CBSE Class 12 Biology (2026–27)
Chapter 8: Microbes in Human Welfare
20 Important Questions and Answers
Microbes play important roles in household products, industrial production, sewage treatment, biogas generation, biocontrol, and biofertilizers. These topics are among the most important areas of the chapter for CBSE examinations.
Q1. What are Lactic Acid Bacteria (LAB)? Explain their role in curd formation.
Answer:
Lactic Acid Bacteria (LAB), mainly species of Lactobacillus, are beneficial microbes used in the conversion of milk into curd. When a small amount of curd is added to warm milk, LAB multiply rapidly and produce lactic acid. The acid coagulates milk proteins, particularly casein, resulting in the formation of curd. LAB also improve the nutritional quality of curd by increasing the availability of vitamin B12. These bacteria suppress the growth of harmful microbes in the intestine and aid digestion. Thus, LAB are important microbes that contribute to food production and human health.
Q2. How are microbes used in the preparation of idli and dosa?
Answer:
The batter used for preparing idli and dosa undergoes fermentation by naturally occurring bacteria and other microorganisms. During fermentation, microbes break down carbohydrates and release gases, causing the batter to rise and become soft and fluffy. This process improves the texture, taste, and digestibility of the food. Fermentation also enhances the nutritional value by making nutrients more available for absorption. The characteristic aroma and spongy texture of idli and dosa are due to microbial activity. Thus, microbes play an essential role in the preparation of many traditional Indian foods.
Q3. Explain the role of yeast in bread making.
Answer:
Yeast, especially Saccharomyces cerevisiae (baker’s yeast), is widely used in bread making. It ferments sugars present in the dough and produces carbon dioxide and small amounts of alcohol. The carbon dioxide gets trapped within the dough, causing it to expand and rise. This makes the bread soft, porous, and light. Fermentation also improves the flavor and texture of bread. Because of its ability to carry out efficient fermentation, yeast has been used in baking industries for centuries. It is one of the most important examples of beneficial microbes in household and commercial food production.
Q4. How are microbes used in the production of alcoholic beverages?
Answer:
Alcoholic beverages such as wine, beer, rum, whisky, and brandy are produced through microbial fermentation. Yeast, mainly Saccharomyces cerevisiae, converts sugars present in grains, fruits, or molasses into ethanol and carbon dioxide under anaerobic conditions. The fermentation process is carried out in large fermenters where environmental conditions are carefully controlled. Different beverages are produced depending on the raw materials and methods used. This microbial process has been utilized for thousands of years and remains an important industrial application of microorganisms. Fermentation is a classic example of how microbes contribute to human welfare through industrial production.
Q5. What are antibiotics? Give one example.
Answer:
Antibiotics are chemical substances produced by certain microorganisms that can kill or inhibit the growth of disease-causing microbes. They are widely used in medicine to treat bacterial infections. One of the most famous antibiotics is penicillin, discovered from the fungus Penicillium. Antibiotics have revolutionized healthcare by reducing mortality from infectious diseases. They are produced commercially using microbial fermentation techniques. However, antibiotics should be used responsibly to prevent the development of antibiotic-resistant bacteria. The discovery and production of antibiotics represent one of the greatest contributions of microbes to human welfare.
Q6. What is sewage treatment? Why is it necessary?
Answer:
Sewage treatment is the process of removing pollutants and organic matter from wastewater before it is released into natural water bodies. Untreated sewage contains harmful microorganisms, organic waste, and toxic substances that can pollute rivers and lakes. Treatment reduces pollution levels and prevents the spread of diseases. Microbes play a crucial role in secondary treatment, where they degrade organic matter. Proper sewage treatment improves environmental quality, protects aquatic life, and safeguards human health. Therefore, it is an essential process for maintaining sanitation and ecological balance in modern societies.
Q7. What are flocs? State their significance.
