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Class - 12 : Biology

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Biology - Chapter : 10

Microbes in Human Welfare

class 12 textbook biology

NCERT Solution for Class 12 Science Biology - Page/Excercise 189

Question : 1 :
Bacteria cannot be seen with 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?

Answer :
Curd can be the best sample because it contains millions of lactic acid bacteria (LAB) as their growth rate is very high at optimum temperature which can be seen under the microscope.

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

Answer :
The organisms (lactic acid bacteria, Propionibacterium and Saccharomyces) related to fermentation of dough, cheese and making and production of beverages produce carbon dioxide.
The bacterium Methanobacterium found in anaerobic sludge produces methane along with CO2 and H2

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

Answer :
Lactic acid bacteria are found in curd and yoghurt.
This bacterium increases the nutritional quality of curd by increasing the content of Vitamin B12.
These bacteria play a beneficial role in checking disease-causing microbes in our stomach.

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

Answer :

Traditional Indian Foods Microbes Used
i.Dosa and idli (from rice and Bengal gram) Fermented by bacteria
ii.Bread (wheat) Baker's yeast (Saccharomyces cerevisiae)
iii.Vada (Bengal gram) Fermented by bacteria

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

Answer :
Microbes produce several antibiotics which are used to treat deadly diseases such as whooping cough, diphtheria and leprosy.
They also produce antisera (or sera which contain antibodies) or antitoxins against a particular pathogen. Antisera are widely used against the diseases diphtheria and lock jaw tetanus.
iii.Microbes produce antibodies containing antitoxins and opsonins which prevent the occurrence of diseases such as small pox, cholera and typhoid.

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

Answer :
i.Penicillium notatum
ii.Trichoderma polysporum

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

Answer :
The municipal waste water with a large amount of human excreta and other organic wastes is called sewage.
Harmful effects: Sewage promotes the growth of phytoplankton. Their excessive growth depletes the oxygen in water. This reduction of oxygen and the presence of poisonous wastes affect the fish population and are unsafe for human consumption. Sewage water contains germs, detergents and other decomposing substances and excreta. These cause severe diseases such as typhoid, cholera, dysentery and jaundice in human population.

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

Answer :
Primary treatment involves the removal of large-sized floating and suspended solids by physical methods, while secondary treatment involves decomposition of organic matter by microbial action which produces methane, hydrogen sulphide and carbon dioxide.

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

Answer :
Yes. Microbes can be used as a source of energy.
The microbes present in activated sludge are anaerobically digested to generate inflammable biogas or gobar gas which is used as a source of energy in rural areas. The biogas thus produced is used for cooking and lighting.

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

Answer :
By the use of biofertilisers and biological pest control methods, the use of chemical fertilisers and pesticides can be reduced. The fertility of the soil depends not only on its chemical composition but also on the presence of useful microbes in it, which enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. The biofertilisers do not pollute the environment and are specific in their action. These days, several biofertilisers are available in the market, and farmers use these regularly in their fields to replenish soil nutrients and to reduce dependence on chemical fertilisers. Similarly, biological pest control methods are being adopted to avoid the use of pesticides.
Biological pest control methods are cheap, specific in their action and these are preferred by farmers. For example, ladybirds are useful to get rid of aphids, the pest of vegetables and fruits.

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 20 mg/L, 8 mg/L and 400 mg/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?

Answer :
The BOD values of the three samples A, Band C were recorded as 20 mg/L, 8 mg/L and 400 mg/L.
Greater the BOD of waste water, more is its polluting potential. So, sample C is more polluting as compared to sample A.
Hence, sample A is the secondary effluent, sample B is river water and sample C is untreated sewage water.

NCERT Solution for Class 12 Science Biology Chapter 10 - Microbes in Human Welfare Page/Excercise 190

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

Answer :
(a) Cyclosporin A (an immunosuppressive drug) is produced by the fungus Trichoderma polysporum.
(b) Statins (blood cholesterol-lowering agents) are produced by the yeast Monascus purpureus.

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

Answer :
(a) SCP: It is protein-rich microbial biomass which can be used as food and feed. It has low fat content. SCP can be produced using algae, fungi, yeast and bacteria.
Examples of common SCP are Spirulina, Yeast and Fusarium graminearum.
(b) Soil: Microbes play an important role in decomposition of organic matter and thus cause mineralisation of soil. They are used as biofertilisers and biopesticides.

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.

Answer :
According to their importance, they are arranged as
Biogas ⇒ Penicillin ⇒ Curd ⇒ Citric acid
Biogas is the most important because it reduces pollution as well as it is used as a fuel. Penicillin is an antibiotic which is used to treat deadly diseases. Curd is a household product and citric acid is a chemical, and they are used as per the requirement of an individual.

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

Answer :
1 Biofertilisers are microorganisms which bring about nutrient enrichment of soil.
They increase the soil fertility as
i.Nitrogen fixing organisms: These are certain bacteria and cyanobacteria which are capable of converting gaseous nitrogen into salts of nitrogen.
ii.Mycorrhiza: It is an association between a fungus and the roots of plants. Mycorrhiza is able to absorb inorganic nutrients from organic matter.
iii. Free-living bacteria: Free-living bacteria such as Azotobacter and Bacillus polymyxa have a capability to fix atmospheric nitrogen and make it available to crops such as cereals, millets and fruits. When Azotobacter is grown along with cotton, rice and maize, it results in increase in their yields.

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