NCERT Solutions for Class 8 Science Chapter 2 Microorganisms Friend and Foe सूक्ष्मजीव: मित्र एवं शत्रु

 NCERT Solutions for Class 8 Science Chapter 2 Microorganisms Friend and Foe सूक्ष्मजीव: मित्र एवं शत्रु

Exercise

Question 1:
Fill in the blanks:
(a) Microorganisms cab be seen with the help of a ________.
(b) Blue green algae fix ______ directly from air to enhance ferrtility of soil.
(c) Alcohol is produced with the help of ________.
(d) Cholera is caused by ________.
Answer
(a) microsope (b) nitrogen (c) yeast (d) bacteria.

Question 2:

Tick the correct answer:

(a) Yeast is used in the production of
(i) sugar
(ii) alcohol
(iii) hydrochloric acid
(iv) oxygen
Answer (ii) alcohol

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast
Answer (ii) Streptomycin

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

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

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

Question 3:

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

Column I		Column II
(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

Answer

(i) - (e); (ii) - (a); (iii) - (b); (iv) - (c); (v) - (d); (vi) - (f);

Question 4:
Can microorganisms be seen with the naked eye? If not, how can they be seen?
Answer
Microorganisms are too small so they cannot be seen with naked eye. They can be seen with the help of a magnifying glass or microscope.

Question 5:
What are the major groups of microorganisms?
Answer
There are five major groups of micro-organisms:
(1) Bacteria 
(2) Fungi 
(3) Protozoa 
(4) Virus 
(5) Algae 

Question 6:
Name the microorganisms which can fix atmospheric nitrogen in the soil.
Answer
Rhizobium 

Question 7:
Write 10 lines on the usefulness of microorganisms in our lives.
Answer
The uses of microorganisms are listed below:
(1) They are helpful in cleaning up the environment. 
(2) Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.
(3) Bacteria are also involved in the making of cheese, pickles and many other food items.
(4) Bacteria and yeast are also helpful for the fermentation of rice idlis and dosa batter.
(5) Yeast reproduces rapidly and produces carbon dioxide during respiration. 
(6) Microorganisms are used for the large-scale production of alcohol, wine and acetic acid (vinegar).
(7) Antibiotics are produced from bacteria and fungi.
(8) Microorganisms are helpful in producing vaccines against disease-causing agents.
(9) Some bacteria are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility.
(10) Microorganisms decompose dead organic waste of plants and animals to convert them into simple substances. 

Question 8:
Write a short paragraph on the harms caused by microorganisms.
Answer
Microorganisms cause diseases in plants and animals. For example, Cholera in humans is caused by bacteria called Vibrio cholera. In animals, the virus causes foot and mouth diseases. Microorganisms spoil food by decomposing the food material. Some microorganisms spoil leather and clothes. Microorganisms are also responsible for food poisoning.

Question 9:
What are antibiotics? What precautions must be taken while taking antibiotics?
Answer
Antibiotics are medicines produced by certain micro-organisms to kill other disease-causing micro-organisms.
Antibiotics should be taken only on the advice of a qualified doctor. Also, one must complete the course prescribed by the doctor.

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