NCERT Solutions Class 12 Biology Chapter 8 Microbes in Human Welfare

In Class 12 Biology Chapter 8, Microbes in Human Welfare, students learn how tiny microorganisms play a big role in improving human life and the environment. This chapter explains how microbes are used in food production, medicine, agriculture, and waste management. It also helps students understand how bacteria and fungi contribute to making antibiotics, vaccines, biogas, and fermented products, showing the positive side of microbes in daily life and industry.

ALLEN has developed comprehensive NCERT Solutions addressing all the exercise questions from the NCERT Textbooks asked at the end of every chapter. Our expert faculty focuses on providing clear explanations of the biological oxygen demand (BOD) and the specific roles of various fungi and bacteria in environmental management. These solutions are designed to help you write precise, point-wise answers that align perfectly with the marking scheme of CBSE. By utilizing ALLEN’s professional resources, you can master the technical details of microbiology with the speed and accuracy required for both board exams and NEET preparation.

1.0Download NCERT Solutions Class 12 Biology Chapter 8 Microbes in Human Welfare : Free PDF

This chapter explains how microbes are used in industries, health care, and environmental protection. The NCERT Solutions for Class 12 Biology Chapter 8 Microbes in Human Welfare help students revise key uses and practise textbook questions easily. Download the free PDF now to study anytime and prepare confidently for exams.

2.0Key Concepts of Class 12 Biology Chapter 8 Microbes in Human Welfare

The chapter focuses on understanding the beneficial uses of microorganisms. Some of the key lessons covered in this chapter are given below:

• Microbes in Household Products: Learn how microbes help in making curd, bread, cheese, and other fermented foods.
• Microbes in Industrial Products: Study the role of microbes in producing alcohol, organic acids, and enzymes.
• Microbes in Medicine: Understand how antibiotics, vaccines, and therapeutic proteins are made.
• Microbes in Sewage Treatment: Learn how microbes help clean wastewater and protect the environment.
• Biogas Production: Understand how bacteria produce biogas from organic waste.
• Biological Control: Study the use of microbes to control plant pests and diseases.
• Biofertilisers: Learn how microbes improve soil fertility and plant growth.

3.0NCERT Class 12 Biology Chapter 8 Microbes in Human Welfare : Detailed Solutions

1. Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why? 

Ans: I would carry a sample of stagnant water (for example, from a flower pot, drain, or pond) or a small amount of curd (yogurt) from home to the biology laboratory.

Reason:

• Stagnant water contains a large number of microorganisms such as bacteria, protozoa, and algae, which can be easily observed under a microscope.
• Similarly, curd contains Lactobacillus bacteria, which are present in high numbers and can be clearly seen after preparing a simple smear and staining.
• These samples are easy to collect, safe to handle under supervision, and reliable for demonstrating the presence of microbes, making them suitable for a school laboratory demonstration.

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

Ans: Below are two examples to prove that microbes release gases during metabolism:

• Yeast in Dough or Sugar Solution: When yeast is added to a sugar solution or dough, it carries out anaerobic respiration (fermentation) and breaks down sugar to form alcohol and carbon dioxide (CO₂). The release of CO₂ causes the dough to rise and become fluffy, which is commonly seen in bread-making.
• Methanogenic Bacteria in Biogas Plants: In biogas plants, methanogenic bacteria break down organic waste in the absence of oxygen and release methane (CH₄), carbon dioxide (CO₂), and hydrogen (H₂). The accumulation of these gases forms biogas, which is used as a fuel.

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

Ans: Lactic Acid Bacteria are commonly found in curd and other fermented milk products such as buttermilk and cheese. 

Applications of LAB:

• Conversion of milk into curd: Lactic acid bacteria (e.g., Lactobacillus) convert lactose into lactic acid, which coagulates milk proteins and forms curd.
• Improves digestion: They act as probiotics, helping in better digestion and maintaining a healthy gut.
• Enhances nutritional value: Fermentation increases the vitamin content, especially vitamin B₁₂, and improves the availability of nutrients.
• Preservation of food: The lactic acid produced creates an acidic environment that inhibits the growth of harmful microbes, thus helping in food preservation.
• Improves flavour and texture: They give fermented foods a pleasant sour taste and better texture.

4. Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes. 

Ans: Many traditional Indian foods are prepared by fermentation, where microbes help in improving taste, texture, and nutritional value.

• Idli and Dosa : The batter is fermented by lactic acid bacteria and yeast, which produce carbon dioxide and organic acids. This makes the batter soft, fluffy, and easily digestible.
• Dhokla : The batter of Bengal gram flour is fermented by lactic acid bacteria, which gives dhokla its spongy texture and sour taste.
• Jalebi : The batter is fermented by yeast and lactic acid bacteria, which helps develop its characteristic flavour and texture.
• Bhatura : Fermentation using yeast or curd (lactic acid bacteria) makes the dough soft and fluffy before frying.
• Appam : Fermented by yeast and lactic acid bacteria, giving a light, porous texture.

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

Ans:

• Production of antibiotics: Many microbes produce substances called antibiotics that can kill or inhibit the growth of harmful bacteria.
• Examples of antibiotic-producing microbes: The fungus Penicillium produces penicillin, while bacteria such as Streptomyces produce streptomycin, tetracycline, and erythromycin.
• Treatment of bacterial infections: These antibiotics are used in medicine to treat diseases like pneumonia, tuberculosis, and wound infections.
• Prevention of spread of infection: By reducing the number of disease-causing bacteria in the body, antibiotics help in controlling the spread of infections.
• Improvement in public health: The use of antibiotics has significantly reduced mortality and severity of bacterial diseases, improving overall human health.

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

Ans: 

• Penicillium notatum : They are used for the production of the penicillin antibiotic.
• Penicillium chrysogenum : They are also used for the large-scale production of penicillin antibiotic.

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

Ans: Sewage can be defined as the wastewater released from homes, hospitals, industries, and other establishments which contains human excreta, food waste, chemicals, and microorganisms.

Harmful effects of Sewage:

• It contains disease-causing microorganisms that can spread illnesses like cholera, typhoid, and dysentery.
• When released into water bodies, it pollutes rivers and lakes, making the water unfit for drinking and domestic use.
• The organic matter in sewage reduces dissolved oxygen in water, which can harm or kill aquatic organisms.
• It can produce foul smell and unhygienic conditions, affecting public health.

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

Ans: The key difference lies in the nature of the process used to remove pollutants. Refer the table below to understand them.

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

Ans: Yes, microbes can indeed be used as a source of energy. The most common example of this is biogas.

Explanation: Under anaerobic conditions, the methanogenic bacteria like methanobacterium act on animal dung and organic waste. They decompose the organic matter and release gases like methane (CH₄), carbon dioxide (CO₂), and hydrogen (H₂). Methane, being a combustible gas can be used a clean and renewable fuel for cooking, lighting, and generating electricity.

10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished. 

Ans: Microbes can reduce the use of chemical fertilisers and pesticides by acting as biofertilisers and biopesticides.

• As Biofertilisers: Microorganisms like Rhizobium (in legume root nodules), Azotobacter, and cyanobacteria fix atmospheric nitrogen and convert it into forms usable by plants. This increases soil fertility and reduces the need for nitrogen-based chemical fertilisers.
• As Biopesticides: Some microbes control pests and plant diseases naturally. For example, Bacillus thuringiensis (Bt) produces toxins that kill insect larvae, and Trichoderma fungi suppress plant pathogens. This reduces dependence on chemical pesticides.

11. Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean? 

Ans: BOD indicates the amount of dissolved oxygen required by microorganisms to break down organic matter in water. Hence, higher BOD implies higher polluted water and Lower BOD implies clearerer water.

Given: Sample A - 20mg/l

Sample B = 8 mg/l

Sample C - 400mg/l

Since Sample C has the highest BOD, this means that Sample C is the most polluted.

Now, Since river water is relatively clean, it will have lower BOD.

