NCERT Solutions for class 12th Biology Chapter 10 Microbes in Human Welfare


Question 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 under a microscope, which sample would you carry and why?

Sol. Bacteria are microscopic single-celled organisms that thrive in diverse environments. They cannot be seen with the naked eyes, but these can be seen with the help of a microscope.

Sample which is easy to carry from home to biology laboratory for the study of microbes present in them is curd because curd contains lactobacilli bacteria.

Curd is formed from milk. A small drop of curd contains millions of lactic acid bacteria (LAB) or Lactobacillus, which can be easily observed under a microscope. The curd forms because of the chemical reaction between the lactic acid bacteria and casein (present in milk along with lactose, a disachharide sugar). During fermentation, the bacteria use enzymes to produce energy (ATP) from lactose. The by-product of ATP production is lactic acid that coagulates and digests milk proteins.

Note: The species of bacteria used in yogurt are Streptococcus thermophilus and Lactobacil­lus bulgaricus. These species eat the sugars in milk. As the bacteria eat milk sugars, the bacteria produce lactic acid.

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

Sol. Microbial metabolism is the means by which a microbe obtains the energy and nutrients (e.g. carbon) it needs to live and reproduce.

Microbes that release gases during metabolism in the given process are as follows:

(i) Fermentation: Bacteria and fungi carry out the process of fermentation and during this process, they release carbon dioxide. Fermentation is the process of converting a complex organic substance into a simpler substance with the action of bacteria or yeast. Fermentation of sugar produces alcohol with the release of carbon dioxide and very little energy.

(ii) Indian food: The dough used for making idli and dosa gives a puffed appearance. This is because of the action of bacteria which releases carbon dioxide (CO2) gas. This carbon dioxide gets trapped in the dough, thereby giving it a puffed appearance.

(iii) Sewage treatment: During secondary treatment of sewage in sewage treatment plants, a number of gases like methane are produced during microbial digestion of organic compounds.

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

Sol. Lactic acid bacteria (LAB) are among the most important groups of microorganisms used in food fermentations. They contribute to the taste and texture offermented products and inhibit food spoilage bacteria by producing growth-inhibiting substances and large amounts of lactic acid.

  • As agents of fermentation LAB are involved in making curd, yogurt, cheese, cultured butter, sour cream, sausage, cucumber pickles, olives and sauerkraut, but some species may spoil beer, wine and processed meats.
  • Application of lactic acid bacteria
    • It plays an important role in curdling of milk. Under suitable condition, these bacteria multiply and produce acids (lactic acid) which coagulate and partially digest the milk proteins and change the milk into curd.
    • It also increases the content of vitamin B12 in curd.
    • They are also found in the human stomach where these bacteria prevent the growth of certain disease-causing microbes.


  • Because LAB obtains energy only from the metabolism of sugars, they are restricted to environments in which sugars are present.
  • Curd is a good source of vitamin B12. Milk is also source of vitamin B12 but while formation of curd its concentration increases due the fact that lactobacilli synthesise vitamin B12.

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

Sol. Traditional Indian foods made by microbes are as follows:

  • Wheat: Bread, Bhatura, Jalebi, Cake etc.
  • Rice: Idli, Dosa, Nan etc.
  • Bengal gram: Dhokla, Khandavi etc.

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

Sol. Microbes are microscopic organism which include bacteria, protozoa, some fungi and algae. Some microbes play an important role in our daily lives by keeping us healthy while some are disease-causing microbes (called pathogens) and can make humans, animals and plants sick by causing infection and disease.

Still microbes play an important role in controlling diseases (caused by harmful bacteria) by producing antibiotics. Antibiotics is a drug produced by one microorganisms that selectively inhibits the growth of another.

Antibiotics either kill or retard the growth of disease-causing microbes. They destroy bacteria by weakening their cell walls as a result of which certain immune cells such as the white blood cells enter the bacterial cell and cause cell lysis. These medicines are commonly obtained from bacteria and fungi.

