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Microbes in Human Welfare – Class 12 Biology notes, important questions and MCQs

NCERT Class 12 Biology, chapter 8. Notes, key terms, important questions with answers and MCQs.

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Chapter overview

Microbes help in household foods, industrial products, sewage treatment, biogas, biocontrol and biofertilisers.

Food and industry

  • Lactobacillus turns milk into curd; Saccharomyces cerevisiae (baker's yeast) for bread and alcohol.
  • Penicillin from Penicillium notatum (Fleming); citric acid from Aspergillus niger; streptokinase from Streptococcus as a clot buster.
  • Cyclosporin A (Trichoderma) is an immunosuppressant; statins (Monascus) lower cholesterol.

Environment and agriculture

  • Sewage treatment: primary (physical) and secondary (aerobic microbes form flocs; BOD falls).
  • Biogas (mainly methane) by methanogens like Methanobacterium in cattle dung.
  • Biocontrol: Bacillus thuringiensis (Bt), Trichoderma; biofertilisers: Rhizobium, Azotobacter, cyanobacteria (Anabaena, Nostoc), mycorrhiza.

Key terms

BOD
biochemical oxygen demand, measure of organic matter in water
Biofertiliser
organism that enriches soil nutrients
Methanogen
bacterium that produces methane

Important questions with answers

Why is Bacillus thuringiensis used in organic farming?2 marks

Its spores contain Bt toxin that kills specific insect larvae (like caterpillars) without harming other insects, plants or humans, so it replaces chemical pesticides.

How is biogas produced? Name its main components.3 marks

Cattle dung (gobar) is fed into a biogas plant; methanogenic bacteria like Methanobacterium break down cellulose anaerobically to produce biogas. It contains mainly methane, with CO₂ and some H₂S and hydrogen; the slurry left is used as manure.

Describe the steps of sewage treatment and explain how BOD indicates the level of pollution.5 marks

Primary treatment: physical removal of floating debris by filtration and grit (soil, pebbles) by sedimentation, giving primary sludge and effluent. Secondary (biological) treatment: the effluent is aerated so aerobic microbes grow as flocs and consume organic matter, greatly reducing BOD. The effluent then goes to a settling tank where flocs settle (activated sludge); part is returned to the aeration tank and the rest is digested anaerobically, producing biogas. Treated water is released to rivers. BOD is the amount of oxygen microbes need to oxidise organic matter in one litre of water; the higher the BOD, the more organic pollution.

Quick revision

  • Lactobacillus → curd.
  • Penicillin → Fleming.
  • Biogas is mostly methane.

Exam tips

  • Higher BOD means more polluted water.
  • Pair each microbe with its product: Lactobacillus–curd; Saccharomyces–bread and alcohol; Aspergillus niger–citric acid; Acetobacter aceti–acetic acid; Clostridium butylicum–butyric acid; Streptococcus–streptokinase.

Practice MCQs

  1. Curd is formed from milk by

    1. Lactobacillus
    2. yeast
    3. Rhizobium
    4. Penicillium
    Show answer

    Answer: (a) Lactobacillus. Lactic acid bacteria.

  2. The main gas in biogas is

    1. methane
    2. CO₂
    3. hydrogen
    4. nitrogen
    Show answer

    Answer: (a) methane. Methane.

  3. Penicillin was discovered by

    1. Pasteur
    2. Fleming
    3. Koch
    4. Jenner
    Show answer

    Answer: (b) Fleming. Alexander Fleming.

  4. Which is a nitrogen-fixing biofertiliser

    1. Trichoderma
    2. Saccharomyces
    3. Penicillium
    4. Rhizobium
    Show answer

    Answer: (d) Rhizobium. Root nodules.

  5. Large holes in Swiss cheese are due to CO₂ released by

    1. Propionibacterium sharmanii
    2. Lactobacillus
    3. Penicillium roqueforti
    4. yeast
    Show answer

    Answer: (a) Propionibacterium sharmanii. Swiss cheese bacterium.

  6. Cyclosporin A is used as

    1. immunosuppressant
    2. antibiotic
    3. clot buster
    4. cholesterol-lowering agent
    Show answer

    Answer: (a) immunosuppressant. Organ transplants.

  7. Higher BOD in a water body indicates

    1. cleaner water
    2. more organic pollution
    3. more dissolved oxygen
    4. lower microbes
    Show answer

    Answer: (b) more organic pollution. More oxygen demand.

  8. In secondary sewage treatment, the masses of bacteria with fungal filaments are called

    1. flocs
    2. sludge only
    3. biogas
    4. effluent
    Show answer

    Answer: (a) flocs. Flocs reduce BOD.

All chapters of Class 12 Biology

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.