NCERT Exercises and Solutions: Microbes in Human Welfare
🎓 Class 12BiologyCBSETheoryCh 8 – Microbes in Human Welfare⏱ ~8 min
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NCERT Exercises and Solutions: Microbes in Human Welfare
10.13 Chapter Summary
Microbes are tiny but indispensable allies of human welfare. They live everywhere — soil, water, air, and our bodies — and contribute in countless beneficial ways. This chapter explored their roles in household products, industrial fermentations, sewage treatment, biogas production, biocontrol, and biofertilisers.
Wine, beer, cider — fermented, not distilled; Whisky, brandy, rum — fermented + distilled. Saccharomyces cerevisiae is the workhorse yeast.
Industrial — Antibiotics
Penicillin discovered by Fleming (1928); developed by Florey + Chain. Cyclosporin A (transplant immunosuppressant) from Trichoderma polysporum; statins from Monascus purpureus.
Trichoderma polysporum — cyclosporin A (antibiotic-like) + Trichoderma harzianum (biocontrol of plant fungi).
This shows how nature uses a few versatile microbes for many different services.
10.15 NCERT Exercise — Worked Solutions
Q1. Bacteria cannot be seen with the naked eye, 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 under a microscope, which sample would you carry and why? L3
I would carry a sample of curd (dahi) from home. Reasons:
Curd is rich in living Lactobacillus bacteria — easy to find dense rod-shaped cells under the microscope.
Sample preparation is simple: spread a thin smear on a slide, stain with methylene blue, observe under 100× oil immersion.
Sample is safe to handle — no pathogens; just food-grade bacteria.
You can also demonstrate the process by comparing fresh milk (very few bacteria) with curd (millions).
Mixed: Kanji (fermented carrot drink), kulhad lassi, mithai, certain pickles, vadi/badi (sun-fermented).
Q5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria? L2
Microbes have provided antibiotics — chemical substances produced by one microbe that kill or inhibit other microbes, especially disease-causing bacteria.
Penicillin from Penicillium notatum — discovered by Fleming (1928); developed by Chain & Florey for WWII soldiers; saved millions from wound infection deaths.
Streptomycin from Streptomyces griseus — TB treatment.
Tetracycline, erythromycin, gentamicin, kanamycin — all from soil microbes.
Before antibiotics, diseases like pneumonia, syphilis, plague and tuberculosis were major killers. Antibiotics added ~20 years to average human life expectancy in the 20th century and made surgery, cancer chemotherapy, and organ transplantation possible.
Q6. Name any two species of fungus used in the production of the antibiotics. L1
Penicillium notatum / P. chrysogenum → Penicillin.
Cephalosporium acremonium → Cephalosporin.
(Many bacteria of the genus Streptomyces are also important antibiotic producers — streptomycin, tetracycline, neomycin etc.)
Q7. What is sewage? In which way can sewage be harmful to us? L2
Sewage = the municipal waste-water generated daily from kitchens, bathrooms, industries and storm drains. It contains large amounts of organic matter and pathogenic microbes.
High organic load → low dissolved O₂ in water bodies → fish kill, dead zones.
Eutrophication of rivers/lakes by N and P.
Foul odour, breeding ground for mosquitoes and houseflies.
Contaminates groundwater used for drinking.
Q8. What is the key difference between primary and secondary sewage treatment? L2
Criterion
Primary
Secondary
Nature
Physical (mechanical)
Biological (microbial)
Process
Filtration, settling
Aeration + flocs
What's removed
Suspended solids, scum
Dissolved organic matter
BOD reduction
30-40%
~90%
Microbes used
None
Aerobic bacteria + fungi (flocs)
Primary alone is insufficient; secondary is essential before discharge.
Q9. Do you think microbes can also be used as a source of energy? If yes, how? L3
Yes. Microbes generate energy by producing biofuels.
Biogas: Methanogens (e.g., Methanobacterium) digest organic matter anaerobically → methane + CO₂. Used as cooking fuel, electricity, vehicle fuel (Bio-CNG).
Bioethanol: Yeast (Saccharomyces cerevisiae) ferments sugar/starch → ethanol, blended with petrol (E5, E10).
