CBSE Class 12 Biology MCQs with Answers & Explanations – Microbes in Human Welfare (NCERT Based)

Course Details

Course: CBSE Class 12 Biology
Unit: Unit III – Biology and Human Welfare
Chapter: Chapter 10 – Microbes in Human Welfare
Prescribed Book: NCERT Biology Class XII
Assessment Type: Multiple Choice Questions (MCQs)
Exam Focus: CBSE Board Examinations (Competency-Based)

About This MCQ Set

• 50 NCERT-aligned MCQs
• Elaborate explanations for concept clarity
• Covers all six sub-topics of the chapter
• Suitable for board exams, school tests & revision

SECTION A: Microbes in Household Products (MCQs 1–8)

Q1. The microorganism responsible for converting milk into curd is:

A. Streptococcus
B. Lactobacillus
C. Saccharomyces
D. Penicillium

Correct Answer: B

Explanation: Lactobacillus converts lactose into lactic acid, which coagulates milk proteins to form curd and improves nutritional quality.

Q2. Which of the following increases vitamin B₁₂ content in curd?

A. Yeast
B. Lactobacillus
C. Rhizobium
D. Methanobacterium

Correct Answer: B

Explanation: Lactic acid bacteria synthesize vitamin B₁₂, enriching curd nutritionally.

Q3. The soft and spongy texture of idli is due to:

A. Lactic acid formation
B. Oxygen release
C. Carbon dioxide release
D. Ethanol formation

Correct Answer: C

Explanation: CO₂ released during fermentation causes the batter to rise, making idli soft and fluffy.

Q4. Toddy is a traditional drink produced by fermentation of:

A. Milk
B. Palm sap
C. Fruit pulp
D. Sugarcane juice

Correct Answer: B

Explanation: Palm sap undergoes natural fermentation by yeast to form toddy.

Q5. Which microorganism is mainly responsible for alcoholic fermentation?

A. Lactobacillus
B. Bacillus
C. Saccharomyces
D. Rhizobium

Correct Answer: C

Explanation: Saccharomyces cerevisiae converts sugars into ethanol and CO₂.

Q6. Which product is NOT prepared using microbial fermentation?

A. Bread
B. Cheese
C. Butter
D. Jam

Correct Answer: D

Explanation: Jam is preserved by high sugar content, not microbial fermentation.

Q7. Fermentation improves food quality by:

A. Destroying proteins
B. Reducing vitamin content
C. Increasing digestibility
D. Removing carbohydrates

Correct Answer: C

Explanation: Fermentation breaks complex molecules into simpler, easily digestible forms.

Q8. Which acid is responsible for the sour taste of curd?

A. Acetic acid
B. Lactic acid
C. Citric acid
D. Butyric acid

Correct Answer: B

Explanation: Lactic acid is produced by lactic acid bacteria during curd formation.

SECTION B: Microbes in Industrial Products (MCQs 9–18)

Q9. Industrial production of ethanol uses:

A. Anaerobic respiration
B. Aerobic respiration
C. Photosynthesis
D. Chemosynthesis

Correct Answer: A

Explanation: Yeast produces ethanol under anaerobic conditions.

Q10. Which substrate is commonly used for large-scale ethanol production?

A. Cellulose
B. Proteins
C. Molasses
D. Lipids

Correct Answer: C

Explanation: Molasses is a sugar-rich by-product of sugar industry.

Q11. Penicillin is produced from:

A. Bacteria
B. Virus
C. Algae
D. Fungus

Correct Answer: D

Explanation: Penicillin is produced by Penicillium fungus.

Q12. Which antibiotic was the first to be discovered?

A. Streptomycin
B. Penicillin
C. Tetracycline
D. Erythromycin

Correct Answer: B

Explanation: Penicillin was discovered by Alexander Fleming.

Q13. Citric acid is commercially produced using:

A. Yeast
B. Aspergillus
C. Rhizobium
D. Lactobacillus

Correct Answer: B

Explanation: Aspergillus niger is used in citric acid production.

Q14. Which enzyme is used in fruit juice clarification?

A. Protease
B. Amylase
C. Pectinase
D. Lipase

Correct Answer: C

Explanation: Pectinase breaks down pectin, clarifying fruit juices.

Q15. Cheese with large holes is formed due to release of:

A. Oxygen
B. Hydrogen
C. Nitrogen
D. Carbon dioxide

Correct Answer: D

Explanation: CO₂ released by Propionibacterium creates holes in cheese.