Examples of antibiotics are streptomycin, tetracycline, and penicillin. So, antibiotics are being used in controlling a number of bacterial diseases like cholera, pneumonia, typhoid, syphilis, whooping cough, diphtheria etc.

Note: Antibiotics cannot control infections caused by viruses, such as colds, flu, most coughs and bronchitis, sore throats.

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

Sol. Antibiotics are medicines that are produced by certain micro-organisms (bacteria and fungi) to kill other disease-causing micro-organisms. The species of fungus used in the production of antibiotics are given in the table below

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


  • Sewage is the municipal waste water containing human excreta which flows through the sewers. It includes both liquid and solid wastes, rich in organic matter and microbes.
  • Sewage can be harmful to us in the following ways:
    • As it contains a large amount of biodegradable organic matter which when decomposed by naturally occurring microbes in the presence of oxygen, then there is sharp decline in dissolved oxygen (DO). This causes suffocation of large number of aquatic animals like prawns, fishes, etc. Such waters become unfit for human consumption.
    • It is a major cause of polluting drinking water. Hence, it is essential that sewage water is properly collected, treated, and disposed.
    • It decreases flora and fauna of the water.
    • Sewage contains pathogenic micro-organisms which are responsible for causing diseases like cholera, typhoid, dysentery, hepatitis, etc

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

Sol. Sewage treatment is the process of removing contaminants from municipal wastewater containing mainly household sewage plus some industrial wastewater. There are three stages of sewage treatment: primary, secondary, and tertiary.

Key differences between primary and secondary sewage treatment are as follows:

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


  • Yes, microbes can be used as a source of energy in the form of biogas (or gobar gas). Microbes can be used extensively to produce biogas/gobar gas from animal dung (gobar) that is used as fuel in rural areas. The major component of biogas is methane (about 50-68%) which is highly inflammable. The other gases are hydrogen (25-35%), nitrogen (1-5%), oxygen (2-7%) and (traces) (0—0.1%).
  • Process of biogas generation: Bacteria such as methano bacterium are used for the generation of gobar gas or biogas. Biogas is produced from degradable organic matter by the activity of various anaerobic microorganisms present in cow dung. The dung is mixed with water to form the slurry and thrown into the tank. The digester of the tank is filled with numerous anaerobic methane-producing bacteria, which produce biogas from the slurry. Biogas can be removed through the pipe which is then used as a source of energy, while the consumed slurry is removed from the outlet and is used as a fertilizer.
  • Biogas produced is used as fuel for heating and cooking, lighting, power for irrigation, and other purposes as an alternative to firewood, kerosene, dung cakes, or even electricity and LPG. It is considered an eco-friendly and pollution-free source of energy.

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


  • Microbes play an important role in organic farming. Organic farming is a farming method that involves growing and nurturing crops without the use of chemical fertilisers and pesticides. It can be done by using biofertilisers.
  • Bio-fertilisers include those organisms that enrich the nutrient quality of the soil and help in improving plant growth through the supply of plant nutrients. The main sources of biofertilisers are bacteria, fungi and cyanobacteria.
  • Many species of bacteria and cyanobacteria have the ability to fix free atmospheric nitrogen.
  • Microbes used as biofertilisers are-
    • Azospirillum, Rhizobium andAzotobacter: They can fix atmospheric nitrogen in the soil.
    • Blue green algae like Anabaena, Nostoc and Oscillatoria : They add organic matter to the soil and responsible to increase its fertility.
  • They can provide plant nutrients at cheaper cost when compared with the chemical fertilisers.
  • Biofertilisers can also act as bio-pesticides to control insect pests in plants, e.g.
    • Bacillus thuringiensis, which produces a toxin that kills the insect pests.
    • Trichoderma, free living fungi, live in the roots of higher plants and protect them from various pathogens.
    • Baculovirus is another bio-pesticide that is used as a biological control agent against insects and other arthropods.

Question 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?

Hint: The greater the BOD of waste water more is its polluting potential.

