Microbes in Human Welfare — Class 12 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 12 Biology chapter "Microbes in Human Welfare" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Microbes in Human Welfare" — 8 important questions with detailed answers for CBSE boar…
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NCERT Solutions for Class 12 Biology — Microbes in Human Welfare. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Microbes in Human Welfare — All 8 Questions Solved
Q1. Define microbes.
Microbes are microscopic organisms that are not visible to the naked eye and require a microscope for observation. They include bacteria, fungi, protozoa, algae, and viruses. These organisms are found everywhere in nature - in soil, water, air, and even inside the human body.
Q2. What is fermentation? Give two examples of fermentation products.
Fermentation is an anaerobic process by which microorganisms break down organic compounds in the absence of oxygen, releasing energy and producing byproducts like ethanol and CO2.
Examples:
- Lactobacillus ferments milk to produce yogurt and cheese
- Saccharomyces cerevisiae (yeast) ferments glucose to produce ethanol and CO2 in beer and wine production
Q3. What is the role of Lactobacillus in human welfare? Name any two products.
Lactobacillus is a beneficial bacterium that plays a crucial role in human nutrition and health:
1. Produces lactic acid through fermentation which preserves food and prevents pathogenic growth
2. Aids in digestion and nutrient absorption in the human gut
3. Produces essential vitamins like B12 and K
4. Acts as a probiotic to maintain healthy gut microbiota
Products:
- Yogurt (fermented milk with L. bulgaricus and L. thermophilus)
- Cheese (various types using different Lactobacillus strains)
- Buttermilk
- Sauerkraut
Q4. Explain the process of sewage treatment and the role of microbes in secondary treatment.
Sewage treatment occurs in three stages:
PRIMARY TREATMENT:
- Physical removal of large particles, grit, and sand through screening and sedimentation
- No microbial action
SECONDARY TREATMENT (BIOLOGICAL):
- Microbes play the main role in decomposing organic matter
- Heterotrophic bacteria oxidize organic compounds to CO2 and H2O
- Nitrifying bacteria convert ammonia to nitrate
- Fungal degradation of complex compounds
- Takes 5-7 days
- Reduces BOD (Biochemical Oxygen Demand) by 80-90%
TERTIARY TREATMENT:
- Removal of remaining nutrients and pathogens
- Disinfection using chlorine or UV light
Beneficial microbes in secondary treatment: Zooglea, Bacillus, Pseudomonas, and activated sludge organisms
Q5. What are probiotics? How do they benefit human health?
Probiotics are live beneficial microorganisms that confer health benefits when administered in adequate amounts. They are mainly lactic acid bacteria and bifidobacteria.
BENEFITS:
1. Improve digestion and lactose tolerance by producing lactase enzyme
2. Enhance nutrient absorption and vitamin synthesis (B vitamins, K)
3. Strengthen intestinal barrier function and prevent pathogen colonization
4. Modulate immune system by promoting beneficial immune responses
5. Produce antimicrobial compounds (bacteriocins) against pathogens
6. Reduce inflammation and improve gut health
7. May help in managing lactose intolerance, IBS, and diarrhea
Common probiotic microbes: Lactobacillus acidophilus, L. casei, Bifidobacterium longum
Food sources: Yogurt, buttermilk, cheese, kimchi, tempeh
Q6. What is biogas? Write the microbial process of its production.
Biogas is a mixture of methane (50-70%), carbon dioxide (30-40%), and traces of H2S and N2. It is a renewable energy source produced through anaerobic digestion of organic matter.
