Class 12 : Biology (English) – Lesson 8: Microbes in Human Welfare

EXPLANATION & SUMMARY

🌟 Introduction
🦠 Microbes are tiny organisms (bacteria, fungi, protozoa, viruses, algae) that are invisible to the naked eye but play a huge role in human welfare.
🌱 They are used in food industries, medicine, agriculture, sewage treatment, and energy production.
💉 Some microbes cause diseases, but many are beneficial and essential for life processes.

🍞 Microbes in Household Products
🍶 Lactic acid bacteria (LAB) (Lactobacillus) → curd, cheese, flavour, and nutrient enrichment (Vitamin B12).
🍞 Yeast (Saccharomyces cerevisiae) → bread, fermentation (CO₂ causes dough to rise).
🍺 Yeast (S. cerevisiae) → alcoholic fermentation (beer, wine, toddy).
🍶 Traditional products: Idli, dosa, dhokla, bhatura → fermentation of batter.

🍻 Microbes in Industrial Products

Alcoholic Beverages
🍷 Wine & 🍺 beer from yeast fermentation.
🥃 Distilled spirits like whisky, rum, brandy from distillation.
Antibiotics (💊 life-saving drugs from microbes)
Penicillin (Penicillium notatum) — discovered by Alexander Fleming.
Streptomycin (Streptomyces griseus).
Tetracycline (Streptomyces aureofaciens).
Enzymes & Organic Acids
Amylases (fungi & bacteria) → detergents, starch breakdown.
Lipases → food processing, cleaning.
Acids: Citric acid (Aspergillus niger), Acetic acid (Acetobacter aceti), Lactic acid (LAB).

🚰 Microbes in Sewage Treatment
🏭 Sewage treatment plants (STPs) use microbes to decompose organic matter.
Primary treatment: Physical removal of solids.
Secondary treatment: Biological degradation by microbes.
Aerobic microbes form activated sludge → reduces BOD (biological oxygen demand).
Methanogens (Archaebacteria) in anaerobic sludge digesters → produce biogas.

🌿 Microbes in Production of Biogas
Biogas plants (Gobar gas plants):
Raw material: dung 🐄 + other organic waste.
Microbes: Methanogens (Methanobacterium).
Gas mixture: CH₄ (major), CO₂, H₂.
🔋 Used as fuel for cooking, lighting, and even electricity.

🌾 Microbes as Biofertilisers
🟢 Symbiotic bacteria: Rhizobium in root nodules (legumes) → nitrogen fixation.
🟣 Free-living bacteria: Azotobacter, Azospirillum.
🟠 Cyanobacteria (Blue-Green Algae): Anabaena, Nostoc → nitrogen fixation in rice fields.
🔵 Fungi: Mycorrhiza (Glomus) → symbiotic association with roots → increases phosphorus uptake.
🌍 Eco-friendly alternative to chemical fertilisers.

🧬 Microbes in Medicine & Biotechnology
Antibiotics 💊: Life-saving against bacterial infections.
Immunosuppressants: Cyclosporin A (Trichoderma polysporum) → organ transplant success.
Statins: Produced by Monascus purpureus, lower blood cholesterol.
Recombinant technology: Microbes engineered for insulin, vaccines, hormones.

🌟 Summary (≈300 words)
Microbes are indispensable allies of humans. In households, they make curd, bread, alcohol, and fermented foods. Industrially, microbes are used to produce alcoholic beverages, antibiotics (penicillin, streptomycin), enzymes, and organic acids.
In sewage treatment, microbes reduce pollution by breaking down organic wastes, while methanogens in sludge digesters generate biogas. Agriculture benefits from biofertilisers: Rhizobium, Azotobacter, cyanobacteria, and mycorrhizal fungi improve soil fertility and reduce dependence on chemicals.
In medicine and biotechnology, microbes produce antibiotics, immunosuppressants (cyclosporin A), cholesterol-lowering agents (statins), and recombinant products like insulin.
Thus, microbes, though microscopic, are macroscopic in impact — supporting food, health, energy, agriculture, and the environment.

🎯 Quick Recap
🧫 Household: Curd, bread, beverages.
🏭 Industry: Antibiotics, enzymes, acids, alcohols.
🚰 Sewage: Microbes purify water, reduce BOD.
🔋 Biogas: Methanogens produce fuel.
🌱 Biofertilisers: Nitrogen-fixers, mycorrhiza.
💊 Medicine: Antibiotics, cyclosporin A, statins.

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QUESTIONS FROM TEXTBOOK

🔹 Q1. 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?
👉 Answer:
🥛 Curd, because it contains Lactic Acid Bacteria (LAB) (Lactobacillus).
Under a microscope, rods-shaped bacteria are visible.
LAB multiply and convert milk lactose into lactic acid.

