Microbes in Human Welfare NCERT solution Class12

Here are CBSE Class 12 Biology (NCERT) Board Exam Answers for Chapter: Microbes in Human Welfare. These answers are written in the style expected in the CBSE board examination.

1. Which sample would you carry to demonstrate microbes under a microscope and why?

Answer:
I would carry a sample of curd because it contains a large number of lactic acid bacteria (LAB), especially Lactobacillus. These bacteria are easily visible under a microscope after proper staining and are present in large numbers due to fermentation.

2. Give examples to prove that microbes release gases during metabolism.

Answer:
Examples are:

  • Yeast produces carbon dioxide during fermentation of dough, causing bread and idli batter to rise.
  • In biogas plants, methanogenic bacteria produce methane (CH₄) along with carbon dioxide and hydrogen sulphide.
  • During the preparation of Swiss cheese, Propionibacterium shermanii releases carbon dioxide, forming large holes.

3. In which food would you find lactic acid bacteria? Mention some of their useful applications.

Answer:
Lactic acid bacteria are found in curd.

Applications:

  • Convert milk into curd.
  • Increase the nutritional value by increasing vitamin B₁₂.
  • Improve digestion.
  • Prevent the growth of harmful microbes in the stomach.

4. Name some traditional Indian foods made of wheat, rice and Bengal gram which involve the use of microbes.

Answer:
Examples include:

  • Idli
  • Dosa
  • Dhokla
  • Jalebi
  • Bhatura

These foods are prepared by microbial fermentation, mainly involving lactic acid bacteria and yeast.

5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria?

Answer:
Microbes produce antibiotics, which kill or inhibit the growth of disease-causing bacteria.
Example:

  • Penicillin, produced by Penicillium notatum, is used to treat many bacterial infections.

6. Name any two species of fungus used in the production of antibiotics.

Answer:

  • Penicillium notatum
  • Penicillium chrysogenum

7. What is sewage? In which way can sewage be harmful to us?

Answer:
Sewage is municipal wastewater containing human excreta, domestic waste and industrial effluents.

Harmful effects:

  • Contains disease-causing microorganisms.
  • Pollutes water bodies.
  • Reduces dissolved oxygen.
  • Spreads water-borne diseases such as cholera, typhoid and dysentery.

8. What is the key difference between primary and secondary sewage treatment?

Answer:

Primary TreatmentSecondary Treatment
Physical processBiological process
Removes floating and suspended solidsRemoves organic matter using microbes
Uses screening and sedimentationUses aerobic microbes in aeration tanks

9. Do you think microbes can also be used as a source of energy? If yes, how?

Answer:
Yes. Methanogenic bacteria convert cattle dung and organic waste into biogas, which mainly contains methane. Biogas is used as a clean and renewable source of energy for cooking, lighting and electricity generation.

10. How can microbes decrease the use of chemical fertilisers and pesticides?

Answer:
Microbes are used as biofertilisers and biopesticides.

  • Biofertilisers such as Rhizobium, Azotobacter and cyanobacteria fix atmospheric nitrogen and improve soil fertility.
  • Biopesticides such as Bacillus thuringiensis (Bt) control insect pests without polluting the environment.

11. Assign the correct labels to the water samples based on BOD values.

Answer:

  • Most polluted sample: C (400 mg/L)
  • Untreated sewage: C
  • River water: B (8 mg/L)
  • Secondary effluent: A (20 mg/L)

Higher BOD indicates greater organic pollution.

12. Name the microbes from which Cyclosporin A and Statins are obtained.

Answer:

  • Cyclosporin A: Trichoderma polysporum
  • Statins: Monascus purpureus

13. Role of microbes in:

(a) Single Cell Protein (SCP)

Answer:

  • Microbes such as Spirulina, yeast and Methylophilus methylotrophus are used as SCP.
  • They are rich in proteins, vitamins and minerals.
  • Used as food supplements and animal feed.

(b) Soil

Answer:

  • Decompose organic matter and recycle nutrients.
  • Fix atmospheric nitrogen.
  • Improve soil fertility.
  • Help in nutrient cycling and humus formation.

14. Arrange the following in decreasing order of importance for the welfare of human society and give reasons.

Answer:

Penicillin > Curd > Biogas > Citric acid

Reasons:

  • Penicillin: Saves millions of lives by treating bacterial diseases.
  • Curd: Nutritious food with beneficial bacteria that improve digestion.
  • Biogas: Renewable, eco-friendly source of energy and helps in waste management.
  • Citric acid: Mainly used as a food preservative and flavouring agent in industries.

15. How do biofertilisers enrich soil fertility?

Answer:
Biofertilisers improve soil fertility by:

  • Fixing atmospheric nitrogen (Rhizobium, Azotobacter, cyanobacteria).
  • Solubilising phosphate.
  • Increasing nutrient availability.
  • Improving soil texture and microbial activity.
  • Reducing the need for chemical fertilisers.

Important Board Exam Keywords

  • Lactic Acid Bacteria (LAB)
  • Fermentation
  • Antibiotics
  • Penicillin
  • Biofertilisers
  • Biopesticides
  • Methanogens
  • Biogas
  • Biological Oxygen Demand (BOD)
  • Activated sludge
  • Rhizobium
  • Lactobacillus
  • Trichoderma polysporum
  • Monascus purpureus
  • Bacillus thuringiensis (Bt)

These answers are concise, NCERT-based, and suitable for CBSE Class 12 Board examinations (2–5 marks).

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