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Class 12-science NCERT Solutions Biology Chapter 8 - Microbes in Human Welfare

Microbes in Human Welfare Exercise 159

Solution 1

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.

Solution 2

i.The organisms (lactic acid bacteria, Propionibacterium and Saccharomyces) related to fermentation of dough, cheese and making and production of beverages produce carbon dioxide.

ii.The bacterium Methanobacterium found in anaerobic sludge produces methane along with CO2 and H2.

Solution 3

Lactic acid bacteria are found in curd and yoghurt.


i.This bacterium increases the nutritional quality of curd by increasing the content of Vitamin B12.

ii.These bacteria play a beneficial role in checking disease-causing microbes in our stomach.

Solution 4

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

Solution 5

i.Microbes produce several antibiotics which are used to treat deadly diseases such as whooping cough, diphtheria and leprosy.

ii.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.

Solution 6

i.Penicillium notatum

ii.Trichoderma polysporum

Solution 7

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.

Solution 8

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.

Solution 9

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.

Solution 10

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.

Solution 11

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.

Microbes in Human Welfare Exercise 160

Solution 12

(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.

Solution 13

(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.

Solution 14

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.

Solution 15

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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