🔹 Q2. Give examples to prove that microbes release gases during metabolism.
👉 Answer:
🍞 Saccharomyces cerevisiae (yeast) releases CO₂ → bread, dosa, idli dough rises.
🍺 In alcoholic fermentation → CO₂ + ethanol are released.
💩 In biogas plants, methanogens release CH₄, CO₂, H₂.

🔹 Q3. In which food would you find lactic acid bacteria? Mention some of their useful applications.
👉 Answer:
Found in curd, yoghurt, cheese.
Uses:
🥛 Fermentation of milk → curd formation.
💪 Enriches curd with Vitamin B12.
🛡️ Suppresses harmful microbes in gut → improves digestion.

🔹 Q4. Name some traditional Indian foods made of wheat, rice, and Bengal gram (or their products) which involve use of microbes.
👉 Answer:
🍚 Rice → idli, dosa.
🌾 Wheat → bhatura, bread.
🌱 Bengal gram flour → dhokla, khandvi.
🍹 Palm sap fermentation → toddy.

🔹 Q5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
👉 Answer:
💊 Antibiotics from microbes kill/inhibit harmful bacteria.
Penicillium notatum → Penicillin.
Streptomyces griseus → Streptomycin.
Streptomyces aureofaciens → Tetracycline.

🔹 Q6. Name any two species of fungus which are used in the production of antibiotics.
👉 Answer:
Penicillium notatum → Penicillin.
Cephalosporium acremonium → Cephalosporins.

🔹 Q7. What is sewage? In which way can sewage be harmful to us?
👉 Answer:
Sewage: Wastewater containing human excreta, organic matter, pathogens.
Harmful effects:
🦠 Spreads water-borne diseases (typhoid, cholera).
☣️ Depletes dissolved oxygen → harms aquatic life.
💩 Causes bad odour, pollution.

🔹 Q8. What is the key difference between primary and secondary sewage treatment?
👉 Answer:
Primary treatment:
Physical removal of large and small particles.
Methods: filtration, sedimentation.
Secondary treatment:
Biological degradation of organic matter by microbes.
Produces activated sludge + reduces BOD.

🔹 Q9. Do you think microbes can also be used as source of energy? If yes, how?
👉 Answer:
✅ Yes. In biogas plants, methanogens like Methanobacterium ferment dung + organic waste.
Produces biogas (CH₄, CO₂, H₂).
Used as 🔥 fuel for cooking, lighting, power.

🔹 Q10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
👉 Answer:
🌱 Biofertilisers:
Rhizobium → nitrogen fixation in legumes.
Azotobacter, Nostoc, Anabaena → free-living nitrogen fixers.
Mycorrhiza (Glomus) → phosphorus uptake.
🐞 Biopesticides:
Bacillus thuringiensis (Bt toxin) → kills insects.
Trichoderma → fungal biocontrol.

🔹 Q11. 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 20 mg/L, 8 mg/L and 400 mg/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?
👉 Answer:
💧 Higher BOD → more polluted.
400 mg/L → Untreated sewage.
20 mg/L → Secondary effluent.
8 mg/L → Relatively clean river water.

🔹 Q12. Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol-lowering agents) are obtained.
👉 Answer:
Cyclosporin A → Trichoderma polysporum.
Statins → Monascus purpureus.

🔹 Q13. Find out the role of microbes in the following:
(a) Single Cell Protein (SCP):
Microbes like Spirulina, Methylophilus methylotrophus → produce protein-rich biomass.
Used as food supplement, low-cost nutrition.
(b) Soil:
Microbes recycle nutrients.
Nitrogen fixation by Rhizobium, Azotobacter.
Decomposition by fungi and bacteria.

🔹 Q14. 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, Curd.
👉 Answer:
Most important:
1️⃣ Penicillin (life-saving antibiotic).
2️⃣ Biogas (renewable energy).
3️⃣ Curd (nutritious food, probiotics).
4️⃣ Citric acid (industrial use).

🔹 Q15. How do biofertilisers enrich the fertility of the soil?
👉 Answer:
🌱 Fix atmospheric N₂ (Rhizobium, Azotobacter).
🌿 Mycorrhiza increases phosphorus uptake.
🍂 Decompose organic matter → release nutrients.
🌍 Reduce chemical fertiliser dependency → eco-friendly.