MICROBIAL PROCESS (ANAEROBIC DIGESTION):
STAGE 1 - HYDROLYSIS:
- Hydrolytic bacteria break down complex polymers (cellulose, proteins, lipids) into simple monomers
- Produces glucose, amino acids, fatty acids
STAGE 2 - ACIDOGENESIS:
- Acidogenic bacteria ferment monomers into volatile fatty acids, alcohols, CO2, and H2
- Produces acetate, propionate, butyrate
STAGE 3 - ACETOGENESIS:
- Acetogenic bacteria convert fatty acids and alcohols to acetate and H2
STAGE 4 - METHANOGENESIS:
- Methanogenic archaea (Methanobacterium, Methanococcus) convert acetate and H2 to methane and CO2
- This is the final and rate-limiting step
Applications: Biogas digesters for energy production, waste management in farms and municipalities
Q7. Describe the role of Aspergillus niger in industrial production. How is citric acid produced from this fungus?
Aspergillus niger is a filamentous fungus widely used in industrial fermentation for producing organic acids.
ROLE:
1. Produces citric acid, one of the most important organic acids
2. Produces other acids like gluconic acid and itaconic acid
3. Produces enzymes like amylase and cellulase
4. Generally recognized as safe (GRAS) status
5. Ferments various carbohydrate sources efficiently
CITRIC ACID PRODUCTION:
Substrate: Molasses (contains sucrose, glucose) or glucose solution
Process:
1. Glucose enters the glycolytic pathway producing pyruvate
2. Pyruvate enters the citric acid cycle (Krebs cycle)
3. Under oxygen-limited or specific pH conditions (pH 5-6), citrate accumulates instead of being further oxidized
4. Citrate is secreted out of the cell
5. Fermentation occurs in aerobic conditions with controlled aeration
6. High glucose concentration and iron limitation promote citrate accumulation
Conditions for maximum production:
- Temperature: 25-30°C
- pH: 5-6 (acidic)
- Aeration: Adequate oxygen supply
- Substrate concentration: High glucose levels
- Iron limitation: Low iron promotes citrate secretion
Q8. What are biofertilizers? Name four types and explain their roles in enhancing soil fertility.
Biofertilizers are microbial preparations containing living cells of beneficial microorganisms that enhance nutrient availability and plant growth. They are eco-friendly alternatives to chemical fertilizers.
TYPES AND ROLES:
1. NITROGEN-FIXING BACTERIA:
- Examples: Azotobacter, Azospirillum, Rhizobium
- Fix atmospheric nitrogen (N2) into ammonia (NH3) through nitrogenase enzyme
- Make nitrogen available to plants in usable form
- Rhizobium forms symbiotic relationships with legumes in root nodules
- Azotobacter is free-living in soil
2. PHOSPHATE-SOLUBILIZING BACTERIA:
- Examples: Bacillus, Pseudomonas, Thiobacillus
- Convert insoluble phosphates in soil to soluble forms
- Produce organic acids that dissolve phosphate minerals
- Make phosphorus available for plant uptake
3. MYCORRHIZAL FUNGI:
- Examples: Glomus, Funneliformis
- Form symbiotic associations with plant roots
- Extend root surface area for nutrient absorption
- Enhance uptake of phosphorus, zinc, and copper
- Improve water retention and plant drought tolerance
4. POTASSIUM-SOLUBILIZING MICROBES:
- Examples: Bacillus mucilaginosus, Frateuria aurantia
- Break down mineral potassium compounds into available forms
- Produce acids and chelating compounds
- Enhance potassium uptake by plants
BENEFITS:
- Reduce dependency on chemical fertilizers
- Improve soil health and structure
- Increase crop yield by 10-25%
- Cost-effective and sustainable
- Reduce environmental pollution
Microbes in Human Welfare — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which microorganism is primarily responsible for converting milk into curd?
- A. Saccharomyces cerevisiae
- B. Lactobacillus
- C. Penicillium roqueforti
- D. Acetobacter aceti
Answer: B. Lactobacillus — Lactobacillus, a type of Lactic Acid Bacteria (LAB), ferments the lactose sugar in milk into lactic acid, which coagulates the milk proteins to form curd.