Sol. Biochemical Oxygen Demand (BOD), as an important water quality parameter, is the amount of dissolved oxygen needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. BOD values indicate the amounts of organic wastes present in the matter.

On the basis of the given information regarding the sample (A, Band C) and their BOD value as given in the table below, it can be concluded that water of sample C is most polluted and water of sample B is relatively clean.

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


  • Cyclosporin A
    • It is produced by fungus Trichoderma polysporum.
    • It was the first immunosuppressant that acted selectively to suppress T-cell immunity.
  • Statins
    • It is a blood cholesterol lowering agents which are produced by yeast Monascus purpureus.
    • It helps lower the level of low-density lipoprotein (LDL) cholesterol in the blood. It may also help to reabsorb cholesterol that has built up in plaques on the artery walls and thereby preventing further blockage in the blood vessels and heart attacks.

Question 13. Find out the role of microbes in the following and discuss it with your teacher.

  • Single cell protein (SCP)
  • Soil

Sol. (a)  Single cell protein (SCP)

  • Single cell protein refers to protein obtained from cells of certain microbes like yeast, fungi, algae and bacteria which are grown on various carbon sources for synthesis.
  • It contains about 45-55% proteins and considered to be an ideal supplement to conventional human food supply and animal feeds.
  • It can be used to solve the problem of protein deficiency in the children of developing countries.
  • Most commonly employed bacterial strains to produce SCP are Methylophillus, Paecilomyces, Chlorella, Spirullina, etc.

(b) Soil

  • Soil contains a number of useful as well as harmful microbes.
  • Microbes play an important role in maintaining soil fertility and help in formation of nutrient-rich humus by decomposition.
  • Many species of bacteria and cyanobacteria have the ability to fix atmospheric nitrogen into usable form. The economically useful bacteria included in the soil are:
    • Ammonifying bacteria: These bacteria decompose the proteins of dead organisms and nitrogenous wastes of animals to ammonia, e.g., Bacillus ramous and B. vulgari.
    • Nitrifying bacteria: They oxidise ammonia to nitrites, e.g. Nitrosomonas, Nitrobacter, etc.
    • Nitrates fixing bacteria: They fix the atmospheric nitrogen as ammonia by the reduction process, e.g., Rhizobium, Azotobacter, Clostridium, etc.

Question 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

Sol. The order of arrangement of products according to their decreasing importance is:

Penicillin > Biogas > Curd> Citric acid.

It is so because of the following roles :

  • Penicillin: It is commonly called as wonder drug. It is a wide-spectrum antibiotic and used to control a number of human diseases like syphilis, gonorrhoea, diphtheria, lung infections, etc. Hence, it is good for health of society.
  • Biagas: It is produced from dung of cattle by the methanogens and considered as an alternative source of energy especially in the rural areas. It is an eco-friendly source of energy and its production also provides slurry which can be used as a manure to increase the soil fertility and crop yield. It is a renewable source and easily maintained and dissipated for community purpose.
  • Curd: It is a milk product, commonly used in almost all the houses. It has only nutritious value. It is obtained by the action of Lactobacillus bacteria on milk.
  • Citric acid: It is least important for society but can be used as a food preservative and as an excipient in pharmaceutical preparations due to its antioxidant properties.

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

Sol. Bio-fertilisers are cost effective and eco-friendly. They are living organisms which help in increasing the fertility of soil as follows:

  1. It includes the selection of useful micro-organisms that help in improving the plant growth through the supply of plant nutrients.
  2. These are introduced to seeds, roots, or soil to mobilise the availability of nutrients by their biological activity. Thus, they are extremely beneficial in enriching the soil with organic nutrients.
  3. Many species of bacteria and cyanobacteria have the ability to fix free atmospheric nitrogen. For examples,
    • Rhizobium: It is a symbiotic bacterium found in the root nodules of leguminous plants.
    • Azospirillium and Azotobocter are free living nitrogen-fixing bacteria, whereas Anabaena, Nostoc, and Oscillatoria are examples of nitrogen-fixing cyanobacteria.

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