NCERT Solutions for Class 12 Biology Chapter 8: Microbes in Human Welfare
These Class 12 Biology Chapter 8 solutions cover Microbes in Human Welfare from the latest NCERT textbook (session 2026–27). Every question in the end-of-chapter Exercises is reproduced exactly as in the book and answered in clear, exam-ready language — covering microbes in household and industrial products, sewage treatment, biogas, biocontrol agents and biofertilisers, with named examples you can write directly in your board paper.
Class: 12Subject: BiologyChapter: 8Chapter Name: Microbes in Human WelfareUnit: Biology in Human WelfareSession: 2026–27
Chapter 8, Microbes in Human Welfare, shows that although some microbes cause disease, a great many are useful to humankind. The chapter is organised into six areas: microbes in household products (curd, dough, cheese), microbes in industrial products (fermented beverages, antibiotics, organic acids, enzymes and bioactive molecules), microbes in sewage treatment (primary and secondary/biological treatment, BOD, activated sludge), microbes in the production of biogas (methanogens such as Methanobacterium), microbes as biocontrol agents (Bacillus thuringiensis, Trichoderma, baculoviruses) and microbes as biofertilisers (Rhizobium, Azotobacter, mycorrhiza, cyanobacteria). The recurring theme is that microbial methods can replace toxic chemicals and recycle wastes, making them central to sustainable human welfare.
Key Concepts & Named Examples
Lactic acid bacteria (LAB): e.g. Lactobacillus; convert milk to curd and increase vitamin B12; also check disease-causing microbes in the gut.
Baker’s/brewer’s yeast:Saccharomyces cerevisiae; leavens bread (CO2) and ferments sugars to ethanol for wine, beer, whisky, brandy and rum.
Antibiotics: chemical substances from microbes that kill or retard disease-causing microbes; Penicillin from Penicillium notatum (discovered by Alexander Fleming).
Questions below are reproduced verbatim from the NCERT textbook. Answers are original and written in CBSE board-exam style.
1. Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
ANSWERI would carry a small amount of curd. Curd is rich in Lactobacillus and other lactic acid bacteria (LAB), which are responsible for converting milk into curd.A drop of curd smeared thinly on a slide and observed under a microscope clearly shows large numbers of these rod-shaped bacteria, making it the simplest household sample to demonstrate the presence of microbes. (Other valid samples are a pinch of fermented dough or a little leftover idli/dosa batter, which contain abundant microbes.)
2. Give examples to prove that microbes release gases during metabolism.
ANSWER(i) The dough used for making bread, idli and dosa is fermented by yeast/bacteria; the puffed-up, spongy appearance is due to release of CO2 gas.(ii) Bacteria such as Propionibacterium sharmanii produce large amounts of CO2, which creates the large holes in ‘Swiss cheese’.(iii) In anaerobic sludge digesters and biogas plants, methanogens (e.g. Methanobacterium) growing on cellulosic material release a mixture of methane, CO2 and H2 (biogas).
3. In which food would you find lactic acid bacteria? Mention some of their useful applications.
ANSWERLactic acid bacteria (LAB), such as Lactobacillus, are found in curd (and other fermented milk foods).Useful applications: (i) they convert milk into curd by producing acids that coagulate and partially digest milk proteins; (ii) they improve the nutritional quality of curd by increasing vitamin B12; (iii) in our stomach, LAB play a beneficial role in checking disease-causing microbes.
4. Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
ANSWER
Raw material
Traditional fermented food
Wheat (its products)
Bread (leavened using baker’s yeast, Saccharomyces cerevisiae)
Rice
Idli and dosa (made from microbially fermented dough)
Bengal gram / black gram (and rice together)
Dhokla and the urad-and-rice batter for idli/dosa, fermented by microbes
In each case, microbial fermentation softens the dough/batter and releases CO2, giving the food a soft, spongy texture.
5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
ANSWERMicrobes produce antibiotics — chemical substances that can kill or retard the growth of disease-causing (pathogenic) microbes.The first antibiotic, Penicillin, was obtained from the mould Penicillium notatum. After it, many other antibiotics were purified from various microbes.Antibiotics have greatly improved our capacity to treat deadly bacterial diseases such as plague, whooping cough, diphtheria and leprosy, which earlier killed millions. Thus microbe-derived antibiotics are one of the most significant medical discoveries for human welfare.
