NCERT Solutions for Class 8 Science Chapter 2

NCERT Solutions for Class 8 Science Chapter 2

NCERT Solutions for Class 8 Science Chapter 2

Chapter 2 Microorganisms: Friend and Foe

Exercise Questions

1. 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

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

b) The following is an antibiotic:
(i) sodium bicarbonate
(ii) streptomycin
(iii) alcohol
(iv) yeast
(ii) streptomycin

(c) Carrier of malaria-causing protozoan is:
(i) female Anopheles mosquito
(ii) cockroach
(iii) housefly
(iv) butterfly
(i) female Anopheles mosquito

(d) The most common carrier of communicable diseases is
(i) ant
(ii) housefly
(iii) dragonfly
(iv) spider
(ii) housefly

(e) The bread or idli dough rises because of:
(i) heat
(ii) grinding
(iii) growth of yeast cells
(iv) kneading
(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
(iii) fermentation

Question 4.
Can microorganisms be seen with the naked eye? If not, how can they be seen?
The microorganisms cannot be seen with our naked eyes because they are very small in size.

Question 5.
What are the major groups of microorganisms?
Microorganisms are classified on the basis of their size into four major groups. These groups are:
(a) Bacteria
(b) Fungi
(c) Protozoa
(d) Some algae

Question 6.
Name the microorganisms which can fix atmospheric nitrogen in the soil.
Rhizobium, Clostridium and Azotobacter.

Question 7.
Write 10 lines on the usefulness of microorganisms in our lives.
Microorganisms are useful to us in many ways. For example,

  • Bacteria like Lactobacillus convert milk into curd.
  • Bacteria are also involved in the making of cheese.
  • Yeast is used in the commercial production of alcohol, wine, and bakery products.
  • Some specific microorganisms are helpful in the manufacturing of antibiotics.
  • Microorganisms act as cleansing agents and decompose the waste products into manure.
  • Dead or weakened microbes are used in the preparation of vaccines.
  • Some bacteria fix atmospheric nitrogen and increase soil fertility.
  • Algae, yeast, fungi or bacteria may be used as an ingredient or a substitute for protein-rich foods that are suitable for human or animal consumption.
  • Some microorganisms are taken as probiotics, which are believed to provide health benefits when consumed.

Question 8.
Write a short paragraph on the harms caused by microorganisms.
Microorganisms are harmful to us in many ways.

For example, microorganisms, called pathogens cause disease in humans, plants and animals. Pathogens or germs enter a healthy body through air, water, contaminated food and infected person by direct or indirect contact or by the carrier.

Common ailments like cold, influenza (flu), cough, polio, chicken pox are caused by viruses. Foot and mouth diseases in the cattle are also caused by viruses. Typhoid, tuberculosis (TB) are caused by bacteria. Anthrax a dangerous human and cattle diseases is also caused by bacteria.

Diseases like dysentery and malaria are caused by protozoa.

Ringworm is caused by fungi. Several microbes causes diseases in plants and thus reduces the yield.

Rust of wheat is a fungal disease spread through air.

Microorganisms that grow on our food sometimes produce toxic substances. These make the food poisonous causing serious illness and even death. This food-borne illness is called food poisoning.

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

Ans: Antibiotics are medicines that kill or stop the growth of the disease-causing microbes. They are manufactured by growing specific microorganisms. They are used to cure a variety of diseases.


a) Antibiotics should be taken only when advised by a qualified doctor.

b) A person must complete the full course as prescribed by the doctor.

c) Antibiotics should be taken in proper doses as advised by the doctor.

d) Antibiotics should not be taken unnecessarily as they may kill the useful bacteria in the body.

see also: Microorganisms Friend and Foe Extra Questions and Answers

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