Class 8 Science


NCERT Solution

Fill in the blanks:

  1. Microorganisms can be seen with the help of a ____________.
  2. Blue green algae fix __________ directly from air to enhance fertility of soil.
  3. Alcohol is produced with the help of __________.
  4. Cholera is caused by __________.

Answer: (a) microscope, (b) nitrogen, (c) yeast, (d) bacteria

Tick the correct answer:

Question 1: Yeast is used in the production of

  1. Sugar
  2. Alcohol
  3. Hydrochloric acid
  4. Oxygen

    Answer: (b) Alcohol

Question 2: The following is an antibiotic

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

    Answer: (b) Streptomycin

Question 3: Carrier of malaria-causing protozoan is

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

    Answer: (a) Female Anopheles mosquito

Question 4: The most common carrier of communicable diseases is

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

    Answer: (b) Housefly

Question 5: The bread or idli dough rises because of

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

    Answer: (c) Growth of yeast cells

Question 6: The process of conversion of sugar into alcohol is called

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

    Answer: (c) Fermentation

Match the organisms in Column I with their action in Column II.

Column IColumn II
(a) Bacteria(1) Fixing nitrogen
(b) Rhizobium(2) Setting of curd
(c) Lactobacillus(3) Baking of bread
(d) Yeast(4) Causing malaria
(e) A protozoan(5) Causing cholera
(f) A virus(6) Causing AIDS
(g) Producing antibodies

Answer: (a) 5, (b) 1, (c) 2, (d) 3, (e) 4, (f) 6

Answer these question:

Question 1: Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer: Such organisms can be seen through microscope.

Question 2: What are the major groups of microorganisms?

Answer: The major groups of microorganisms are: bacteria, algae, fungi and protozoa

Question 3: Name the microorganisms which can fix atmospheric nitrogen in the soil.

Answer: Rhizobium, Blue-green algae

Question 4: Write 10 lines on the usefulness of microorganisms in our lives.

Answer: Usefulness of microorganisms:

Question 5: Write a short paragraph on the harms caused by microorganisms.

Answer: Many microorganisms are harmful for us. Some microbes enter our body and cause diseases. Some of the infectious diseases can be highly debilitating in nature. Some microbes infect cattle and poultry and thus cause heavy loss to us. Some microbes damage the crops. Microbes also spoil our food. For example; fruits, vegetables, meat, fish, milk, etc. can easily get spoiled due to microbes.

Question 6: What are antibiotics? What precautions must be taken while taking antibiotics?

Answer: A substance which kills or stops the growth of bacteria is called antibiotic. Following precautions need to be taken while taking antibiotics: