Microrganisms : Friend and Foe (Class - 8) NCERT Science Sollution

Class – 8

Subject – Science

Chapter – 2 : Microorganisms : Friends and Foe

Exercise   (Question and Answer)


Question 1. Fill in the blanks.

(a) Microorganisms can be  seen with the help of a _microscope_.

(b) Blue green algae  fix _nitrogen_ directly from air and 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


Answer - (ii) Yeast


(b) The following is an  antibiotic

(i) Sodium bicarbonate.   (ii) Streptomycin (iii) Alcohol.                        (iv) Yeast


Answer - (ii) Streptomycin



(c) Carrier of malaria-causing protozoan is

(i) female  Anopheles  mosquito (ii) cockroach

(iii) housefly                                  (iv) butterfly


Answer - (i) female Anopheles mosquito


(d) The most  common carrier of communicable diseases is

(i) ant                                 (ii) housefly

(iii) dragonfly                   (iv) spider


Answer - (ii) housefly.



(e) The bread or  idli  dough  rises because of

(i) heat.                                        (ii) grinding

(iii) growth of  yeast cells.      (iv) kneading


Answer - (iii) growth of yeast



(f) The process of conversion of  sugar into alcohol is called

(i) nitrogen fixation.          (ii) moulding 

(iii) fermentation               (iv) infection


Answer - (iii) fermentation



Question 3. Match the organisms in Column  A  with their action in Column  B.


Answer -

Column  A 

Column  B


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 can not be seen with naked eyes. They can be seen with the help of microscope.


Question 5. What are the major groups of  microorganisms?


Answer - The major group of microorganisms are as follows -

1. Viruses

2. Bacteria

3. Algea

4. Fungi

5. Protozoa



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


Answer - The microorganism which can fix atmospheric nitrogen in the soil is Cyanobacteria (blue green algae).



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


Answer - (i) Micro-organisms are used in the making of curd.

(ii) Microorganisms are used in the making of  bread and cake.

(iii) Micro-organisms are used in the production of alcohol.

(iv) Microorgamisms are used in the synthesis of acetic acid (vinegar).

(v) Micro-organisms are used in the preparation of medicines (or drugs) called antibiotics.

(vi) Micro-organisms are used in agriculture to increase the fertility of soil by fixing atmospheric nitrogen gas (to form nitrogen compounds).

(vii) microorganisms are used in the manufacturing of vaccine. ex- corona vaccine.

(viii) Micro-organisms clean up the environment by decomposing the organic matter of dead plants, dead animals and animal wastes into harmless and useful simple substances. 

(ix) Some microorganisms like blue green algea produce oxygen in the process of photosynthesis.

(x) Some Algae and Fungi are used as food. 



Question 8.  Write a short paragraph on the harmful effects of microorganisms.


Answer - Micro-organisms can be harmful in many ways. For example, some of the micro-organisms cause diseases in human beings, other animals and plants. Those micro-organisms which cause diseases are called pathogens. Thus, pathogens are disease-causing micro-organisms. Pathogens can be bacteria, viruses, protozoa or fungi, etc. Micro-organisms cause diseases such as tuberculosis (TB), tetanus, diphtheria, whooping cough, cholera, typhoid, AIDS, food poisoning, malaria, smallpox and chickenpox, etc. Some micro-organisms spoil food, clothing and leather objects. 


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


Answer - A medicine which stops the growth of, or kills the disease-causing micro-organisms is called an antibiotic. The source of antibiotic medicines are micro-organisms. 

Some of the precautions to be observed in the use of antibiotics are as follows :

(i) Antibiotics should be taken only on the advice of a qualified doctor.

(ii) A person must complete the ‘full course’ of antibiotics prescribed by the doctor.

(iii) The antibiotics should be taken in proper doses as advised by the doctor. If a person takes antibiotics in wrong doses (or when not needed), it may make the antibiotics less effective when the person might need it in future.

(iv) Antibiotics should not be taken unnecessarily. Antibiotics taken unnecessarily may kill the useful bacteria in the body and harm us.








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