Hydrogen: Certain microalgae and bacteria produce H₂ gas via biophotolysis.
Microbial fuel cells (MFCs): Electroactive bacteria transfer electrons to an electrode while metabolising organic matter — generates electricity directly from wastewater.
Q10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished. L3
Baculoviruses (NPV) — species-specific viral kills only the target pest.
Pheromone traps + predator release also part of IPM.
Benefits: Lower chemical residues, healthier soil, sustainable productivity, biodiversity preservation.
Q11. Three water samples — pond water, distilled water, river water — are checked for BOD. Arrange in increasing order of BOD and justify. L4
Increasing BOD: Distilled water < Pond water < River water (polluted river).
Distilled water — pure H₂O, no microbes, no organic matter. BOD ≈ 0.
Pond water — some microbes and algae; modest organic matter. BOD low to moderate.
River water — particularly downstream of cities — receives sewage and effluents; high microbial load. BOD high.
Note: The order assumes the river is in/near a city; an unpolluted Himalayan stream could have lower BOD than a stagnant pond.
Q12. Name some microbes that are part of "Bioactive Molecules" used by humans for medical or industrial use. L1
Bioactive molecule
Microbe
Use
Penicillin
Penicillium notatum
Antibiotic
Cyclosporin A
Trichoderma polysporum
Immunosuppressant for transplants
Statins
Monascus purpureus
Cholesterol lowering
Streptokinase
Streptococcus (engineered)
Clot buster (heart attack)
Citric acid
Aspergillus niger
Food/beverage acidulant
Acetic acid
Acetobacter aceti
Vinegar production
Butyric acid
Clostridium butylicum
Plastics, perfumes
Lactic acid
Lactobacillus
Food, biodegradable plastics
Lipase
Various microbes
Bio-detergent enzyme
Competency-Based Questions
Q1. Choose the INCORRECT statement: L2 Understand
(a) Roquefort cheese uses Penicillium roqueforti
(b) Swiss-cheese holes are due to CO₂ from Propionibacterium
(c) Lactobacillus produces lactic acid that causes milk to coagulate
(d) Bread rises due to acetic-acid fermentation by yeast
Incorrect = (d). Bread rises due to alcoholic fermentation (sugar → CO₂ + ethanol) by yeast, NOT acetic-acid fermentation. The other three are correct.
Q2. Which microbe is the source of cyclosporin A, and what is the role of this drug? L2 Understand
Trichoderma polysporum. Cyclosporin A is an immunosuppressant — given to organ-transplant patients to prevent the body's T-cells from rejecting the new organ. It blocks the calcineurin pathway, dampening IL-2 production. Made transplantation routinely successful from the early 1980s.
Q3. What is the role of activated sludge in a sewage treatment plant? L3 Apply
Activated sludge = settled flocs of aerobic microbes (bacteria + fungi). After settling, a part is recycled back to the aeration tank as inoculum — maintaining the high microbial density needed to oxidise incoming sewage organics. The excess sludge is sent to anaerobic digesters, where methanogens convert it into biogas + slurry manure. Thus activated sludge serves both biological treatment (recycled) and energy generation (digested).
Q4. Analyse: A farmer applied chemical fertiliser for 10 years; soil yield dropped despite increasing doses. Diagnose and recommend a microbial solution. L4 Analyse
Diagnosis: Continuous chemical fertilisation depletes organic matter, lowers microbial biomass, salinises soil, alters pH — soil becomes lifeless. More fertiliser cannot fix a soil that lacks living biological activity.
Microbial solution:
Stop chemical urea/DAP gradually; introduce biofertilisers — Rhizobium for legume crop rotation, Azospirillum/Azotobacter for cereals.
Mycorrhiza inoculant (Glomus) at sowing — restores P uptake and root health.
Cyanobacterial cake (Anabaena-Azolla) for rice fields.
Compost + farmyard manure — feeds the resident soil microbes.
Within 2-3 seasons, organic matter rebuilds and yield recovers.
Q5. HOT (Create): Design a 7-day "Microbes Around Us" awareness week for a school. List one activity per day. L6 Create
Day-wise plan:
Day 1 — Curd lab: Set curd at different temperatures; observe under microscope.