MCQ 2. The 'brewer's yeast' used for fermenting malted cereals and fruit juices to produce ethanol is:
- A. Propionibacterium sharmanii
- B. Saccharomyces cerevisiae
- C. Clostridium butylicum
- D. Trichoderma polysporum
Answer: B. Saccharomyces cerevisiae — Saccharomyces cerevisiae, commonly known as brewer's yeast, is widely used in the production of alcoholic beverages through fermentation.
MCQ 3. The first antibiotic, Penicillin, was discovered by Alexander Fleming from which microorganism?
- A. Streptomyces griseus
- B. Penicillium notatum
- C. Bacillus subtilis
- D. Aspergillus niger
Answer: B. Penicillium notatum — Alexander Fleming accidentally discovered penicillin from the mould Penicillium notatum (later identified as Penicillium chrysogenum) growing in his culture plates.
MCQ 4. A high Biological Oxygen Demand (BOD) value of a water body indicates:
- A. Low microbial activity and high dissolved oxygen
- B. High microbial activity and high dissolved oxygen
- C. Low microbial activity and low dissolved oxygen
- D. High microbial activity and low dissolved oxygen
Answer: D. High microbial activity and low dissolved oxygen — High BOD indicates a high amount of organic matter in the water, which leads to increased microbial decomposition and a consequent depletion of dissolved oxygen.
MCQ 5. What are the primary processes involved in the primary treatment of sewage?
- A. Aeration and flocculation
- B. Filtration and sedimentation
- C. Anaerobic digestion and gas production
- D. Chlorination and disinfection
Answer: B. Filtration and sedimentation — Primary treatment is a physical process that involves the removal of large and small particles from sewage through sequential filtration and sedimentation.
MCQ 6. In the secondary treatment of sewage, microbial flocs play a crucial role by:
- A. Removing inorganic waste
- B. Reducing the Biological Oxygen Demand (BOD)
- C. Producing methane gas
- D. Filtering out suspended solids
Answer: B. Reducing the Biological Oxygen Demand (BOD) — Microbial flocs, consisting of bacteria and fungi, consume the major part of the organic matter in the effluent, significantly reducing its BOD.
MCQ 7. Which group of bacteria is responsible for producing biogas (methane) in anaerobic sludge digesters?
- A. Nitrifying bacteria
- B. Denitrifying bacteria
- C. Methanogens
- D. Lactic Acid Bacteria
Answer: C. Methanogens — Methanogens are anaerobic bacteria, such as Methanobacterium, that decompose organic matter to produce methane along with carbon dioxide and hydrogen.
MCQ 8. The main component of biogas, which is combustible and used as fuel, is:
- A. Carbon dioxide
- B. Hydrogen sulphide
- C. Methane
- D. Ethane
Answer: C. Methane — Methane (CH4) constitutes the major portion (50-70%) of biogas and is the primary component responsible for its energy value.
MCQ 9. Which bacterium is used to produce the Bt toxin that provides resistance to insect pests in genetically modified crops like Bt cotton?
- A. Agrobacterium tumefaciens
- B. Bacillus thuringiensis
- C. Rhizobium leguminosarum
- D. Azotobacter chroococcum
Answer: B. Bacillus thuringiensis — Bacillus thuringiensis is a bacterium whose gene for producing insecticidal toxins is incorporated into plants to confer resistance against certain insect pests.
MCQ 10. Which of the following is a fungal biocontrol agent used effectively against several plant pathogens?
- A. Bacillus thuringiensis
- B. Nucleopolyhedrovirus
- C. Trichoderma
- D. Azospirillum
Answer: C. Trichoderma — Trichoderma species are free-living fungi that are effective biocontrol agents of several plant pathogens found in the root ecosystems.
