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

Question 1:

Microorganisms can be seen with the help of a __________ .



Question 2:

Blue green algae fix __________ directly from air to enhance the fertility of the soil.



Question 3:

Alcohol is produced with the help of __________ .



Question 4:

Cholera is caused by __________ .



Question 5:

Yeast is used in the production of

  1. sugar
  2. alcohol
  3. hydrochloric acid
  4. oxygen


Question 6:

The following is an antibiotic:

  1. Sodium bicarbonate
  2. Streptomycin
  3. Alcohol
  4. Yeast


Question 7:

Carrier of malaria-causing protozoan is

  1. female anopheles mosquito
  2. cockroach
  3. housefly
  4. butterfly

female anopheles mosquito

Question 8:

The most common carrier of communicable diseases is

  1. ant
  2. housefly
  3. dragonfly
  4. spider


Question 9:

The bread or idli dough rises because of

  1. heat
  2. grinding
  3. growth of yeast cells
  4. kneading

growth of yeast cells

Question 10:

The process of conversion of sugar into alcohol is called

  1. nitrogen fixation
  2. moulding
  3. fermentation
  4. infection


Question 11:

Match the organisms in column A with their action in column B.

  1. Bacteria
  2. Rhizobium
  3. Lactobacillus
  4. Yeast

(i), (d), (ii). (a), (iii). (b), (iv). (c)

Question 12:

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


No, microorganisms cannot be seen with naked eyes because they are too small. We can see them under a microscope.

Question 13:

What are the major groups of microorganisms?


The major groups of micro-organisms are bacteria, fungi, protozoa, algae and virus.

Question 14:

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


Rhizobium bacteria and blue green alga can fix atmospheric nitrogen in the soil.

Question 15:

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


The beneficial aspects of microorganisms are as follows:
a) Microorganisms are the decomposers and they help in the recycling of substances.
b) Blue green algae and Rhizobium bacteria convert atmospheric nitrogen into nitrogenous compounds and increase the fertility of the soil.
c) Lactobacillus helps in the formation of milk products like curd and cheese.
d) Yeast helps in the formation of alcohol and bakery products like bread and cakes.
e) Products like idli and dosa are prepared by the action of fungi.
f) Medicines such as penicillin are made from fungi.
g) Mushrooms are edible fungi.
h) Algal cell extract such as agar are used for preparing medicines, food and cosmetics.
i) Kelp, a brown alga, is a rich source of iodine and potassium.
j) Many types of seaweed are used as food in China and Japan.

Question 16:

Write a short paragraph on the harmful effects of microorganisms.


Besides being useful, microorganisms are responsible for causing a large variety of diseases in plants, animals and human beings. They spoil food by breaking down food molecules and making new products called amines, which are generally smelly. This changes the chemical composition, texture and physical appearance of the food. Such food, if consumed, leads to many diseases including food poisoning. In plants, they cause diseases like wheat rust and citrus canker. In animals they cause diseases like foot and mouth disease and anthrax. They spoil articles like clothes, leather, etc.

Question 17:

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


Antibiotics are chemicals that inhibit the growth of microorganisms without harming the host. Precautions to be taken while taking antibiotics are:
a) Antibiotics should be taken only on the advice of a qualified doctor.
b) One must finish the entire course prescribed by the doctor.
c) Antibiotics, however, are not effective against cold and flu as these diseases are caused by viruses.