Chapter 2 Microorganisms: Friend and Foe

By | May 29, 2018
Class 8 | Science | Ch 2 | Microorganisms: Friend and Foe | NCERT Solutions

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 __________.

Answer: (a) Microscope (b) Nitrogen (c) Yeast (d) Bacteria

2. Tick the correct answer.
(a) Yeast is used in the production of
(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

Answer: (ii) alcohol

(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 cells

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

Answer: (iii) fermentation

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


Column A                     Column 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

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


No, micro-organism are too small to be seen with naked eyes.

They can seen only with the help of microscope or magnifying glass.

5. What are the major groups of microorganisms?


Micro-organism are classified into 5 major groups:







They are very small,single-celled micro-organism which have cell wall but do not have an organises nucleus and other structures.

For Ex:Lactobacillus,Rhizobium.


They are the smallest micro-organism which can develop only inside the cells of the host organism .Viruses can reproduce and multiply only inside the cells of other organism .They are the agents of diseases.

The diseases caused by virus are common cold, influenza,measles,polio,chicken pox,small pox etc.

For Ex:HIV(Human Immuno deficiency) virus causes AIDS, Common cold virus.


They are a group of single-celled micro-organism which are classified as animals.

For Ex:Amoeba,paramecium,Entamoeba,plasmodium etc.

Diseases like dysentery and malaria are caused by protozoa.

Plasmodium is a protozoan which cause a disease called malaria .


Algae is a large group of simple,plant like organism.They contain chlorophyll and produce food by photosynthesis.

For Ex:Chlamydomonas,Spirogyra,blue-green algae.


They are a large group of organism which do not have chlorophyll and do not photosynthesise.

For Ex:Yeast,mould, penicillium,aspergillus etc.

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

Answer: Some of the micro-organism present in the soil can fix nitrogen gas from the atmosphere to form nitrogen compounds.These nitrogen compounds mix with the soil and increase the fertility of soil.

Some bacteria like rhizobium and blue green algae are able to fix nitrogen gas from the atmosphere to enrich the soil with nitrogen compounds and increase its fertility.

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


They are utilised in the making of curd, bread and cake.They are used in the production of alcohol, wine and  acetic acid.They are used in the preparation of medicines called antibiotics.Antibiotics are the chemicals produced by micro-organism to kill bacteria.some of the micro-organism present in the soil can fix nitrogen gas from the atmosphere to form nitrogen compounds.They are used in agriculture to increase the fertility of soil.Micro-organism are used to reduce pollution.They clean up the environment by decomposing the organic matter of dead plants, dead animals and animal waste into harmless and simple substances.Certain micro-organism are used in the biological treatment of sewage and effluents.They are used in the production of industrial chemical products such as many enzymes , drugs, vitamins etc.

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


Those micro-organism which cause diseases are called pathogens.They can be bacteria, fungi, virus or protozoa.Micro-organism cause diseases such as cholera, typhoid, tuberculosis, malaria, smallpox, chickenpox etc.Some micro-organism spoil food, clothing and leather objects.Some of the examples of diseases caused in animals by micro-organism are Foot and mouth disease,Anthrax and Aspergillosis.Foot and mouth disease of animal is caused by a virus.The cattle suffering from this disease get blisters on feet and mouth.Anthrax is a dangerous disease of animals which is caused by a bacterium.The causative micro-organism is bacterium(Bacillus anthracis).Aspergillosis is a disease of animals(like poultry)which is caused by a fungus.Some of the common plant diseases caused by micro-organism are:Rust of wheat, citrus canker, yellow vein mosaic of bhindi. The plant disease called rust of wheat is caused by fungi.It is transmitted through air and seeds.It occurs in wheat plant.The plant disease called citrus canker is caused by bacteria.It is transmitted through air.It occurs in citrus trees such as lemon, lime, orange etc.The plant disease called yellow vein mosaic of bhindi is caused by a virus.It is transmitted through insects.The disease of plants caused by micro-organism reduce the yield and quality of various crops.The plant disease can be controlled by the use of certain chemicals which kill the disease-causing micro-organism.

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


A medicine which stops the growth of,or kill the disease causing micro-organism is called antibiotics.

The source of antibiotic medicine is micro-organism.The antibiotic are manufactured by growing specific micro-organism.

For Ex: Penicillin, Streptomycin, Erythromycin, tetracycline etc.

Some of the precautions to be taken while taking an antibiotics are : 

1)They should be taken only on the advice of a qualified doctor.

2)They should be taken in proper dose.

3)A person must complete the full course of antibiotics prescribed by the doctor.

4)They should not be taken unnecessarily.

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