Q16. Which product is NOT an industrial microbial product?

A. Antibiotics
B. Enzymes
C. Vaccines
D. Insecticides

Correct Answer: D

Explanation: Insecticides are chemical products, not microbial industrial products.

Q17. Which microbe is used in streptomycin production?

A. Streptococcus
B. Streptomyces
C. Lactobacillus
D. Bacillus

Correct Answer: B

Explanation: Streptomyces species produce streptomycin.

Q18. Industrial fermenters are preferred because they:

A. Reduce oxygen
B. Increase contamination
C. Provide optimal growth conditions
D. Reduce yield

Correct Answer: C

Explanation: Fermenters maintain pH, temperature, and nutrients optimally.

SECTION C: Microbes in Sewage Treatment (MCQs 19–28)

Q19. The primary aim of sewage treatment is to:

A. Increase pathogens
B. Reduce organic matter
C. Produce drinking water
D. Increase nutrients

Correct Answer: B

Explanation: Sewage treatment removes organic pollutants and pathogens.

Q20. Primary sewage treatment involves:

A. Biological oxidation
B. Mechanical filtration
C. Methanogenesis
D. Fermentation

Correct Answer: B

Explanation: Physical processes like sedimentation and filtration are used.

Q21. Flocs are formed by:

A. Algae and fungi
B. Bacteria and protozoa
C. Viruses and bacteria
D. Protozoa only

Correct Answer: B

Explanation: Flocs consist of bacteria held together by fungal filaments.

Q22. Activated sludge is formed during:

A. Primary treatment
B. Secondary treatment
C. Tertiary treatment
D. Chlorination

Correct Answer: B

Explanation: Secondary treatment uses aerobic microbes forming activated sludge.

Q23. BOD measures:

A. Oxygen required by microbes
B. Oxygen dissolved in water
C. Carbon dioxide content
D. Nitrogen content

Correct Answer: A

Explanation: BOD indicates organic pollution level.

Q24. Low BOD value indicates:

A. High pollution
B. Clean water
C. Untreated sewage
D. Toxic waste

Correct Answer: B

Explanation: Lower BOD means fewer organic pollutants.

Q25. Anaerobic digesters produce:

A. Oxygen
B. Nitrogen
C. Biogas
D. Ozone

Correct Answer: C

Explanation: Anaerobic microbes convert organic matter into biogas.

Q26. Which gas is NOT present in biogas?

A. Methane
B. Carbon dioxide
C. Hydrogen sulphide
D. Oxygen

Correct Answer: D

Explanation: Biogas is formed under anaerobic conditions, so oxygen is absent.

Q27. Sludge digestion helps in:

A. Increasing pathogens
B. Producing fuel
C. Increasing BOD
D. Water pollution

Correct Answer: B

Explanation: Anaerobic digestion produces biogas.

Q28. Treated sewage water is released into:

A. Drinking water supply
B. Industries
C. Rivers after disinfection
D. Agriculture fields directly

Correct Answer: C

Explanation: Disinfection ensures pathogen-free release.

SECTION D: Microbes in Biogas Production (MCQs 29–35)

Q29. Methanogens belong to:

A. Eubacteria
B. Archaebacteria
C. Cyanobacteria
D. Mycoplasma

Correct Answer: B

Explanation: Methanogens are archaebacteria adapted to anaerobic conditions.

Q30. Major component of biogas is:

A. CO₂
B. H₂
C. CH₄
D. NH₃

Correct Answer: C

Explanation: Methane forms the major portion of biogas.

Q31. Gobar gas plant uses:

A. Plant waste
B. Industrial waste
C. Cow dung
D. Plastic waste

Correct Answer: C

Explanation: Cow dung is rich in organic matter for methanogens.

Q32. Biogas slurry is useful as:

A. Pesticide
B. Fertiliser
C. Herbicide
D. Fungicide

Correct Answer: B

Explanation: Slurry is nutrient-rich organic manure.

Q33. Methanogens are found naturally in:

A. Human stomach
B. Rumen of cattle
C. Plant roots
D. Soil surface

Correct Answer: B

Explanation: Ruminants host methanogens aiding cellulose digestion.

Q34. Biogas production is an example of:

A. Aerobic respiration
B. Anaerobic digestion
C. Photosynthesis
D. Nitrification

Correct Answer: B

Explanation: Biogas is produced under anaerobic conditions.