6. Name any two species of fungus, which are used in the production of the antibiotics.
ANSWER(i)Penicillium notatum — source of the antibiotic Penicillin.(ii)Penicillium chrysogenum — a related high-yielding species used commercially for Penicillin production.(Other accepted fungal sources include species of Cephalosporium, which yield cephalosporin antibiotics.)
7. What is sewage? In which way can sewage be harmful to us?
ANSWERSewage: the large quantity of waste water (municipal waste water) generated everyday in cities and towns, whose major component is human excreta. It contains large amounts of organic matter and microbes, many of which are pathogenic.How it is harmful: if discharged untreated into natural water bodies such as rivers and streams, the pathogenic microbes in sewage spread water-borne diseases. The organic matter also raises the BOD of the water, depleting dissolved oxygen and polluting the water body, harming aquatic life and the people who use that water.
8. What is the key difference between primary and secondary sewage treatment?
ANSWER
Primary treatment
Secondary (biological) treatment
Involves physical removal of particles from sewage by filtration and sedimentation.
Involves biological action of microbes that digest the organic matter.
Floating debris is removed by filtration; grit settles to form primary sludge; the supernatant is the effluent.
Effluent is aerated in tanks; aerobic microbes grow as flocs and consume organic matter, greatly reducing BOD; activated sludge settles out.
Key difference: primary treatment is a physical process (removal of solids), whereas secondary treatment is a biological process (microbial breakdown of dissolved/suspended organic matter to lower the BOD).
9. Do you think microbes can also be used as source of energy? If yes, how?
ANSWERYes. Microbes are used to produce biogas, an inflammable mixture of gases containing predominantly methane, which serves as a source of energy.Certain bacteria called methanogens (e.g. Methanobacterium) grow anaerobically on cellulosic material — present in cattle dung (gobar) and in the anaerobic sludge of sewage treatment — and produce methane along with CO2 and H2.In a biogas plant, a slurry of dung is collected in a tank where these microbes produce gas under a floating cover; the biogas is then piped to nearby houses and used for cooking and lighting, while the spent slurry is used as fertiliser.
10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
ANSWERReplacing chemical fertilisers (biofertilisers): microbes enrich soil nutrients naturally. Rhizobium in root nodules of legumes, and free-living Azospirillum and Azotobacter, fix atmospheric nitrogen into forms plants can use. Mycorrhizal fungi (genus Glomus) absorb phosphorus from soil and pass it to the plant. Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix nitrogen and add organic matter, especially in paddy fields. Using these biofertilisers reduces the need for chemical fertilisers.Replacing chemical pesticides (biocontrol agents): microbes can kill pests selectively. Bacillus thuringiensis (Bt), sprayed as dried spores, releases a toxin in the gut of insect larvae and kills them without harming other insects; Trichoderma controls several plant pathogens; baculoviruses (genus Nucleopolyhedrovirus) are species-specific insecticides. These biocontrol measures greatly reduce dependence on toxic chemical pesticides, lowering environmental pollution.
11. Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
ANSWERA higher BOD indicates more organic matter and therefore greater pollution. So the most polluted sample is C (400 mg/L).Assigning labels (river water cleanest, untreated sewage most polluted, secondary effluent in between):
Sample
BOD value
Identity
A
20 mg/L
Secondary effluent (partly treated)
B
8 mg/L
River water (relatively clean)
C
400 mg/L
Untreated sewage water (most polluted)
12. Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
ANSWERCyclosporin A (an immunosuppressive agent used in organ-transplant patients) is produced by the fungus Trichoderma polysporum.Statins (blood-cholesterol lowering agents) are produced by the yeast Monascus purpureus. Statins act by competitively inhibiting the enzyme responsible for synthesis of cholesterol.
13. Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)(b) Soil
ANSWER(a) Single cell protein (SCP): SCP is protein-rich biomass obtained from microbes such as Spirulina (a cyanobacterium), certain bacteria, yeasts and fungi grown in culture. These microbes multiply rapidly and convert simple substrates into protein, which can be used as a food/feed supplement. SCP is an economical, protein-rich source that helps reduce pressure on conventional agriculture and is being explored to address protein-malnutrition.(b) Soil: microbes maintain soil fertility and structure. Decomposer bacteria and fungi break down dead organic matter and recycle nutrients (mineralisation). Nitrogen-fixing microbes (Rhizobium, Azotobacter, cyanobacteria) enrich soil nitrogen; mycorrhizal fungi help plants absorb phosphorus; and microbes such as Trichoderma protect roots from pathogens. Together they keep the soil nutrient-rich and biologically healthy.
14. Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer.
Biogas, Citric acid, Penicillin and Curd
ANSWERDecreasing order of importance: Penicillin > Biogas > Curd > Citric acid.Reasons:Penicillin ranks first because it is a life-saving antibiotic that has controlled many deadly bacterial diseases and saved countless lives.Biogas ranks next as a renewable, clean source of energy used for cooking and lighting, especially in rural areas, while also recycling organic waste.Curd follows as a widely consumed, nutritious food that supplies protein and vitamin B12 and supports gut health.Citric acid is placed last; though useful as a food preservative and industrial chemical, it is the least essential to human survival among the four. (A reasoned alternative order is acceptable.)
15. How do biofertilisers enrich the fertility of the soil?
ANSWERBiofertilisers are organisms (bacteria, fungi and cyanobacteria) that enrich the nutrient quality of soil.(i) Nitrogen enrichment:Rhizobium in the root nodules of leguminous plants fixes atmospheric nitrogen into organic forms usable by the plant; free-living Azospirillum and Azotobacter also fix nitrogen in the soil. In paddy fields, cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix nitrogen and add organic matter.(ii) Phosphorus enrichment: mycorrhizal fungi (genus Glomus) form symbiotic associations with plant roots and absorb phosphorus from the soil, passing it to the plant; they also give resistance to root pathogens and tolerance to salinity and drought.By adding nitrogen, phosphorus and organic matter naturally, biofertilisers replenish soil nutrients and reduce dependence on chemical fertilisers.
Extra Practice Questions
Short Answer Type Questions
Q1. What is BOD and what does a high BOD value indicate?
ANSWERBOD (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. A higher BOD indicates more organic matter and hence greater polluting potential of the water.
Q2. Why are large holes seen in ‘Swiss cheese’?
ANSWERThe large holes are due to the production of a large amount of CO2 by the bacterium Propionibacterium sharmanii during ripening of the cheese.
Q3. Name the microbe and the use of streptokinase.
ANSWERStreptokinase is produced by the bacterium 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 myocardial infarction (heart attack).
Q4. What is activated sludge, and what is done with it?
ANSWERActivated sludge is the sediment of bacterial flocs that settles in the settling tank after secondary treatment. A small part is pumped back into the aeration tank as inoculum; the major part is sent to anaerobic sludge digesters where it produces biogas.
Q5. What are methanogens? Give one example and one site where they occur.
ANSWERMethanogens are bacteria that grow anaerobically on cellulosic material and produce methane (along with CO2 and H2). Example: Methanobacterium. They occur in the anaerobic sludge during sewage treatment and in the rumen of cattle.
Long Answer Type Questions
Q1. Describe the role of microbes in industrial products with suitable named examples.
ANSWERMicrobes are used industrially to make beverages, antibiotics, organic acids, enzymes and other bioactive molecules. Fermented beverages:Saccharomyces cerevisiae (brewer’s yeast) ferments malted cereals and fruit juices to ethanol; wine and beer are made without distillation while whisky, brandy and rum are distilled. Antibiotics: Penicillin from Penicillium notatum. Organic acids: citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), butyric acid (Clostridium butylicum), lactic acid (Lactobacillus) and ethanol (Saccharomyces cerevisiae). Enzymes: lipases in detergents remove oily stains; pectinases and proteases clarify bottled fruit juices; streptokinase from Streptococcus is a clot buster. Other bioactive molecules: cyclosporin A (immunosuppressant) from Trichoderma polysporum and statins (cholesterol-lowering) from Monascus purpureus. These products show how microbes contribute massively to industry and medicine.
Q2. Explain the two stages of sewage treatment in detail.
ANSWERPrimary treatment is the physical removal of particles. Floating debris is removed by sequential filtration, and grit (soil and small pebbles) is removed by sedimentation. The settled solids form primary sludge and the supernatant forms the effluent, which goes for secondary treatment. Secondary (biological) treatment passes the primary effluent into large aeration tanks where it is constantly agitated and air is pumped in. This allows vigorous growth of useful aerobic microbes as flocs (masses of bacteria with fungal filaments), which consume most of the organic matter and greatly reduce the BOD. The treated effluent then enters a settling tank where bacterial flocs sediment as activated sludge; a small part is recycled as inoculum, and the rest goes to anaerobic sludge digesters that produce biogas. The final effluent is released into rivers and streams. Microbial sewage treatment has not been rivalled by any man-made technology.
