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SCIENCE CLASS-8 

CHAPTER -2 

MICROORGANISMS - FRIEND AND FOE 

EXERCISES

Question number 1

Fill in the blanks.
(a) Microorganisms cam be seen with the help of a __________________.
(b) Blue green algae fix ________________ directly from the air to enhance fertility of soil.
(c) Alcohol is produced with the help of ________________.
(d) Cholera is caused by __________________.

Answer number 1


(a) microscope.
(b) nitrogen.
(c)yeast.
(d)bacteria.

Question number 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 if 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 cell
(iv)kneading

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

(i)nitrogen fixation
(ii)moulding
(iii)fermentation
(iv)infection

Answer number 2

(a) alcohol
(b) streptomycin
(c) female anopheles mosquito
(d) housefly
(e) growth of yeast cell
(f) fermentation

Question number 3


Match the organism in Column A with their action in Column B
             A                                       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

 Answer number 3

          A                                B
(i)Bacteria                   (e)Causing cholera
(ii)Rhizobium             (a)Fixing nitrogen
(iii)Lactobacillus        (b)Setting of curd
(iv)Yeast                       (c)Baking of bread
(v)A protozoan           (d)Causing malaria
(vi)A virus                   (f)Causing AIDS

Question number 4


Can microorganisms be seen with the naked eyes? If not, how can they be seen

Answer number 4


Microorganisms can not be seen by naked eyes, because they are very small. They can be seen with the help of microscope.

Question number 5


What are the major groups of microorganisms?

Answer number 5


Major groups of microorganisms are as follow -
  • Bacteria
  • Fungi
  • Protozoa
  • Algae
  • Viruses

Question number 6


Name the microorganism which can fix atmospheric nitrogen in the soil.

Answer number 6


The name of microorganism which can fix atmospheric nitrogen in the soil is Rhizobium.

Question number 7


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

Answer number 7

  1. They are helpful in cleaning up of the environment. The organic waste (vegetable peels, remains of animals, faeces etc. ) are broken down into harmless and useful substances by bacteria.
  2. In agriculture they are used to increase the soil fertility by fixing nitrogen.
  3. They are used in the preparation of medicines.
  4. Microorganisms are useful in making curd and bread. Lactobacillus promotes the formation of curd. It multiplies in the milk and converts it into curd.
  5. Bacteria are involved in the making cheese, pickle and many other food items.
  6. Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar)
  7. Antibiotics are being produced by bacteria  and fungi.
  8. Microorganisms are helpful in producing vaccines which protect from several diseases such as cholera, tuberculosis, small pox and hepatitis.
  9. Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.
  10. Microorganisms helps in cleaning the environment by decomposing dead organic waste of plants and animals to convert them into simple substances.
Question number 8

Write a short paragraph on the harms caused by microorganisms.

Answer number 8


Microorganisms are harmful in many ways. Microorganisms causes diseases in human beings, plants and animals such microorganisms are called pathogens. Pathogens enters our body through air we breathe, the water we drink or the food we eat. They can spread communicable diseases such as cholera, common cold, chicken pox and tuberculosis.
               Microorganisms spoil food by decomposing the food material. They spoil leather and clothes also. They are also responsible for food poising.

 Question number 9

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

Answer number 9


 Antibiotics hill or stop the growth of the disease causing microorganisms.
Antibiotics should be taken only on the advice of a qualified doctor. Also antibiotic course must be completed prescribed by the doctor. Antibiotics should not be taken unnecessarily because it may hill the beneficial bacteria present in our body.

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