent enters aeration tanks.
2. Air is pumped into the tank.
3. Mechanical agitation supports microbial growth.
4. Aerobic microbes form flocs.
5. Microbes consume organic matter.
6. BOD reduces significantly.

Final fact:
Secondary treatment depends on microbial activity.

28. What are flocs in sewage treatment?

Flocs are masses of bacteria associated with fungal filaments that form mesh-like structures. They develop during secondary sewage treatment.

1. Aerobic microbes grow in aeration tanks.
2. Bacteria and fungal filaments join together.
3. These microbial masses consume organic matter.
4. They help reduce BOD.

Final answer:
Flocs are microbial masses used in biological sewage treatment.

29. What is BOD in Class 12 Biology?

BOD is the amount of oxygen consumed by microbes to oxidise organic matter in one litre of water. It measures organic pollution.

Full form:
Biochemical Oxygen Demand

Meaning:

1. Higher BOD means more organic matter.
2. Higher BOD means higher polluting potential.
3. Sewage treatment reduces BOD.

Final fact:
The BOD test indirectly measures organic matter in water.

30. Why does BOD decrease during sewage treatment?

BOD decreases because aerobic microbes consume most organic matter in the effluent. Less organic matter means less oxygen demand.

1. Microbes grow in aeration tanks.
2. They use organic matter as food.
3. Organic load decreases.
4. Oxygen demand decreases.

Final answer:
Lower BOD indicates treated and less polluting water.

31. What is activated sludge?

Activated sludge is the sediment formed when bacterial flocs settle after secondary sewage treatment. It contains active microbial biomass.

1. Treated effluent enters a settling tank.
2. Flocs settle at the bottom.
3. The sediment is called activated sludge.
4. A small part returns to the aeration tank as inoculum.

Final fact:
Most activated sludge goes to anaerobic sludge digesters.

32. What happens in anaerobic sludge digesters?

Anaerobic sludge digesters contain bacteria that digest bacteria and fungi in activated sludge without oxygen. They produce biogas.

1. Activated sludge enters large anaerobic tanks.
2. Anaerobic bacteria digest organic material.
3. Methane, hydrogen sulphide, and carbon dioxide form.
4. These gases form biogas.

Final fact:
Biogas can be used as an energy source.

BOD Class 12 Biology and Water Pollution Questions

BOD questions often appear as application-based questions with water samples. The key is simple: the sample with the highest BOD is the most polluted.

33. Three water samples have BOD values of 20 mg/L, 8 mg/L, and 400 mg/L. Which is most polluted?

The sample with BOD 400 mg/L is the most polluted. Higher BOD means higher organic matter and higher microbial oxygen demand.

Given data:
Sample A = 20 mg/L
Sample B = 8 mg/L
Sample C = 400 mg/L

Reasoning:

1. River water has the lowest BOD.
2. Secondary effluent has moderate BOD.
3. Untreated sewage has the highest BOD.

Final result:
Sample C is untreated sewage and the most polluted.

34. Assign river water, untreated sewage, and secondary effluent to BOD values 8, 20, and 400 mg/L.

River water has 8 mg/L, secondary effluent has 20 mg/L, and untreated sewage has 400 mg/L.

Correct matching:

1. 8 mg/L = relatively clean river water.
2. 20 mg/L = secondary effluent.
3. 400 mg/L = untreated sewage water.

Final answer:
Untreated sewage has the highest BOD.

Biogas Production Class 12 Biology Questions

Biogas questions are popular because they combine microbes, rural India, cattle dung, methane, and renewable energy. NCERT directly connects methanogens with gobar gas plants.

35. What is biogas?

Biogas is a mixture of gases produced by microbial activity and used as fuel. It mainly contains methane.

Main gases:

1. Methane
2. Carbon dioxide
3. Hydrogen sulphide

Final fact:
Biogas is inflammable and can be used for cooking and lighting.

36. What are methanogens?

Methanogens are anaerobic bacteria that produce methane from cellulosic material. Methanobacterium is a common methanogen.

