NCERT Solutions Std 8 Science Microorganisms Friend and Foe

Microorganisms: Friend and Foe


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.


2). Tick the correct answer.

(a) Yeast is used in the production of

(i) sugar                                    (ii) alcohol

(iii) hydrochloric acid                 (iv) oxygen

Ans: alcohol

(b) The following is an antibiotic

(i) Sodium bicarbonate              (ii) Streptomycin

(iii) Alcohol                                (iv) Yeast

Ans: Streptomycin

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito    (ii) cockroach

(iii) housefly                              (iv) butterfly

Ans: female Anopheles mosquito

(d) The most common carrier of communicable diseases is

(i) ant                                       (ii) housefly

(iii) dragonfly                             (iv) spider

Ans: housefly

(e) The bread or idli dough rises because of

(i) heat                                      (ii) grinding

(iii) growth of yeast cells            (iv) kneading

Ans: growth of yeast cells

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

(i) nitrogen fixation                    (ii) moulding

(iii) fermentation                       (iv) infection

Ans: fermentation

3). Match the organisms 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



(i) Bacteria → Causing cholera

(ii) Rhizobium→ Fixing nitrogen

(iii) Lactobacillus → Setting of curd

(iv) Yeast → Baking of bread

(v) A protozoan → Causing cholera                

vi) A virus → Causing AIDS

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

Ans: No, microorganisms cannot be seen with the naked eye. We can see them only with the help of a microscope.

5). What are the major groups of microorganisms?

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

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

Ans: Rhizobium

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

Ans: Microorganisms are used for various purposes.

i). Microorganisms are used in the preparation of curd, bread and cake.

ii). They are used in the production of alcohol.

iii). They are also used in cleaning up of the environment.

iv). They are used in the preparation of medicines

v). In agriculture, they are used to increase the fertility of the soil.

vi). They are used in the production of vinegar.

vii) They are also used in food preservatives.

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

Ans: Microorganisms are harmful in many ways. Some of microorganisms cause diseases in human beings, plants and other animals. Such disease-causing microorganisms are called pathogens. Cholera, tuberculosis, common cold, chicken pox are some of the diseases caused by microorganisms. Anthrax is a serious disease caused in humans and cattles by microorganisms. Some microorganisms spoil food, clothing and leather.

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

Ans: The medicines that kill or stop the growth of the disease-causing microorganisms are called as antibiotics. Streptomycin, tetracycline and erythromycin are some of the antibiotics that are made from fungi and bacteria. The anitbiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. They are also used to control many plant diseases.

Additional Questions:

Fill in the blanks.

i) Viruses are also microscopic but are not considered as microorganisms.

ii) Microorganisms are used to increase soil fertility by fixing nitrogen.

iii). Lactobacillus promotes the formation of curd.

Write whether ‘True’ or ‘False’.

i). Dieses like polio and chicken pox are caused by fungus.

Ans: False

ii). Diseases like dysentery and malaria are caused by protozoa.

Ans: True

iii). Typhoid is a bacterial disease.

Ans: True

iv). Microorganisms cannot live in all type of environment.

Ans: False.

v). Microorganisms are used for the large scale production of alcohol.

Ans: True

vi). The atmospheric nitrogen can be directly taken by plants and animals

Ans: False

Answer in one sentence.

1). Name the microorganism used in the production of alcohol.

Ans: Yeast

2). What do you mean by fermentation?

Ans: The process of conversion of sugar into alcohol is known as fermentation.

3). What are pathogens?

Ans: Disease-causing microorganisms are called pathogens.

4). What are communicable diseases?

Ans: Microbial diseases that can be spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases.

5). Write the examples of communicable diseases.

Ans: Diseases like cholera, common cold, chicken pox and tuberculosis are some of the examples of communicable diseases.

6). What are viruses?

Ans: Viruses are microscopic organisms that reproduce only inside the cells of host organisms.

7) What do you mean by nitrogen fixation.

Ans: The process by which atmospheric nitrogen is converted into nitrates by the action of microorganisms is called nitrogen fixation.

Answer in short

1) Write in brief about viruses.

Ans: Viruses are microscopic organisms that reproduce only inside the cell of a host organism, which may be a bacterium, plant or animal. Common ailments like cold, Influenza and most coughs are caused by viruses. Serious diseases like polio and chicken pox are also caused by viruses.

2). Write a note on carriers.

Ans: The insects and animals that help in the transmission of disease-causing microorganisms are called carriers. Housefly in one such carrier. Another example of a carrier is the female Anopheles mosquito which carried the parasite of malaria. Female Aedes mosquito acts as a carrier of the dengue virus.

3). Write a note on pasteurization.

Ans: The milk is heated to about 700C for 15 to 30 seconds and suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process is called pasteurization. This process was discovered by Louis Pasteur. Pasteurised milk can be consumed without boiling as it is free from harmful microbes.

Name the following.

1). Two habitats of microorganisms.

Ans: i). soil                          ii). Water

2). Diseases caused by viruses.

Ans: i). Polio                        ii). Influenza

3). Diseases caused by Protozoa.

Ans: i). Dysentry                 ii). Malaria

4). Diseases caused by bacteria.

Ans: i). Typhoid                   ii). Tuberculosis

5). Multicellular microorganisms..

Ans: i). Algae                       ii). Fungi

6). Two Antibiotics.

Ans: i). Streptomycin           ii). Tetracycline

7). Microorganism promoting the formation of curd.

Ans: i). Bacterium Lactobacillus

8). Carrier of Dengue virus..

Ans: Female Aedes mosquito.

9). Preservatives

Ans: i). Sugar                      ii). Salt  

10). Chemical preservatives.

Ans: i). Sodium benzoate     ii). Sodium metabisulphite




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