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Microbes in Human Welfare
Biology · Class 12NCERT pp. 149–1582 sets × 25 questions
Key facts and formulas for Microbes in Human Welfare, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Key facts for this chapter
| Idea | Rule or fact |
|---|
| Prion, viroid, virus | Prion = protein only; viroid = RNA only; viruses grow only in living host cells |
| Curd and cheese | Lactic acid bacteria make curd, raise vitamin B12; rennet acts on casein; Swiss cheese holes: Propionibacterium (CO2); Roquefort: fungus |
| Acids by microbes | Citric: Aspergillus niger; acetic: Acetobacter aceti; butyric: Clostridium butylicum; lactic: Lactobacillus; ethanol: Saccharomyces |
| Drinks | Wine and beer are not distilled; whisky, brandy and rum are distilled |
| Antibiotics | Penicillin: Fleming found it, Chain and Florey proved it; streptomycin from Streptomyces; penicillin blocks cell wall synthesis |
| Bioactive molecules | Streptokinase: clot buster; cyclosporin A: immunosuppressant; statins: block cholesterol enzyme (Monascus) |
| Sewage treatment | Primary: physical; secondary: aeration, flocs, activated sludge; tertiary: nitrates and phosphates; high BOD means more pollution |
| Biogas | Methanogens (archaebacteria) in sludge and rumen; gases: CH4, H2S, CO2; slurry used as fertiliser |
| Biocontrol | Ladybird: aphids; dragonfly: mosquitoes; Bt: caterpillars (alkaline gut dissolves crystal); Trichoderma: pathogens; cryIAc: bollworm |
| Biofertilisers | Rhizobium: nodules; Azotobacter, Azospirillum: free-living; Anabaena: paddy, Azolla; Glomus: phosphorus; nitrogenase, leghaemoglobin, heterocysts |
Three sample questions
Q1. Which of the following infectious agents is made of protein only and has no nucleic acid?
- Viroid
- Bacteriophage
- Adenovirus
- Prion
Hint and solution
Hint: The name itself says it: proteinaceous infectious particle.
- Prion = proteinaceous infectious particle — The question asks for an agent that has protein but no nucleic acid.
- Viruses: nucleic acid + protein coat — Bacteriophage and adenovirus are viruses, so both carry DNA or RNA inside a protein coat.
- Viroid: RNA only — A viroid is just a small RNA, so it has nucleic acid and no protein.
- Only the prion is protein alone — So the answer is the prion, which is made only of protein. Option D.
Answer: D. Prions are proteinaceous infectious agents.
Q2. Consider the following.
I. Bacteria and many fungi form colonies on a nutrient medium.
II. Viruses form colonies on simple nutrient media.
III. A bacterial colony can be seen without a lens.
Which are correct?
- I and III only
- I and II only
- II and III only
- I, II and III
Hint and solution
Hint: Which of these microbes can multiply without a living host cell?
- Colony = visible mass from one cell dividing — Bacteria and many fungi divide repeatedly on a nutrient medium and form a visible colony. So I is true.
- Viruses multiply only in living host cells — Viruses cannot be grown on simple nutrient media. So II is false.
- Colony is seen with the naked eye — A colony is a mass of millions of cells, so no lens is needed. III is true.
- I and III are correct — So the answer is the pair of statements I and III. Option A.
Answer: A. Viruses multiply only in living host cells, so II is wrong.
Q3. Match the virus with its host or effect.
Column I: a. Bacteriophage; b. Adenovirus; c. Tobacco Mosaic Virus
Column II: (i) Respiratory infection in humans; (ii) Infects bacteria; (iii) Rod-shaped plant virus
- a-(i), b-(ii), c-(iii)
- a-(iii), b-(i), c-(ii)
- a-(ii), b-(i), c-(iii)
- a-(ii), b-(iii), c-(i)
Hint and solution
Hint: Phage means eater of bacteria; TMV has the word tobacco, a plant.
- Bacteriophage → infects bacteria — A bacteriophage is a virus whose host is a bacterium. So a matches (ii).
- Adenovirus → respiratory infection — Adenovirus causes respiratory infections in humans. So b matches (i).
- TMV → rod-shaped plant virus — Tobacco Mosaic Virus infects plants and is rod-like. So c matches (iii).
- a-(ii), b-(i), c-(iii) — So the combination is a with ii, b with i, c with iii. Option C.
Answer: C. Bacteriophage infects bacteria; adenovirus causes respiratory infections; TMV is a rod-shaped plant virus.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.