NCERT Solutions for Class 8 Science Chapter 2 : Microorganisms : Friend and Foe


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


The microorganisms that fix atmospheric nitrogen in the soil are Rhizobium and blue-green algae. These microorganisms are commonly called biological nitrogen fixers that fix atmospheric nitrogen to enrich soil with nitrogen and increase the fertility of soil.


What are the major groups of microorganisms?


Microorganisms are classified into four major groups. The four major groups of microorganisms are:

  1. Bacteria
  2. Fungi
  3. Protozoa
  4. Algae


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


No, microorganisms cannot be seen with the naked eye. They are too small to be seen with naked eye. They can be seen through magnifying lens and microscope. Microorganisms, such as protozoans, present in water can be seen under a microscope. Fungus, that grows on bread, can be seen with the help of a magnifying lens.


Fill in the blanks.
(a) Microorganisms 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) yeast
(d) bacteria


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


Antibiotics are the medicines that are produced by the microorganisms to kill the disease causing microorganisms. The microorganisms that are used to prepare the antibiotics are bacteria and fungi.

The precautions that are to be taken while taking antibiotics are:

  1. It should be taken under a medical supervision.
  2. A complete course of antibiotics should be taken as prescribed by the physician.
  3. A right amount of antibiotics should be taken at right time as prescribed.
  4. Intake of wrong dose would make the antibiotic less effective.
  5. If not required, antibiotics should not be consumed as it may make body resistant, when it is needed in future and kill the beneficial bacteria present in the body.


Write a short paragraph on the harmful effects of microorganisms.


Microorganisms cause harm in many ways. Some of the harmful effects of microorganisms include causing diseases in humans, animals and plants and spoiling of food, clothes and leather. The disease causing microorganisms are called pathogens. Some bacterial microorganisms cause tuberculosis, typhoid, cholera, etc. in human beings. Diseases, like measles, chickenpox, polio, hepatitis, etc., are caused by viruses in human beings. In cattle, anthrax is caused by bacteria and foot and mouth disease is caused by fungi. In plants, disease like citrus canker, rust of wheat and yellow vein mosaic of bhindi are caused by bacteria, fungi and virus, respectively and reduces the yield of crops. Some microorganisms, like bacteria and fungi, spoil food by producing toxic substances. When these microorganisms are ingested by human beings along with the spoiled food, food poisoning is caused.


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

(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


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) (ii) alcohol
(b) (ii) Streptomycin
(c) (i) female Anopheles mosquito
(d) (ii) housefly
(e) (iii) growth of yeast cells
(f) (iii) fermentation


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


Following are some functions of microorganisms that are important to us:

  1. At home, microorganism like yeast is used in the preparation of bread, cake, idli, dosa, etc.
  2. In baking industry, microorganism like yeast is used in making breads, pastries and cakes.
  3. A bacterium, Lactobacillus, helps in the setting of curd from milk.
  4. Microorganism like yeast is used in the production of alcohols and wine on large scale.
  5. In agriculture, microorganisms like Rhizobium and blue green algae help in the fixation of atmospheric nitrogen to enrich the fertility of soil.
  6. In medicine, microorganisms such as bacteria and fungi play an important role in the preparation of antibiotics.
  7. Microorganisms, such as certain bacteria, help in cleaning the environment by decomposing organic wastes into harmless and usable substances.
  8. They are also used in the preparation of vaccines to protect humans and other animals from various diseases.
  9. Certain microorganisms play an important role in the sewage treatment plant.
  10. Antibiotics prepared from microorganisms are also used to prevent microbial infection in the animals of livestock and poultry and to control various diseases in plants.
Start Your
7-Days Free Trial
and experience it yourself
Start Trial
Play StoreApp Store
Flying Girl
A subscription
that’s worth your
Get the best classroom experience on
India’s leading learning platform

in touch

Have a query about our services?
Get in touch with us and we will
guide you through a new world of
redefined learning