Answer:
Flocs are masses of bacteria associated with fungal filaments that form mesh-like structures during secondary sewage treatment. They develop in aeration tanks where wastewater is continuously supplied with oxygen. The microorganisms present in flocs actively decompose organic matter in sewage, reducing pollution levels. As flocs grow, they settle down as activated sludge. Their activity lowers the Biological Oxygen Demand (BOD) of wastewater, indicating improved water quality. Therefore, flocs are essential components of biological sewage treatment and help make wastewater safer before its release into the environment.
Q8. Define Biological Oxygen Demand (BOD).
Answer:
Biological Oxygen Demand (BOD) is the amount of oxygen required by microorganisms to decompose organic matter present in water. It serves as an indicator of the level of organic pollution. Higher BOD values indicate greater pollution because more oxygen is needed for microbial decomposition. During sewage treatment, microbial activity reduces organic matter and lowers the BOD of wastewater. A low BOD value signifies cleaner water that is safer for aquatic organisms. Therefore, BOD is an important parameter used to assess the efficiency of sewage treatment processes and the quality of water.
Q9. What is activated sludge?
Answer:
Activated sludge is the sediment formed after flocs settle at the bottom of the secondary treatment tank in sewage treatment plants. It contains a large population of aerobic microorganisms that continue decomposing organic matter. A portion of the activated sludge is pumped back into the aeration tank to act as an inoculum for fresh sewage treatment. The remaining sludge is transferred to anaerobic digesters, where further microbial decomposition occurs. Activated sludge plays a key role in reducing pollution and enhancing the efficiency of wastewater treatment systems.
Q10. What is biogas? Mention its composition.
Answer:
Biogas is a combustible gaseous fuel produced through the anaerobic decomposition of organic waste by methanogenic bacteria. It is considered an eco-friendly and renewable source of energy. The major component of biogas is methane, which is responsible for its fuel value. It also contains carbon dioxide and small amounts of hydrogen and hydrogen sulphide. Biogas is produced from cattle dung, agricultural waste, sewage sludge, and other biodegradable materials. It is widely used for cooking, heating, and electricity generation in rural areas. Thus, microbes contribute significantly to sustainable energy production.
Q11. Who are methanogens? Give one example.
Answer:
Methanogens are anaerobic microorganisms that produce methane gas during the decomposition of organic matter. They thrive in oxygen-free environments such as marshes, biogas plants, and the rumen of cattle. One common example is Methanobacterium. These microbes break down cellulose-rich materials and generate methane as a metabolic by-product. Their activity forms the basis of biogas production, which provides a renewable source of energy. Methanogens are therefore highly important in waste management, energy generation, and maintaining ecological nutrient cycles.
Q12. What are biocontrol agents?
Answer:
Biocontrol agents are living organisms used to control pests, pathogens, and weeds without the use of harmful chemical pesticides. These include bacteria, fungi, viruses, and beneficial insects. Biological control is environmentally friendly and helps maintain ecological balance. It reduces pollution and prevents the harmful effects associated with excessive pesticide use. Biocontrol agents are an important component of sustainable agriculture and organic farming. Their use promotes safer food production and conservation of beneficial organisms in agricultural ecosystems.
Q13. Explain the role of Bacillus thuringiensis (Bt) as a biocontrol agent.
Answer:
Bacillus thuringiensis (Bt) is a soil bacterium widely used as a biological control agent against insect pests. It produces protein crystals containing toxins that become active in the alkaline gut of caterpillars. When insect larvae feed on Bt-treated plants, the toxin damages their digestive tract and causes death. Bt specifically targets harmful insect pests without affecting beneficial insects, humans, or animals. Because of its selective action and environmental safety, Bt is extensively used in agriculture and has also been utilized in the development of Bt crops.
Q14. What are biofertilizers?
