NCERT Solutions Class 8 Science Microorganisms Friends and Foe

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NCERT Solutions for Class 8 Science for chapter 2 Microorganisms Friends and Foe


Question-1 Fill in the blanks:

a) Microorganisms can be seen with the help of a _____
b) Blue-green algae fix ____ directly from air to enhance fertility of soil.
c) Alcohol is produced with the help of ____.
d) Cholera is caused by ____


a) Microorganisms can be seen with the help of a microscope.

b)Blue-green algae fix nitrogen directly from air to enhance fertility of soil.

c) Alcohol is produced with the help of yeast.

d)Cholera is caused by bacteria.

Question-2 Tick the correct answer:

a) Yeast is used in the production of
i. sugar
ii. alcohol
iii. hydrochloric acid
iv. oxygen

b) The following is an antibiotic
i. sodium bicarbonate
ii. streptomycin
iii. alcohol
iv. yeast

c) Carrier of malaria causing protozoan is
i. female Anopheles mosquito
ii. cockroach
iii. housefly
iv. butterfly

d) The most common carrier of communicable disease is
i. ant
ii. housefly
iii. dragonfly
iv. spider

e) The bread or idli dough rises because of
i. heat
ii. grinding
iii. growth of yeast cells
iv. kneading

f) The process of conversion of sugar into alcohol is called
i. nitrogen fixation
ii. moulding
iii. fermentation
iv. infection


a) (ii) alcohol
b) (ii) Streptomycin
c) (i) female Anopheles mosquito
d) (Ii) housefly
e) (iii) growth of yeast cells
f) (iii) fermentation

Question-3 Match the organisms in Column I with their action I Column II.

Column I

Column II


Fixing Nitrogen


Setting of curd


Baking of bread


Causing Malaria

A protozoan

Causing Cholera

A virus

Causing AIDS


Producing antibodies


Column I

Column II


Causing Cholera


Fixing Nitrogen


Setting of curd



Baking of bread

A protozoan

Causing Malaria

A virus

Causing AIDS

Question-4 Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer: No, microorganisms cannot be seen with the naked eye. They are too small to be seen with naked eye. They can be seen through magnifying lens and microscope. Microorganisms such as protozoans present in water can be seen under a microscope. Fungus that grows on bread can be seen with the help of a magnifying lens.

Question-5 What are the major groups of microorganisms?

Answer: Microorganisms are classified into four major groups. The four major groups of microorganisms are:

1. Bacteria
2. Fungi
3. Protozoa
4. Algae

Question-6 Name the microorganisms which fix atmospheric nitrogen in the soil.

Answer: The microorganisms that fix atmospheric nitrogen in the soil are Rhizobium and blue-green algae. These microorganisms are commonly called biological nitrogen fixers that fix atmospheric nitrogen to enrich soil with nitrogen and increase the fertility of soil.

Question-7 Write 10 lines on the usefulness of microorganisms in our lives.

Answer: Following are some functions of microorganisms that are important to us:

1. At home, microorganism like yeast is used in the preparation of bread, cake, idli, dosa, etc.
2.In baking industry, microorganism like yeast is used in making breads, pastries and cakes.
3. A bacterium Lactobacillus helps in the setting of curd from milk.
4. Microorganism like yeast is used in the production of alcohols and wine on large scale.
5. In agriculture, microorganisms like Rhizobium and blue green algae help in the fixation of atmospheric nitrogen to enrich the fertility of soil.
6. In medicine, microorganisms such as bacteria and fungi play an important role in the preparation of antibiotics.
7. Microorganisms such as certain bacteria help in cleaning the environment by decomposing organic wastes into harmless and usable substances.
8. They are also used in the preparation of vaccines to protect humans and other animals from various diseases.
9. Certain microorganisms play an important role in sewage treatment plant.
10. Antibiotics prepared from microorganisms are also used to prevent microbial infection in the animals of livestock and poultry and to control various diseases in plants.

Question-8 Write a short paragraph on the harms caused by microorganisms.

Answer: Microorganisms cause harm in many ways. Some of the harmful effects of microorganisms include causing diseases in humans, animals and plants and spoiling of food, clothes and leather. The disease causing microorganisms are called pathogens. Some bacterial microorganisms cause tuberculosis, typhoid, cholera, etc in human beings. Diseases like measles, chickenpox, polio, hepatitis, etc are caused by viruses in human beings. In cattle, anthrax is caused by virus and foot and mouth disease is caused by fungi. In plants, disease like citrus canker, rust of wheat and yellow vein mosaic of bhindi are caused by bacteria, fungi and viruses respectively and reduces the yield of crops. Some microorganisms like bacteria and fungi spoil food by producing toxic substances. When these microorganisms are ingested by human beings along with the spoiled food cause food poisoning.

Question-9 What are antibiotics? What precautions must be taken while taking antibiotics?

Answer: Antibiotics are the medicines that are produced by the microorganisms to kill the disease causing microorganisms. The microorganisms that are used to prepare the antibiotics are bacteria and fungi. The precautions that are to be taken while taking antibiotics are:

1. It should be taken under a medical supervisio
2. A complete course of antibiotics should be taken as prescribed by the physicia
3. A right amount of antibiotics should be taken at right time as prescribed.
4. Intake of wrong dose would make the antibiotic less effective.
5. If not required, antibiotics should not be consumed as it may make body resistant, when it is needed in future and kill the beneficial bacteria present in the body.


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