Day 2 — Bread workshop: Make dough with and without yeast; compare rise.
Day 3 — Fleming play: Skit on accidental discovery of penicillin.
Day 4 — Sewage walk: Visit a local STP or Yamuna outfall; discuss BOD.
Day 5 — Biogas mini-plant: Build a 5-L bottle digester with cow dung; collect gas in balloon; light it.
Day 6 — Rhizobium dig: Pull up gram/pea plants from school garden; identify nodules; section under microscope.
Day 7 — Microbe Mela: Stalls showcasing each chapter topic; quiz competition; awards.
Assessment: Student journals, photos, video presentations.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Anaerobic sludge digesters at STPs produce biogas as a useful by-product.
R: Methanogens like Methanobacterium ferment the sludge in absence of oxygen, generating methane.
Answer: (A). Both true and R explains A. Biogas is captured and used as fuel — turning a pollution liability into an energy asset.
A: Bt cotton plants do not need foliar insecticide spraying for bollworms.
R: Bt cotton expresses the Cry gene from Bacillus thuringiensis in its tissues, producing the insecticidal protein on its own.
Answer: (A). Both true and R explains A. The plant's own tissues contain the toxin — feeding larvae die without external spraying.
A: Cyclosporin A is a powerful antibiotic widely used to treat bacterial infections.
R: Cyclosporin A is isolated from Trichoderma polysporum.
Answer: (D). A is FALSE — cyclosporin A is NOT an antibiotic. It is an immunosuppressant used in organ-transplant patients. R is TRUE.
Frequently Asked Questions - NCERT Exercises and Solutions: Microbes in Human Welfare
What are the most-asked NCERT exercise questions in Chapter Microbes in Human Welfare?
NCERT Class 12 Biology Chapter on Microbes in Human Welfare exercises cover definitions, mechanisms, labelled diagrams, comparison tables, and application-based questions. The MyAiSchool solution set provides full step-by-step solutions for every NCERT question, aligned with the CBSE board exam pattern. Students should master scientific terminology, mechanism flowcharts, and concept comparison to score full marks.
How should students approach diagram-based questions in Microbes in Human Welfare?
For diagram-based questions in NCERT Class 12 Biology Chapter on Microbes in Human Welfare: (1) draw clean, proportional, large diagrams with sharp pencil lines, (2) label all parts horizontally on the right side using a ruler, (3) use scientific terminology (Latin/Greek names where applicable), (4) write a 1-2 line description if asked. The MyAiSchool solutions provide editable reference diagrams aligned with NCERT textbook figures.
What types of CBSE board questions come from Microbes in Human Welfare?
CBSE Class 12 Biology board questions from Microbes in Human Welfare typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer mechanism/comparison questions, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool exercise set tags each question by mark weight and Bloom level (L1-L6).
How do I compare two biological processes in 5-mark questions?
For 5-mark comparison questions in NCERT Class 12 Biology on Microbes in Human Welfare: (1) use a two-column table with feature headings down the left side, (2) compare on at least 5-6 features (definition, location, mechanism, control, outcome, significance), (3) include one labelled diagram if relevant, (4) end with biological significance. The MyAiSchool solutions follow this CBSE-aligned tabular format consistently for full marks.
What are common mistakes in Microbes in Human Welfare exercises?
Common mistakes in NCERT Class 12 Biology Microbes in Human Welfare include: (1) confusing similar terms (mitosis vs meiosis, syngamy vs triple fusion, etc.), (2) skipping intermediate steps in mechanisms, (3) missing examples or species names, (4) writing essays when tables are expected, (5) forgetting biological significance. The MyAiSchool solutions highlight these traps with red flags so students avoid losing marks.
How does the MyAiSchool solution differ from other NCERT solution sets?
MyAiSchool Class 12 Biology Microbes in Human Welfare solutions use NEP 2024-aligned pedagogy with Bloom Taxonomy tagged questions (L1 Remember to L6 Create), step-by-step working with biological reasoning, fully labelled SVG diagrams, comparison tables, interactive simulations, and Competency-Based Questions (CBQs) for board exam practice. Each solution is verified against NCERT and CBSE marking schemes.
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