MCQ 11. Nucleopolyhedroviruses (NPVs), used as biocontrol agents, are known for their:
- A. Broad-spectrum insecticidal applications
- B. Ability to harm non-target organisms
- C. Species-specific, narrow-spectrum activity
- D. Effectiveness against fungal diseases in plants
Answer: C. Species-specific, narrow-spectrum activity — NPVs are excellent candidates for species-specific, narrow-spectrum insecticidal applications, as they do not harm plants, mammals, birds, fish, or non-target insects.
MCQ 12. Which bacterium forms a symbiotic association with leguminous plants and fixes atmospheric nitrogen in root nodules?
- A. Azotobacter
- B. Rhizobium
- C. Anabaena
- D. Frankia
Answer: B. Rhizobium — Rhizobium is a symbiotic bacterium that lives in the root nodules of leguminous plants and converts atmospheric nitrogen into usable forms for the plant.
MCQ 13. An example of a free-living, non-symbiotic nitrogen-fixing bacterium that enriches the nitrogen content of soil is:
- A. Rhizobium
- B. Azotobacter
- C. Nostoc
- D. Frankia
Answer: B. Azotobacter — Azotobacter is a free-living aerobic bacterium that fixes atmospheric nitrogen in the soil without forming a symbiotic relationship with plants.
MCQ 14. Cyanobacteria, such as Anabaena and Nostoc, are important biofertilisers, especially in paddy fields, primarily due to their ability to:
- A. Solubilize phosphorus
- B. Fix atmospheric nitrogen
- C. Improve soil aeration
- D. Produce plant growth hormones
Answer: B. Fix atmospheric nitrogen — Cyanobacteria are autotrophic microbes that can fix atmospheric nitrogen, significantly contributing to the nitrogen enrichment of soil, particularly in flooded rice fields.
MCQ 15. The fungus Aspergillus niger is commercially used for the production of:
- A. Lactic acid
- B. Citric acid
- C. Butyric acid
- D. Acetic acid
Answer: B. Citric acid — Aspergillus niger is a filamentous fungus widely employed in industrial processes for the large-scale production of citric acid.
MCQ 16. Which bacterium is responsible for the conversion of ethanol to acetic acid, commonly known as vinegar?
- A. Lactobacillus bulgaricus
- B. Clostridium butylicum
- C. Acetobacter aceti
- D. Saccharomyces cerevisiae
Answer: C. Acetobacter aceti — Acetobacter aceti is an aerobic bacterium that oxidizes ethanol to acetic acid, which is the main component of vinegar.
MCQ 17. The enzyme 'lipase' is often incorporated into detergent formulations for its ability to:
- A. Clarify fruit juices
- B. Remove protein stains
- C. Break down fats and oils
- D. Tenderize meat
Answer: C. Break down fats and oils — Lipases are enzymes that break down lipids (fats and oils), making them effective in removing oily stains from laundry.
MCQ 18. Pectinases and proteases are commercially used to:
- A. Increase the shelf life of milk
- B. Clarify bottled fruit juices
- C. Produce antibiotics
- D. Synthesize vitamins
Answer: B. Clarify bottled fruit juices — Pectinases and proteases are used to break down pectin and proteins present in fruit juices, respectively, making them clearer and more appealing.
MCQ 19. Statins, produced by the yeast Monascus purpureus, are important bioactive molecules because they:
- A. Act as immunosuppressive agents
- B. Dissolve blood clots
- C. Lower blood cholesterol levels
- D. Function as broad-spectrum antibiotics
Answer: C. Lower blood cholesterol levels — Statins competitively inhibit the enzyme responsible for cholesterol synthesis, thereby lowering blood cholesterol levels and preventing cardiovascular diseases.
MCQ 20. Cyclosporin A, an immunosuppressive agent used in organ transplant patients, is produced by which microorganism?
- A. Monascus purpureus
- B. Streptomyces griseus
- C. Trichoderma polysporum
- D. Aspergillus niger
Answer: C. Trichoderma polysporum — Cyclosporin A is a bioactive molecule produced by the fungus Trichoderma polysporum, and it is a potent immunosuppressive agent.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.