Class 8 Science Chapter 2 Microorganisms: Friend and Foe | Textbook Questions and Answers

1. Fill in the blanks.

(a) Microorganism can be seen with the help of a …………………………..

b) Blue green algae fix ……………………… directly from air and enhance fertility of soil.

c) Alcohol is produced with the help of …………………………..

d) Cholera is caused by ……………………..


a) microscope

b) nitrogen

c) microorganisms

d) bacteria

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 diseases 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) alcohol; (b) streptomycin; (c) female Anopheles mosquito; (d) housefly; (e) growth of yeast cells; (f) fermentation

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

(i) Bacteria                      (a) fixing nitrogen

(ii) Rhizobium                 (b) setting of curd

(iii) Lactobacillus           (c) baking of bread

(iv) Yeast                         (d) causing malaria

(v) A protozoan             (e) causing cholera

(vi) A virus                      (f) causing AIDS

                                           (g) producing antibodies


(i) Bacteria – causing cholera

(ii) Rhizobium – fixing nitrogen

(iii) Lactobacillus – setting of curd

(iv) Yeast – baking of bread

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(v) A protozoan – causing malaria

(vi) A virus – causing AIDS

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

No. Microorganisms cannot be seen with the naked eye. Some of these such as fungus can be seen with a magnifying glass. Others can only be seen with the help of a microscope.

5. What are the major groups of microorganisms?

The major groups of microorganisms are bacteria, fungi, protozoa and algae.

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

The bacterium Rhizobium can fix atmospheric nitrogen. This bacterium lives in the root nodules of leguminous plants like peas and beans. Blue green algae can also fix atmospheric nitrogen.

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

Microorganisms benefit us in several ways. They are used in the making of curd, bread and wine. Microorganisms help to clean up the environment by causing dead and decaying organic matter to decompose. Thus, they also increase the fertility of the soil. In addition, some bacteria and blue green algae are capable of fixing atmospheric nitrogen and making it available to plants. Microorganisms have medicinal use as well. Antibiotics which are used to kill harmful bacteria are produced from bacteria and fungi.

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

Microorganisms are harmful in many ways. Some of them cause diseases in humans, animals and plants. Some microorganisms spoil food and clothing. Food poisoning is another harmful effect of microbes. If people eat food spoiled by bacteria or fungi, they will fall ill. The growth of microbes in food substances also reduce their shelf life.

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

These are medicines used to kill or stop the growth of disease causing bacteria. Antibiotics are produced from bacteria and fungi.

Antibiotics are used to cure diseases caused by bacteria. Antibiotics should be taken only on the advice of a qualified doctor. Also, their course must be completed. If antibiotics are taken in the wrong doses or when they are not needed, it will lead to a condition called drug resistance. This makes the antibiotic less effective against bacteria the next time you take it. Antibiotics may also kill friendly bacteria in the body. For these reasons it is important to use antibiotics only when it is absolutely necessary.

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