Therefor the correct labels for the sample are as follows:

River Water = Sample B = 8mg/l

Secondary Effluent = Sample A = 20mg/l

Untreated Sewage = Sample C = 400mg/l

12. Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained. 

Ans: 

• Cyclosporin A (an immunosuppressive drug) are obtained from the fungus Trichoderma polysporum
• Statins (blood cholesterol lowering agents) are obtained from the fungus Monascus purpureus.

13. Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP) (b) Soil 

Ans: Single Cell Protein (SCP):  It refers to the protein-rich microbial biomass obtained from microorganisms such as bacteria, yeast, fungi, and algae. These microbes are grown on organic wastes or inexpensive raw materials and harvested as a nutritious food or feed supplement.

Role:

• They provide a cheap and rich source of protein
• They help in recycling organic waste
• They are also used as animal feed and sometimes as human food supplements
• Examples of microbes: Spirulina, Chlorella, yeast (Saccharomyces)

Soil: Microbes play a vital role in maintaining soil fertility and ecosystem balance.

Role:

• Decompose organic matter and convert it into simpler nutrients (nutrient recycling)
• Fix atmospheric nitrogen (e.g., Rhizobium, Azotobacter) making it available to plants
• Improve soil structure and aeration
• Suppress plant pathogens by acting as natural biocontrol agents
• Examples of microbes: Rhizobium

14. Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. 

Biogas, Citric acid, Penicillin and Curd 

Ans: The order of least importance is Penicilin, Biogas, Curd and Citric Acid. The reason for the arrangement is stated below:

1. Penicillin (Most Important): Penicillin is an antibiotic produced by microbes and is used to treat a wide range of bacterial infections. It has saved millions of lives by controlling deadly diseases such as pneumonia, syphilis, and wound infections. Its role in public health and medical treatment makes it the most important for human welfare.

2. Biogas: Biogas is a renewable source of energy produced by the action of microbes on organic waste. It provides a clean and affordable fuel for cooking and lighting, especially in rural areas. It also helps in waste management and produces slurry that can be used as a natural fertiliser, improving environmental sustainability.

3. Curd: Curd is a nutritious food product formed by the action of Lactobacillus bacteria on milk. It improves digestion, enhances nutritional value, and acts as a probiotic that supports gut health. While important for daily nutrition, its impact is more limited compared to life-saving medicines and energy resources.

4. Citric Acid (Least Important): Citric acid is produced industrially using microbes like Aspergillus and is mainly used in the food and pharmaceutical industries as a preservative, flavouring agent, and stabiliser. Although economically useful, it is less directly essential to survival and welfare compared to the others.

15. How do biofertilisers enrich the fertility of the soil? 

Ans: Biofertilisers are living microorganisms that improve soil fertility by increasing the availability of nutrients to plants in a natural and sustainable way.

• Nitrogen fixation: Microbes like Rhizobium (in legume root nodules), Azotobacter, and cyanobacteria (Nostoc, Anabaena) convert atmospheric nitrogen (N₂) into ammonia and nitrates, which plants can easily absorb for making proteins and nucleic acids.
• Phosphate solubilisation: Some bacteria and fungi convert insoluble phosphates in the soil into soluble forms, making phosphorus available to plants for root development and energy transfer.
• Mycorrhizal association: Fungi such as Glomus form mycorrhiza with plant roots and help in better absorption of water and minerals, especially phosphorus, increasing plant growth and resistance to drought and diseases.
• Improvement of soil health: Biofertilisers enhance soil structure, microbial activity, and organic matter content, leading to long-term soil fertility.

4.0Key Features and Benefits of Class 12 Biology Chapter 8 Microbes in Human Welfare

• Clear explanations help students connect biology concepts with environmental and health-related issues.
• Solutions explain practical applications clearly, helping students link microbes with real-world benefits.
• Practice questions strengthen understanding of industrial and medical uses of microorganisms.
• For improved recall of examples and microbial processes during exams, regular revision of the solutions are beneficial.
• Strong fundamentals also support preparation for NEET and studies in biotechnology and microbiology.