1. They grow without oxygen.
2. They break down cellulosic material.
3. They produce methane, CO2, and H2.
4. They occur in anaerobic sludge and cattle rumen.

Final answer:
Methanogens produce methane-rich biogas.

37. Why is cattle dung used in biogas plants?

Cattle dung is used because it contains methanogens from the rumen of cattle. These bacteria help produce methane.

1. Cattle eat cellulosic plant material.
2. Methanogens live in their rumen.
3. Dung contains these bacteria.
4. Dung slurry produces biogas in anaerobic tanks.

Final fact:
Cattle dung is commonly called gobar.

38. Describe the structure and working of a biogas plant.

A biogas plant uses dung slurry and anaerobic microbes to produce biogas. It is common in rural areas.

Structure:

1. A concrete tank 10-15 feet deep holds bio-waste.
2. Dung slurry enters the tank.
3. A floating cover rises as gas forms.
4. An outlet pipe supplies biogas to houses.
5. Another outlet removes spent slurry.

Final fact:
Spent slurry can be used as fertiliser.

39. Why are biogas plants common in rural India?

Biogas plants are common in rural India because cattle dung is easily available in large quantities. Rural households can use biogas for cooking and lighting.

1. Villages often have cattle.
2. Cattle dung supplies methanogens.
3. Biogas provides local fuel.
4. Slurry works as fertiliser.

Final fact:
IARI and KVIC helped develop biogas technology in India.

Microbes as Biocontrol Agents Class 12 Questions

Biocontrol is searched because students confuse it with biofertilisers. Biocontrol controls pests and diseases, while biofertilisers enrich soil nutrients.

40. What is biocontrol?

Biocontrol is the use of biological methods to control plant diseases and pests. It reduces dependence on chemical pesticides.

1. It uses predators, parasites, pathogens, or microbes.
2. It avoids indiscriminate killing of useful organisms.
3. It supports ecological balance.

Final answer:
Biocontrol uses living organisms to manage pests.

41. Why is biocontrol better than chemical pesticides?

Biocontrol is better because it reduces toxic chemical use and protects beneficial organisms. Chemical pesticides can pollute soil, groundwater, crops, fruits, and vegetables.

1. Chemicals may harm humans and animals.
2. Chemicals may kill useful insects.
3. Biocontrol keeps pests at manageable levels.
4. It supports sustainable agriculture.

Final fact:
Biocontrol fits integrated pest management.

42. How do ladybird beetles and dragonflies help in biocontrol?

Ladybird beetles control aphids, while dragonflies control mosquitoes. They act as natural predators.

1. Ladybird beetles feed on aphids.
2. Dragonflies feed on mosquitoes.
3. Both reduce pest populations naturally.

Final answer:
They are useful predatory biocontrol agents.

43. How is Bacillus thuringiensis used as a biocontrol agent?

Bacillus thuringiensis controls butterfly caterpillars by releasing toxin in the larval gut. It is commonly called Bt.

Steps:

1. Dried Bt spores are mixed with water.
2. The mixture is sprayed on vulnerable plants.
3. Insect larvae eat the spores.
4. Toxin releases in the larval gut.
5. Larvae die while other insects remain unharmed.

Final fact:
Bt toxin genes are also used in Bt cotton.

44. How does Trichoderma act as a biocontrol agent?

Trichoderma acts as a biocontrol agent against several plant pathogens. It is a free-living fungus common in root ecosystems.

1. It lives around plant roots.
2. It suppresses plant pathogens.
3. It supports disease control in crops.

Final fact:
Trichoderma species are useful fungal biocontrol agents.

45. What are baculoviruses?

Baculoviruses are pathogens that attack insects and other arthropods. Most biocontrol baculoviruses belong to the genus Nucleopolyhedrovirus.

1. They show species-specific action.
2. They have a narrow spectrum.
3. They do not harm plants, mammals, birds, fish, or non-target insects.

Final fact:
Baculoviruses are useful in integrated pest management.