Q3. How are microbes used as biocontrol agents? Discuss with examples.
ANSWERBiocontrol is the use of biological methods to control plant diseases and pests, reducing dependence on toxic insecticides, pesticides and weedicides that pollute the environment. Bacteria:Bacillus thuringiensis (Bt) is sold as dried spores; sprayed onto crops like brassicas and fruit trees, it is eaten by insect larvae and releases a toxin in their gut that kills the larvae while leaving other insects unharmed. Bt toxin genes have also been introduced into plants (e.g. Bt-cotton) to make them pest-resistant. Fungi:Trichoderma species are free-living fungi of root ecosystems and are effective biocontrol agents of several plant pathogens. Viruses: baculoviruses (genus Nucleopolyhedrovirus) are species-specific, narrow-spectrum insecticides with no negative impact on plants, mammals, birds, fish or non-target insects, making them ideal for integrated pest management (IPM). Insect predators such as ladybird (against aphids) and dragonfly (against mosquitoes) are also used.
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Curd is more nutritious than milk.
Reason: Lactic acid bacteria growing in milk increase the amount of vitamin B12 in curd.
A-R 2. Assertion: Antibiotics are described as ‘pro-life’ for human beings.
Reason: Antibiotics kill or retard the growth of disease-causing microbes.
A-R 3. Assertion: Secondary sewage treatment greatly reduces the BOD of the effluent.
Reason: Aerobic microbes growing as flocs consume the organic matter in the effluent.
A-R 4. Assertion: Baculoviruses are ideal for integrated pest management.
Reason: They are broad-spectrum and kill all insects including non-target and beneficial insects.
A-R 5. Assertion: Biofertilisers can reduce the use of chemical fertilisers.
Reason: Microbes such as Rhizobium and cyanobacteria fix atmospheric nitrogen and enrich the soil.
Answer key: 1-(A), 2-(A), 3-(A), 4-(C), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing the microbes — remember LAB (Lactobacillus) makes curd, while yeast (Saccharomyces cerevisiae) makes bread and ethanol.
Thinking a high BOD means clean water — it is the opposite: high BOD = more organic matter = more polluted.
Calling primary treatment a biological process — primary is physical (filtration/sedimentation); secondary is biological.
Mixing up products and sources, e.g. cyclosporin A (Trichoderma polysporum) vs statins (Monascus purpureus).
Confusing biofertilisers (enrich nutrients) with biocontrol agents (kill pests) — Rhizobium is a biofertiliser, Bt is a biocontrol agent.
Writing ‘biogas is pure methane’ — biogas is a mixture predominantly of methane with CO2 and H2.
Exam tips for this chapter
This is a high-scoring, example-heavy chapter, so memorise the microbe–product–use trios and write the scientific name in italics with correct spelling. For sewage and biogas, draw a clear flow (primary → secondary → activated sludge → anaerobic digester → biogas) and always link BOD to pollution level. Use tables to compare primary vs secondary treatment, and biofertilisers vs biocontrol agents. In ‘arrange in order’ and ‘design/discuss’ questions, give a short reason for every point — reasons carry the marks.
Frequently Asked Questions
What is Class 12 Biology Chapter 8 about?
Chapter 8, Microbes in Human Welfare, explains the useful roles of microbes — in household products (curd, bread, cheese), industrial products (beverages, antibiotics, organic acids, enzymes), sewage treatment, biogas production, biocontrol of pests and as biofertilisers.
Which microbe converts milk into curd?
Lactic acid bacteria (LAB), chiefly Lactobacillus, grow in milk and produce acids that coagulate milk proteins to form curd, also increasing its vitamin B12 content.
What is the difference between primary and secondary sewage treatment?
Primary treatment is a physical process that removes solid particles by filtration and sedimentation, while secondary treatment is a biological process in which aerobic microbes consume the organic matter and reduce the BOD of the effluent.
Are these Class 12 Biology Chapter 8 solutions free?
Yes. All ClearStudy NCERT Solutions for Class 12 Biology are free and follow the official NCERT textbook for session 2026–27.