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OTHER IMPORTANT QUESTIONS FOR EXAMS

(CBSE MODEL QUESTION PAPER)

ESPECIALLY MADE FROM THIS CHAPTER ONLY

🟢 Section A — Very Short MCQs (1 mark each)
🔹 Q1. 🍞 Yeast used for bread-making is:
🔵 (A) Saccharomyces cerevisiae
🟢 (B) Rhizopus arrhizus
🟠 (C) Penicillium notatum
🔴 (D) Aspergillus niger
✅ Answer: (A) Saccharomyces cerevisiae

🔹 Q2. 🍶 Acetic acid (vinegar) is produced by:
🔵 (A) Acetobacter aceti
🟢 (B) Monascus purpureus
🟠 (C) Trichoderma polysporum
🔴 (D) Azotobacter
✅ Answer: (A) Acetobacter aceti

🔹 Q3. 💊 The first antibiotic discovered was:
🔵 (A) Streptomycin
🟢 (B) Tetracycline
🟠 (C) Penicillin
🔴 (D) Rifampicin
✅ Answer: (C) Penicillin

🔹 Q4. 🔋 The chief gas in biogas is:
🔵 (A) CO₂
🟢 (B) H₂
🟠 (C) CH₄
🔴 (D) N₂
✅ Answer: (C) CH₄

🔹 Q5. 🧪 Cyclosporin A (immunosuppressant) is obtained from:
🔵 (A) Trichoderma polysporum
🟢 (B) Streptomyces griseus
🟠 (C) Monascus purpureus
🔴 (D) Penicillium chrysogenum
✅ Answer: (A) Trichoderma polysporum

🔹 Q6. 🩺 Statins (cholesterol-lowering) are produced by:
🔵 (A) Monascus purpureus
🟢 (B) Rhizobium
🟠 (C) Aspergillus niger
🔴 (D) Lactobacillus
✅ Answer: (A) Monascus purpureus

🔹 Q7. 🐛 A microbial biopesticide used against insect larvae is:
🔵 (A) Bacillus thuringiensis
🟢 (B) Rhizobium
🟠 (C) Azotobacter
🔴 (D) Spirulina
✅ Answer: (A) Bacillus thuringiensis

🔹 Q8. 🚰 BOD (Biological Oxygen Demand) indicates:
🔵 (A) Hardness of water
🟢 (B) Organic pollution load
🟠 (C) pH of water
🔴 (D) Salinity
✅ Answer: (B) Organic pollution load

🔹 Q9. 🌾 Biofertiliser commonly used in paddy fields:
🔵 (A) Anabaena/Nostoc (cyanobacteria)
🟢 (B) Penicillium
🟠 (C) Escherichia coli
🔴 (D) Plasmodium
✅ Answer: (A) Anabaena/Nostoc (cyanobacteria)

🔹 Q10. 🧫 Citric acid is industrially produced by:
🔵 (A) Aspergillus niger
🟢 (B) Acetobacter aceti
🟠 (C) Saccharomyces cerevisiae
🔴 (D) Rhizopus stolonifer
✅ Answer: (A) Aspergillus niger

🟡 Section B — Short Answer I (2 marks each)
🔹 Q11. 🧈 What is SCP? Give one example and one use.
✅ Answer: Single Cell Protein = protein-rich microbial biomass (e.g., Spirulina); used as low-cost dietary supplement.

🔹 Q12. 🧫 Two benefits of Lactic Acid Bacteria (LAB) in curd.
✅ Answer: (i) Ferments milk to curd & improves digestibility; (ii) Increases vitamin B₁₂ and inhibits gut pathogens.

🔹 Q13. 🧯 Two advantages of biocontrol over chemical pesticides.
✅ Answer: Specific to target pests; eco-friendly with no residue bioaccumulation.

🔹 Q14. 🌱 Mention two roles of mycorrhiza (Glomus) in plants.
✅ Answer: Enhances phosphorus uptake; provides resistance to root-borne pathogens/drought tolerance.

🔴 Section C — Short Answer II (3 marks each)
🔹 Q15. 🚰 Distinguish between primary and secondary sewage treatment.
✅ Answer:
1️⃣ Primary: Physical removal of floating & settleable solids by screening, grit removal, sedimentation.
2️⃣ Secondary: Biological oxidation by aerobic microbes → activated sludge; BOD drastically reduced.
3️⃣ Secondary effluent may be disinfected before discharge.

🔹 Q16. 🔋 Outline biogas production in an anaerobic digester.
✅ Answer:
Step 1: Hydrolysis of complex organic matter.
Step 2: Acidogenesis/acetogenesis → organic acids, H₂, CO₂.
Step 3: Methanogenesis by archaea (Methanobacterium) → CH₄-rich biogas.