Q35. Biogas is considered eco-friendly because it:

A. Increases pollution
B. Uses renewable waste
C. Produces smoke
D. Reduces oxygen

Correct Answer: B

Explanation: It converts waste into clean energy.

SECTION E: Microbes as Biocontrol Agents (MCQs 36–42)

Q36. Bt toxin is produced by:

A. Virus
B. Fungus
C. Bacterium
D. Alga

Correct Answer: C

Explanation: Bacillus thuringiensis produces Bt toxin.

Q37. Bt toxin specifically affects:

A. Humans
B. Plants
C. Insect larvae
D. Birds

Correct Answer: C

Explanation: Bt toxin damages insect gut lining.

Q38. Biocontrol agents are preferred because they are:

A. Toxic
B. Non-specific
C. Eco-friendly
D. Expensive

Correct Answer: C

Explanation: They reduce chemical pesticide use.

Q39. Which fungus acts as a biocontrol agent?

A. Aspergillus
B. Penicillium
C. Trichoderma
D. Rhizopus

Correct Answer: C

Explanation: Trichoderma suppresses plant pathogens.

Q40. Integrated pest management includes:

A. Only chemicals
B. Only microbes
C. Biological and chemical methods
D. Mechanical methods only

Correct Answer: C

Explanation: IPM uses multiple eco-safe methods.

Q41. Baculoviruses are useful because they are:

A. Pathogenic to plants
B. Species-specific
C. Harmful to humans
D. Airborne

Correct Answer: B

Explanation: They target specific insect pests only.

Q42. Biocontrol helps in:

A. Soil erosion
B. Environmental balance
C. Pest resistance
D. Crop failure

Correct Answer: B

Explanation: It maintains ecological balance.

SECTION F: Microbes as Biofertilisers (MCQs 43–50)

Q43. Rhizobium fixes nitrogen in:

A. Soil
B. Root nodules
C. Leaves
D. Stems

Correct Answer: B

Explanation: Rhizobium lives symbiotically in legume root nodules.

Q44. Which plant benefits most from Rhizobium?

A. Wheat
B. Rice
C. Pea
D. Maize

Correct Answer: C

Explanation: Legumes form root nodules with Rhizobium.

Q45. Cyanobacteria used as biofertilisers include:

A. Chlorella
B. Anabaena
C. Spirogyra
D. Volvox

Correct Answer: B

Explanation: Anabaena fixes atmospheric nitrogen.

Q46. Mycorrhiza is a symbiotic association between:

A. Algae and fungi
B. Roots and fungi
C. Bacteria and roots
D. Fungi and bacteria

Correct Answer: B

Explanation: Mycorrhiza enhances nutrient absorption.

Q47. Mycorrhiza mainly helps in absorption of:

A. Nitrogen
B. Potassium
C. Phosphorus
D. Iron

Correct Answer: C

Explanation: Fungal hyphae increase phosphorus uptake.

Q48. Biofertilisers improve soil fertility by:

A. Killing microbes
B. Adding toxic chemicals
C. Increasing nutrient availability
D. Reducing organic matter

Correct Answer: C

Explanation: They enrich soil biologically.

Q49. Which is NOT a biofertiliser?

A. Rhizobium
B. Anabaena
C. Mycorrhiza
D. DDT

Correct Answer: D

Explanation: DDT is a chemical pesticide.

Q50. Biofertilisers are preferred because they:

A. Pollute soil
B. Are expensive
C. Are sustainable
D. Reduce crop yield

Correct Answer: C

Explanation: They support sustainable agriculture.

Why This MCQ Set Is Ideal for CBSE Class 12

✔ 100% NCERT-aligned
✔ Covers all exam-weighted concepts
✔ Includes concept-clearing explanations
✔ Suitable for board exams & revision tests

🎯 Targeting Exams Section (for Content Body)

Targeted Exams

• CBSE Class 12 Biology Board Examination

• CBSE School Unit Tests & Pre-Board Exams

• Competitive Exam Foundation (NEET – Concept Building)

• Olympiad & NTSE Biology Preparation (Conceptual Level)

Why This Content Helps:
These MCQs are strictly framed as per NCERT guidelines and reflect the latest CBSE competency-based question pattern, making them ideal for objective revision, practice tests, and last-minute exam preparation.

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