Microbes as Biofertilisers Class 12 Questions

Biofertilisers are searched with Rhizobium, cyanobacteria, and mycorrhiza because these are exact NCERT examples. They help reduce dependence on chemical fertilisers in Indian agriculture.

46. What are biofertilisers?

Biofertilisers are organisms that enrich the nutrient quality of soil. They include bacteria, fungi, and cyanobacteria.

Main sources:

1. Bacteria
2. Fungi
3. Cyanobacteria

Final fact:
Biofertilisers help reduce chemical fertiliser use.

47. How does Rhizobium act as a biofertiliser?

Rhizobium fixes atmospheric nitrogen in root nodules of leguminous plants. The plant uses this fixed nitrogen as nutrient.

1. Rhizobium forms symbiotic association with legumes.
2. Root nodules develop.
3. Atmospheric nitrogen changes into organic forms.
4. Plant growth improves.

Final answer:
Rhizobium enriches soil nitrogen through symbiotic nitrogen fixation.

48. Name free-living nitrogen-fixing bacteria used as biofertilisers.

Azospirillum and Azotobacter are free-living nitrogen-fixing bacteria used as biofertilisers. They enrich soil nitrogen.

1. They live freely in soil.
2. They fix atmospheric nitrogen.
3. They improve nutrient content.

Final answer:
Azospirillum and Azotobacter

49. What is mycorrhiza?

Mycorrhiza is a symbiotic association between fungi and roots of higher plants. Many species of Glomus form mycorrhiza.

1. The fungus absorbs phosphorus from soil.
2. It passes phosphorus to the plant.
3. The plant supports the fungus with food.
4. The association improves plant growth.

Final fact:
Mycorrhiza helps plants resist root-borne pathogens, salinity, and drought.

50. How do cyanobacteria act as biofertilisers?

Cyanobacteria act as biofertilisers by fixing atmospheric nitrogen and adding organic matter to soil. They are especially useful in paddy fields.

Examples:

1. Anabaena
2. Nostoc
3. Oscillatoria

Final fact:
Blue-green algae increase soil fertility in paddy fields.

Microbes in Human Welfare NCERT Questions for Board Practice

NCERT exercise questions from this chapter are highly search-relevant because many board answers use the same examples. These answers keep the wording direct and exam-ready.

51. Which sample can show microbes under a microscope from home?

Curd is a suitable sample because it contains lactic acid bacteria. These bacteria can be observed under a microscope.

1. Curd is easily available.
2. It contains Lactobacillus and other LAB.
3. LAB multiply in milk during curd formation.

Final answer:
Curd is a good household sample to demonstrate microbes.

52. Give examples to prove that microbes release gases during metabolism.

Microbes release gases during fermentation and anaerobic digestion. These gases create visible effects.

Examples:

1. Idli and dosa batter rises due to CO2.
2. Bread dough rises due to CO2 from yeast.
3. Swiss cheese holes form due to CO2 from Propionibacterium sharmanii.
4. Methanogens produce methane in biogas plants.

Final fact:
Microbial metabolism can release CO2 and methane.

53. How have microbes helped control bacterial diseases?

Microbes help control bacterial diseases by producing antibiotics. Antibiotics kill or slow disease-causing bacteria.

Examples of diseases controlled:

1. Plague
2. Whooping cough
3. Diphtheria
4. Leprosy

Final fact:
Penicillin was the first antibiotic discovered.

54. What is the key difference between primary and secondary sewage treatment?

Primary treatment is physical, while secondary treatment is biological. Primary treatment removes particles, and secondary treatment reduces BOD using microbes.

Final fact:
Secondary treatment depends on microbial flocs.

55. How do microbes decrease the use of chemical fertilisers and pesticides?

Microbes reduce chemical use by acting as biofertilisers and biocontrol agents. They improve soil nutrients and control pests naturally.

1. Rhizobium fixes nitrogen in legume roots.
2. Cyanobacteria enrich paddy fields.
3. Glomus improves phosphorus absorption.
4. Bt controls insect larvae.
5. Trichoderma controls plant pathogens.

Final answer:
Microbes support organic and sustainable farming.

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