Microbes in Human Welfare – Class 12 Biology Chapter 8 Notes & NCERT PDF

Class 12BiologyChapter 8NCERT book: Biology - Class 12

Notes and a simple summary of Chapter 8 of the NCERT Class 12 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 8: Microbes in Human Welfare

Introduction

Microbes are major components of biological systems. They live everywhere: in soil, water, air, inside bodies of animals and plants, and even in geysers (thermal vents) at about 100°C, deep in soil, under thick snow and in highly acidic environments. They include protozoa, bacteria, fungi, microscopic plant and animal viruses, viroids and prions (proteinaceous infectious agents). Bacteria and fungi can be grown on nutritive media to form visible colonies. Chapter 7 showed microbes causing disease; this chapter shows how useful they are at home, in industry, in sewage treatment, energy production, pest control and soil fertility.

Key Concepts

8.1 Microbes in Household Products

Curd: Lactobacillus and other lactic acid bacteria (LAB) grow in milk. They produce acids that coagulate and partially digest milk proteins. A little curd added as inoculum (starter) contains millions of LAB that multiply at suitable temperature. Curd has more vitamin B₁₂ than milk. In our stomach LAB check disease-causing microbes.

Dosa and idli dough is fermented by bacteria; its puffed-up look is due to CO₂.

Bread dough is fermented by *baker's yeast, Saccharomyces cerevisiae***.

Toddy, a traditional drink of southern India, is fermented palm sap. Fish, soyabean and bamboo shoots are also fermented into foods.

Cheese: texture, flavour and taste depend on the microbes used. Large holes of Swiss cheese come from lots of CO₂ produced by the bacterium *Propionibacterium sharmanii. Roquefort cheese* is ripened by a specific fungus.

8.2 Microbes in Industrial Products

Industrial-scale production uses very large vessels called fermentors.

8.2.1 Fermented beverages: Saccharomyces cerevisiae, here called brewer's yeast, ferments malted cereals and fruit juices to ethanol. Wine and beer are made without distillation; whisky, brandy and rum are made by distillation of the fermented broth.

8.2.2 Antibiotics: chemical substances produced by some microbes that kill or retard growth of other (disease-causing) microbes. 'Anti' means against and 'bio' life; for humans they are 'pro life'.

Alexander Fleming, working on Staphylococci, saw a mould on an unwashed culture plate around which the bacteria could not grow.

He named the chemical Penicillin, after the mould *Penicillium notatum*; it was the first antibiotic and a chance discovery.

Ernest Chain and Howard Florey established its full potential; it treated soldiers wounded in World War II.

Fleming, Chain and Florey won the Nobel Prize in 1945.

Antibiotics helped treat plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

8.2.3 Chemicals, enzymes and bioactive molecules:

Organic acids: Aspergillus niger (fungus) – citric acid; Acetobacter aceti (bacterium) – acetic acid; Clostridium butylicum (bacterium) – butyric acid; Lactobacillus (bacterium) – lactic acid.

Ethanol: Saccharomyces cerevisiae.

Enzymes: lipases in detergents remove oily stains; pectinases and proteases clarify bottled fruit juices.

Streptokinase from Streptococcus, modified by genetic engineering, is a 'clot buster' for patients with myocardial infarction.

Cyclosporin A, an immunosuppressant for organ-transplant patients, comes from the fungus *Trichoderma polysporum*.

Statins from the yeast *Monascus purpureus* lower blood cholesterol by competitively inhibiting the enzyme that synthesises cholesterol.

8.3 Microbes in Sewage Treatment

Municipal waste water, mainly human excreta, is sewage; it has much organic matter and many pathogenic microbes, so it is treated in sewage treatment plants (STPs) by heterotrophic microbes naturally present in it.

Primary treatment (physical):

Floating debris is removed by sequential filtration.

Grit (soil, small pebbles) is removed by sedimentation.

Settled solids form primary sludge; the supernatant is the effluent, sent to secondary treatment.

Secondary (biological) treatment:

Primary effluent goes to large aeration tanks, agitated mechanically with air pumped in.

Useful aerobic microbes grow into flocs (bacteria with fungal filaments forming mesh-like structures) and consume most organic matter, reducing BOD.

The effluent passes to a settling tank where flocs sediment as activated sludge.

A small part of activated sludge returns to the aeration tank as inoculum.

The major part goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi, producing methane, hydrogen sulphide and CO₂ (biogas).

Treated effluent is released into rivers and streams.

BOD (biochemical oxygen demand) is the amount of oxygen consumed if all organic matter in one litre of water were oxidised by bacteria. It indirectly measures organic matter; higher BOD means greater polluting potential. The Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build more STPs.

8.4 Microbes in Production of Biogas

Biogas is a gas mixture, predominantly methane, produced by microbes and used as fuel. Bacteria growing anaerobically on cellulosic material produce methane with CO₂ and H₂; they are called methanogens, e.g., *Methanobacterium. They occur in anaerobic sludge and in the rumen* of cattle, where they help break down cellulose. Cattle dung (gobar) is rich in them, hence 'gobar gas'.

Biogas plant: a concrete tank 10–15 feet deep receives bio-wastes and dung slurry; a floating cover rises as gas forms; an outlet pipe supplies gas to nearby houses for cooking and lighting; spent slurry is removed and used as fertiliser. The technology was developed in India mainly by IARI and KVIC.

