CBSE Class 12 Biology Case-Based Questions with Answers – Microbes in Human Welfare (NCERT Aligned)

Course Details

Course: CBSE Class 12 Biology
Unit: Unit III – Biology and Human Welfare
Chapter: Chapter 10 – Microbes in Human Welfare
Prescribed Textbook: NCERT Biology Class XII
Exam Focus: CBSE Board Examination (Case-Based / Source-Based Questions)

About This Study Material

This section includes 25 Case-Based Questions with Answers, strictly prepared as per the NCERT syllabus and CBSE examination pattern.
Each case focuses on real-life applications of microbes, followed by competency-based questions that test conceptual understanding, application, and analytical skills.

SECTION A: Microbes in Household Products

Case 1

A family regularly consumes curd prepared at home using a small amount of previously prepared curd as a starter culture.

Q1. Name the microorganism involved in curd formation.
Answer: Lactobacillus species are responsible for converting milk into curd.

Q2. How does this microbe improve the nutritional quality of milk?
Answer: Lactobacillus increases vitamin B₁₂ content and improves digestibility by converting lactose into lactic acid.

Case 2

Idli and dosa batter rises overnight due to microbial activity.

Q3. Which microorganisms are responsible for fermentation in idli batter?
Answer: Lactic acid bacteria and yeast carry out fermentation in idli and dosa batter.

Q4. Why does the batter become soft and fluffy?
Answer: Carbon dioxide released during fermentation causes the batter to rise and become soft.

Case 3

Traditional foods like toddy are prepared using natural fermentation.

Q5. Name the microorganism involved in toddy preparation.
Answer: Yeast (Saccharomyces cerevisiae) is responsible for toddy fermentation.

SECTION B: Microbes in Industrial Products

Case 4

An industry produces ethanol on a large scale using fermentation tanks.

Q6. Name the microorganism used for ethanol production.
Answer: Saccharomyces cerevisiae (brewer’s yeast).

Q7. Which substrate is commonly used in ethanol production?
Answer: Sugars such as glucose and molasses are used as substrates.

Case 5

Antibiotics are produced commercially using microbes.

Q8. Which antibiotic is produced by Penicillium?
Answer: Penicillin.

Q9. What is the role of antibiotics in human health?
Answer: Antibiotics inhibit or kill disease-causing microorganisms.

Case 6

A dairy industry uses microbes to enhance cheese flavor.

Q10. Name one microbe used in cheese production.
Answer: Propionibacterium shermanii.

SECTION C: Microbes in Sewage Treatment

Case 7

Municipal sewage undergoes treatment before being released into water bodies.

Q11. What is the primary purpose of sewage treatment?
Answer: To reduce organic matter and pathogenic microbes before disposal.

Q12. Which process forms flocs during secondary treatment?
Answer: Activated sludge process.

Case 8

A sewage treatment plant shows reduced BOD after treatment.

Q13. What does reduced BOD indicate?
Answer: Effective removal of organic pollutants from sewage.

Case 9

Activated sludge is reused in sewage treatment.

Q14. Why is activated sludge recycled?
Answer: It acts as an inoculum rich in aerobic microbes.

SECTION D: Microbes in Biogas Production

Case 10

A village uses gobar gas plants for cooking fuel.

Q15. Name the microorganism involved in biogas production.
Answer: Methanogens such as Methanobacterium.

Q16. Which gas is mainly produced in biogas?
Answer: Methane.

Case 11

Biogas plants reduce organic waste accumulation.

Q17. Mention one environmental benefit of biogas plants.
Answer: They reduce pollution and recycle organic waste.

Case 12

Slurry from biogas plants is used in fields.

Q18. Why is biogas slurry beneficial for agriculture?
Answer: It is rich in nutrients and acts as organic manure.

SECTION E: Microbes as Biocontrol Agents

Case 13

Farmers prefer biological pest control over chemical pesticides.

Q19. Name a bacterial biocontrol agent used against insect pests.
Answer: Bacillus thuringiensis (Bt).

Case 14

Bt cotton plants show resistance to insect pests.

Q20. How does Bt toxin affect insects?
Answer: It damages the gut lining of insect larvae, causing death.

Case 15

A fungus is used to control plant pathogens.

Q21. Name the fungus used as a biocontrol agent.
Answer: Trichoderma species.

SECTION F: Microbes as Biofertilisers

Case 16

Farmers use microbial inoculants to improve soil fertility.

Q22. Name a nitrogen-fixing bacterium found in legume roots.
Answer: Rhizobium.

Case 17

Rice fields show better yield after blue-green algae application.

Q23. Name one cyanobacterium used as biofertiliser.
Answer: Anabaena.

Case 18

Mycorrhiza is used in sustainable agriculture.

Q24. What is the role of mycorrhiza in plants?
Answer: It enhances phosphorus absorption and water uptake.

Case 19

Biofertilisers reduce dependence on chemical fertilisers.

Q25. Give one advantage of biofertilisers over chemical fertilisers.
Answer: They are eco-friendly and improve long-term soil fertility.

Why This Resource is Ideal for CBSE Board Exams

✔ Strictly NCERT-based
✔ Case-based & competency-oriented
✔ Covers all six NCERT sub-topics
✔ Suitable for 5-mark CBSE case-study questions
✔ Helpful for revision, practice, and exam confidence