🔹 Q17. 📊 BOD labeling (numerical): Three water samples have BOD = 8 mg/L, 22 mg/L, 380 mg/L. Assign river water, secondary effluent, untreated sewage.
✅ Answer:
Step 1: Highest BOD ⇒ most polluted = 380 mg/L → Untreated sewage.
Step 2: Intermediate BOD ⇒ 22 mg/L → Secondary effluent.
Step 3: Lowest BOD ⇒ 8 mg/L → Relatively clean river water.

🔴 Section C — Short Answer II
🔹 Q18. 🌱 How do microbes enrich soil fertility?
✅ Answer:
Rhizobium in legume nodules → N₂ fixation.
Free-living Azotobacter → nitrogen enrichment.
Cyanobacteria (Anabaena, Nostoc) → fix N₂ in rice paddies.
Mycorrhiza (Glomus) → increases phosphorus uptake.

🔹 Q19. 🧴 Two industrially important enzymes from microbes and their uses.
✅ Answer:
Amylase (Aspergillus) → starch hydrolysis, detergents.
Lipase (Candida) → fat hydrolysis, food processing.

🔹 Q20. 🍷 Outline steps in wine production.
✅ Answer:
Grapes crushed → juice fermented by Saccharomyces cerevisiae.
Ethanol + CO₂ produced anaerobically.
Aged in barrels before packaging.

🔹 Q21. 💊 Explain how antibiotics are produced.
✅ Answer:
Microbes cultured in fermentors.
Secondary metabolites secreted.
Extracted & purified → drugs (Penicillin, Streptomycin).

🔹 Q22. 🧬 Mention two microbes used in biotechnology.
✅ Answer:
E. coli → cloning vectors, insulin.
Agrobacterium tumefaciens → Ti plasmid transfers genes to plants.

🔹 Q23. 📉 Why is secondary sewage treatment important?
✅ Answer:
Removes organic load via microbes.
Reduces BOD → prevents pollution.
Produces activated sludge for reuse.

🔹 Q24. 💉 Name one microbial product as:
(a) Immunosuppressant
(b) Cholesterol-lowering agent
✅ Answer:
(a) Cyclosporin A (Trichoderma polysporum)
(b) Statins (Monascus purpureus)

🔹 Q25. 🦠 Write two harmful microbes with diseases.
✅ Answer:
Mycobacterium tuberculosis → Tuberculosis.
Salmonella typhi → Typhoid.

🔹 Q26. 🌾 Define biofertiliser with one example.
✅ Answer: Microbes that enrich soil nutrients. Example: Rhizobium.

🟡 Section D — Long Answer
🔹 Q27. 🚰 Describe stages of sewage treatment.
✅ Answer:
1️⃣ Primary: screening, grit removal, sedimentation.
2️⃣ Secondary: aerobic microbes → activated sludge, reduced BOD.
3️⃣ Anaerobic digestion: methanogens produce methane.
4️⃣ Final effluent disinfected → discharged.

🔹 Q28. 🍞 Explain microbial role in household products.
✅ Answer:
Lactobacillus → curd, cheese.
Saccharomyces cerevisiae → bread, alcohol.
Fermentation of rice + dal → idli, dosa.
Fermented flour → dhokla, bhatura.

🟢 Section E — Case Study (MCQs with options)
Case Study Passage:
A dairy industry wants to reduce environmental waste and simultaneously produce useful products. The company decides to use microbes for (i) converting milk to cheese and curd, (ii) treating wastewater, and (iii) generating biogas from sludge.
Q29. Which microbe is used in curd formation?
🔵 (A) Rhizobium
🟢 (B) Lactobacillus
🟠 (C) Aspergillus
🔴 (D) Penicillium
✅ Answer: (B) Lactobacillus

Q30. Which stage of sewage treatment reduces BOD?
🔵 (A) Primary treatment
🟢 (B) Secondary treatment
🟠 (C) Tertiary chlorination
🔴 (D) Screening
✅ Answer: (B) Secondary treatment

Q31. Which microbe group produces methane in biogas plants?
🔵 (A) Actinomycetes
🟢 (B) Methanogens
🟠 (C) Cyanobacteria
🔴 (D) Fungi
✅ Answer: (B) Methanogens

Q32. Which by-product of sludge digestion is used as manure?
🔵 (A) Methane
🟢 (B) Effluent slurry
🟠 (C) Carbon dioxide
🔴 (D) Lactic acid
✅ Answer: (B) Effluent slurry

Q33. Which fungus is used in cheese production for flavouring?
🔵 (A) Aspergillus niger
🟢 (B) Penicillium roqueforti
🟠 (C) Saccharomyces cerevisiae
🔴 (D) Rhizopus stolonifer
✅ Answer: (B) Penicillium roqueforti

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