8.5 Microbes as Biocontrol Agents

Biocontrol is the use of biological methods to control plant diseases and pests, reducing dependence on toxic chemicals. The organic farmer keeps pests at manageable levels through checks and balances rather than eradicating them, since beneficial predators depend on them.

Ladybird beetles control aphids; dragonflies control mosquitoes.

**Bacillus thuringiensis (Bt): dried spores mixed with water are sprayed on brassicas and fruit trees; caterpillars eat them, the toxin is released in the larval gut and kills them, leaving other insects unharmed. Bt toxin genes introduced into plants give pest-resistant crops such as Bt-cotton**.

**Trichoderma:** free-living fungi in root ecosystems, biocontrol agents of plant pathogens.

Baculoviruses (mostly genus *Nucleopolyhedrovirus) attack insects and arthropods; they are species-specific and narrow-spectrum, harmless to plants, mammals, birds, fish and non-target insects, ideal for integrated pest management (IPM)*.

8.6 Microbes as Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of soil; main sources are bacteria, fungi and cyanobacteria.

Symbiotic bacteria: *Rhizobium* in root nodules of legumes fixes atmospheric nitrogen.

Free-living bacteria: *Azospirillum* and *Azotobacter* fix nitrogen in soil.

Mycorrhiza: fungi (many of genus *Glomus) absorb phosphorus* and pass it to plants; plants also gain resistance to root-borne pathogens and tolerance to salinity and drought.

Cyanobacteria: autotrophic nitrogen fixers such as **Anabaena, Nostoc, Oscillatoria; important biofertilisers in paddy fields**, adding organic matter too.

Diagrams to Know

Secondary treatment: aeration tank → settling tank → activated sludge returned as inoculum, remainder to anaerobic digester.

Biogas plant: dung slurry inlet, digester tank, floating gas holder, gas outlet pipe, spent slurry outlet.

Fermentor: large closed vessel for industrial-scale microbial growth.

Key-Facts Sheet

Penicillin: Penicillium notatum; Nobel Prize 1945.

Cyclosporin A: Trichoderma polysporum; statins: Monascus purpureus.

Swiss cheese: Propionibacterium sharmanii.

Methanogen: Methanobacterium.

Board Exam Focus

Key difference between primary (physical) and secondary (biological) treatment.

BOD puzzle: samples A, B and C show 20, 8 and 400 mg/L. C (400) is untreated sewage and most polluted; B (8) is the relatively clean river water; A (20) is the secondary effluent.

Examples showing microbes release gases (dough, Swiss cheese, biogas).

How microbes reduce use of chemical fertilisers and pesticides.

Arrange biogas, citric acid, penicillin and curd by importance, with reasons.

Key Terms

Microbe (सूक्ष्मजीव)

Fermentation (किण्वन)

Fermentor (किण्वक)

Antibiotic (प्रतिजैविक)

Sewage (वाहित मल)

Activated sludge (सक्रियित आपंक)

Floc (ऊर्णक)

Biochemical oxygen demand (जैव रासायनिक ऑक्सीजन माँग)

Biogas (जैव गैस)

Methanogen (मीथेनोजन)

Biocontrol (जैव नियंत्रण)

Biofertiliser (जैव उर्वरक)

Mycorrhiza (कवकमूल)

Common Mistakes

Saying yeast makes curd; curd is made by lactic acid bacteria.

Saying beer is distilled; wine and beer are not distilled, whisky, brandy and rum are.

Thinking low BOD means more pollution; higher BOD means more organic matter.

Stating Bt kills the insect on contact; the toxin acts only after release in the larval gut.

Mixing up sources: cyclosporin A is from Trichoderma polysporum, statins from Monascus purpureus.

💡 Key Learning Points

✓Lactic acid bacteria make curd (adding vitamin B₁₂), yeast leavens bread and ferments beverages, and specific microbes give cheeses their texture and flavour.
✓Industrial microbes produce antibiotics (penicillin), organic acids, ethanol, enzymes (lipases, pectinases, proteases, streptokinase) and drugs like cyclosporin A and statins.
✓Sewage is treated physically in primary treatment and biologically in secondary treatment, where aerobic flocs reduce BOD and anaerobic digesters produce biogas.
✓Methanogens such as Methanobacterium in cattle dung produce methane-rich biogas used for cooking and lighting in rural areas.
✓Biocontrol agents (Bt, Trichoderma, baculoviruses, ladybirds) and biofertilisers (Rhizobium, Azotobacter, Glomus mycorrhiza, cyanobacteria) reduce reliance on chemicals.

👨‍🏫 Teaching Tips

→Start with a kitchen hunt: ask students to list foods at home made using microbes, then classify them by organism.
→Set curd with a little starter in class and observe after a day; discuss inoculum and temperature.
→Draw the sewage treatment plant as a flow chart and have students trace a drop of water from sewer to river.
→Solve the BOD sample puzzle together, reasoning from 'more organic matter means more oxygen used'.
→Play a matching game: cards with microbes on one set and products on another.
→Arrange a visit or video tour of a biogas plant or STP, and ask students to label each part.
→Debate organic farming versus chemical farming, using biocontrol and biofertiliser examples as evidence.

📋 Assessment Questions

Can the student name microbes used for curd, bread, Swiss cheese, citric acid, penicillin, cyclosporin A and statins?
Can the student describe primary and secondary sewage treatment and define BOD?
Can the student explain how biogas is produced and describe a biogas plant?
Can the student explain the action of Bacillus thuringiensis and baculoviruses as biocontrol agents?
Can the student give examples of biofertilisers and explain how they enrich soil?

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