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

1. Fill in the blanks.

(a) Microorganisms can be seen with the help of a ____________.

Answer: microscope

(b) Blue green algae fix __________ directly from air and enhance fertility of soil.

Answer: nitrogen

(c) Alcohol is produced with the help of __________.

Answer: yeast

(d) Cholera is caused by __________.

Answer: bacteria

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

3. Match the organisms in Column A with their action in Column B.

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:

Column I	Column II
(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

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

Answer: No. Microorganisms are so small that they are invisible to naked eye. To see microorganisms microscope can be used.

5. What are the major groups of microorganisms?

Answer: Microorganisms are divided into bacteria, fungi, protozoa, virus and algae.

• Bacteria – Bacterias are single cell microorganisms that cause disease. They are spiral or rod shaped.
• Fungi – Fungi are multicellular microorganisms that cause disease. Bread mould is well known example of fungi.
• Protozoa – Protozoa are single celled or multi celled organisms. Some examples of protozoa are plasmodium, amoeba, etc.
• Virus – These microorganisms reproduces inside host organism.
• Algae – These are multicellular photosynthetic like spirogyra.

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

Answer: Rhizobium and some blue green algae have ability to fix atmospheric nitrogen into the soil.

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

Answer: Benefits of microorganisms –

• Yeast is used in preparation of bread.
• Yeast is used in fermentation process for production of alcohol.
• Curd is formed because of lactobacillus.
• Some microorganisms acts as decomposers which clean environment by decomposing dead and decaying matter.
• Bacteria like rhizobium are helpful to fix atmospheric nitrogen in soil.
• Useful in preparation of medicines. Antibiotics are prepared from bacteria which helps to kill disease causing bacterias.
• Microorganisms are helpful in treatment of sewage.
• Bacterias are used in food processing like baking, wine making, etc.

8. Write a short paragraph on the harmful effects of microorganisms.

Answer: Microorganism can cause harmful diseases. Bacteria cause disease like cholera, tuberculosis, etc. viruses causes foot and mouth diseases. Various microbes attack on plants that affect the productivity of crops. Some microbes enter in body and produce harmful substances, that causes food poisoning.

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

Answer: Antibiotics are the medicines produced from microorganisms which kill other harmful microorganisms and prevents us from diseases. Some antibiotics are – streptomycin, penicillin, etc. Precautions to be taken while taking antibiotics –

• Should be taken on recommendation of medical practitioner.
• Should be taken as and when as prescribed by the doctor.
• Should be taken in right quantity.

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