Chapter-2: Microorganisms Friend and Foe

Q.1. Fill in the blanks:

(i) ___________ is used to see microorganisms.

(ii) ___________ causes cholera.

(iii)___________ is used to produce alcohol.

(iv)___________ is fixed directly from air to enhance the soil fertility by blue green algae.

Ans. (i) Microscope (ii) Bacteria (iii) Yeast (iv) Nitrogen


Q.2. Choose the correct answer:

(i) Production of yeast involves

(a) sugar (b) alcohol (c) oxygen (d) hydrochloric acid


(ii) Which of the following is an antibiotic?

(a) streptomycin (b) Sodium Bicarbonate (c) Alcohol (d) Yeast


(iii) Protozoan causing malaria is caused by

(a) cockroach (b) female Anopheles mosquito (c) butterfly (d) housefly


(iv) Communicable disease are commonly carried by

(a) housefly (b) ant (c) spider (d) dragonfly


(v) The rise of bread and idli dough is because of

(a) kneading (b) grinding (c) heat (d) growth of yeast cells


(vi)The conversion process involving sugar into alcohol is called

(a) molding (b) infection (c) nitrogen fixation (d) fermentation


(i) b. alcohol   (ii) a. streptomycin   (iii) b. female anopheles mosquito   (iv) a. housefly

(v) growth of yeast cell    (vi) d. fermentation


Q.3. Are microorganism visible to the naked eye? If not, how are they seen?


Microorganism cannot be seen through our naked eye. Magnifying glasses or microscopes are used to see them.

Ex- With the help of a microscope, the fungus which grows on bread can be seen.


Q.4. Match the actions with their organisms

                  Table A                        Table B
(a) Bacteria (i)Causes AIDS


(b) A virus (ii)Causes cholera


(c) A protozoan (iii)Bread baking


(d) Rhizobium (iv)Antibodies production


(e)Lactobacillus (v)Nitrogen fixing


(f) Yeast (vi) causes malaria


(vii) curd setting



(a) Bacteria causes cholera

(b) Rhizobium fixes nitrogen

(c) Lactobacillus helps in curd setting

(d) Yeast helps in baking of bread

(e) A protozoan causes malaria

(f) A virus causes AIDS


Q.5. Explain major groups of microorganism.

Ans. It consist of five major groups :

(a) Algae – They  are Spirogyra, Chlamydomonas which are multicellular, photosynthetic organism.


(ii) Virus – They are microbes which causes disease and they reproduce only inside the host.


(iii) Protozoa – They are unicellular or multicellular organisms such as Amoeba, plasmodium.


(iv) Bacteria – They are either spiral or rod shaped. These micro-organisms are disease causing single celled organisms.


Q.6. What is the micro-organism which is used in the fixation of atmospheric nitrogen into the soil?

Ans. Fixation of atmospheric nitrogen and converting them into useful nitrogen compounds is done by bacteria such as blue-green algae and Rhizobium. Plants use these nitrogenous compounds to synthesise the necessary plant proteins and other compounds.


Q.7. How are micro-organisms useful in our lives?


Micro-organisms are not visible to the naked eye but they are necessary to plants and environment .

In baking, pickling, winemaking and other food making process, these micro-organisms are used. In the preparation of wine and bread, alcoholic fermentation by yeast is used. For the curd formation, a bacterium Lactobacillus is used.

Bacteria and fungi helps to decompose dead bodies and excreta which forms inorganic compounds that are absorbed by plants.

Bacteria such as Rhizobium and blue-green algae help to fix atmospheric nitrogen in soil.

In the preparation of medicines, microbes are used. Micro-organisms produce antibiotics to kill bacteria. Streptomycin is an antibiotic.

Treatment of sewage and industrial effluents are done by certain microbes.


Q.8. What are the harmful effects of microorganisms?


Micro-organisms can cause diseases. Tuberculosis, cholera, typhoid are caused by bacteria in humans. In cattle, virus causes foot and mouth disease. Diseases in plants are caused by several microbes. Microbes produce toxic substances when they enter our body and this leads to food poisoning. Food are spoiled by microorganisms like fungi. Under moist conditions, when bread is left unused, it gets spoiled by a fungus causing a white cotton like growth on its surface.


Q.9. Explain about antibiotics. When taking them, what precautions must be taken?


Micro-organisms produce antibodies to eliminate other micro-organisms that cause disease. Bacteria and fungi are used to obtain these medicines. Common antibiotics are Streptomycin, tetracycline, etc.

Precautions when taking antibiotics:

(a) Antibiotics are to be taken only under the surveillance of a well-qualified doctor.

(b) As per the prescription given by the doctor, the antibiotics are to be taken.

(c) Antibiotics are to be taken in right dose at the right time. The drug is ineffective if it is taken in wrong dose and may kill the useful bacteria present in the body if it is taken in excess.

  • Gaurav Mishra

    thanks byju

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