Answer:
Biofertilizers are living microorganisms that enrich soil fertility by increasing the availability of nutrients to plants. They include bacteria, fungi, and cyanobacteria. Unlike chemical fertilizers, biofertilizers are eco-friendly and improve long-term soil health. They help in nitrogen fixation, phosphorus solubilization, and nutrient recycling. Their use enhances crop productivity while reducing environmental pollution. Biofertilizers play an important role in sustainable agriculture and are increasingly promoted in organic farming systems. Thus, they represent a beneficial application of microbes in human welfare.
Q15. Describe the role of Rhizobium as a biofertilizer.
Answer:
Rhizobium is a nitrogen-fixing bacterium that lives symbiotically in the root nodules of leguminous plants such as pea, gram, and bean. It converts atmospheric nitrogen into ammonia, which plants can readily utilize for growth. In return, the plant provides food and shelter to the bacteria. This mutual relationship improves soil fertility and reduces the need for chemical nitrogen fertilizers. Rhizobium is one of the most widely used biofertilizers in agriculture and contributes significantly to sustainable crop production and environmental conservation.
Q16. What is mycorrhiza? State its advantages.
Answer:
Mycorrhiza is a symbiotic association between fungal species and plant roots. The fungal partner helps the plant absorb water and minerals, particularly phosphorus, from the soil. In return, the fungus receives carbohydrates produced by the plant through photosynthesis. Mycorrhizal associations improve plant growth, increase resistance to root pathogens, and enhance tolerance to drought and salinity. A commonly cited fungal genus involved in mycorrhiza is Glomus. These associations are valuable biofertilizers that support sustainable agriculture and improve soil productivity.
Q17. Name the cyanobacteria used as biofertilizers. Why are they important?
Answer:
Important cyanobacterial biofertilizers include Anabaena, Nostoc, and Oscillatoria. These microorganisms can fix atmospheric nitrogen and enrich soil fertility naturally. They are especially useful in paddy fields where they improve crop productivity. In addition to nitrogen fixation, cyanobacteria contribute organic matter to the soil, enhancing its structure and nutrient content. Their use reduces dependence on chemical fertilizers and promotes environmentally sustainable agriculture. Because of these advantages, cyanobacteria are widely employed as natural biofertilizers in rice cultivation.
Q18. Differentiate between biocontrol agents and biofertilizers.
Answer:
Biocontrol agents and biofertilizers are both beneficial microorganisms but serve different purposes. Biocontrol agents help control pests, pathogens, and weeds, reducing crop damage. Examples include Bacillus thuringiensis and Trichoderma. Biofertilizers, on the other hand, improve soil fertility by supplying nutrients such as nitrogen and phosphorus. Examples include Rhizobium, Azotobacter, and cyanobacteria. While biocontrol agents protect crops from harmful organisms, biofertilizers enhance plant nutrition and growth. Together, they contribute to sustainable agriculture and reduce dependence on chemical pesticides and fertilizers.
Q19. Why is organic farming gaining importance?
Answer:
Organic farming emphasizes the use of natural methods such as biofertilizers, biocontrol agents, compost, and crop rotation instead of synthetic chemicals. It improves soil health, maintains biodiversity, and minimizes environmental pollution. Organic products are often considered safer because they contain fewer chemical residues. The approach also promotes sustainable agricultural practices and conserves natural resources. Microbes play a vital role in organic farming through nutrient recycling and biological pest control. Therefore, organic farming is increasingly recognized as an environmentally friendly and sustainable method of food production.
Q20. How do microbes contribute to human welfare?
Answer:
Microbes contribute significantly to human welfare in various fields. They are used in food production, such as curd, bread, idli, and alcoholic beverages. In industries, microbes produce antibiotics, enzymes, and other valuable products. They help in sewage treatment by decomposing organic waste and reducing pollution. Methanogenic bacteria generate biogas, a renewable energy source. Microbes also act as biocontrol agents and biofertilizers, promoting sustainable agriculture. These diverse applications demonstrate that microbes are not only disease-causing organisms but also indispensable resources that support human health, environmental management, and economic development.