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NCERT Class 8 Science Chapter 2 Micro Organisms: Friends and Foe
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Micro Organisms: Friends and Foe
Chapter: 2
NCERT TEXT BOOK EXERCISES
Q. 1. Fill in the blanks.
(a) Microorganisms can be seen with the help of a __________.
Ans. microscope.
(b) Blue green algae fix _________ directly from air to enhance fertility of soil.
Ans. atmospheric nitrogen.
(c) Alcohol is produced with the help of _________.
Ans. yeast.
(d) Cholera is caused by _________.
Ans. bacteria.
Q. 2. Tick the correct answer:
(i) Yeast is used in the production of:
(a) sugar.
(b) alcohol.
(c) hydrochloric acid.
(d) oxygen.
Ans. (b) alcohol.
(ii) The following is an antibiotic:
(a) Sodium bicarbonate.
(b) Streptomycin.
(c) Alcohol.
(d) Yeast.
Ans. (b) Streptomycin.
(iii) Carrier of malaria-causing protozoan is:
(a) female Anopheles mosquito.
(b) cockroach.
(c) dragonfly.
(d) butterfly.
Ans. (a) female Anopheles mosquito.
(iv) The most common carrier of communicable disease is:
(a) ant.
(b) housefly.
(c) dragonfly.
(d) spider.
Ans. (b) housefly.
(v) The bread or idli dough rises because of:
(a) heat.
(b) grinding.
(c) growth of yeast cells.
(d) kneading.
Ans. (c) growth of yeast cells.
(vi) The process of conversion of sugar into alcohol is called:
(a) nitrogen fixation.
(b) moulding.
(c) fermentation.
(d) infection.
Ans. (c) fermentation.
Q. 3. Match the organisms in Column I with their action in Column II.
| Column I | Column II |
| (i) Bacteria | (a) Nitrogen fixation. |
| (ii) Rhizobium | (b) Setting of curd. |
| (iii) Lactobacillus | (c) Baking of bread. |
| (iv) Yeast | (d) Causes Malaria. |
| (v) A protozoan | (e) Causing Cholera. |
| (vi) A virus | (f) Causing AIDS. |
| (g) Producing antibodies. |
Ans.
| Column I | Column II |
| (i) Bacteria | (e) Causing Cholera. |
| (ii) Rhizobium | (a) Nitrogen fixation. |
| (iii) Lactobacillus | (b) Setting of curd. |
| (iv) Yeast | (c) Baking of bread. |
| (v) A protozoan | (d) Causes Malaria. |
| (vi) A virus | (f) Causing AIDS |
Q. 4. Can microorganisms be seen with the naked eye? If not, how can they be seen?
Ans. No, microorganisms cannot be seen with the naked eye because they are too small. We can only see them under microscope.
Q. 5. What are the major groups of Microorganisms?
Ans. Microorganisms can mainly be divided into five groups:
(i) Bacteria.
(ii) fungi.
(iii) protozoa.
(iv) algae.
(v) virus.
Q. 6. Name the microorganisms which can fix atmospheric nitrogen in the soil.
Ans. Bacteria like Rhizobium, Azotobacter, Blue green algae like Anabaena and Nostoc can fix atmospheric nitrogen in soil.
Q. 7. Write 10 lines on the usefulness of microorganisms in our lives.
Ans. Microorganisms are very useful to us.
They help us in the following ways:
(i) Microorganisms help us in food sector. They are used for curdling of milk, preparation of bread, cake, etc.
(ii) Microorganisms are used to produce alcohol at large scale.
(iii) They are also used to produce wine.
(iv) Yeast is used to prepare vinegar.
(v) They are also used as preservatives for food items.
(vi) Used to make different medicines especially the antibiotics.
(vii) Microorganisms are used to prepare vaccines for various diseases.
(viii) Useful for agriculture sector, as they enhance the fertility of soil by fixing the biological nitrogen.
(ix) They work as natural cleansers as they decompose the dead bodies of plants and animals.
(x) Microorganisms prepare manures by decomposing dead bodies of plants and animals.
Q. 8. Write a short paragraph on the harms caused by microorganisms.
Ans. Harmful effects of Microorganisms: Microorganisms can prove very harmful to us, as they cause a number of human and animal diseases. Diseases in humans like common cold, tuberculosis, measles, chicken pox, Polio, Cholera, Typhoid, Hepiatitis B, Malaria, etc., are caused by microorganisms. Some serious disease slike Anthrax is also caused in animals by the microbes. Microbes grow on food products and render them unfit for consumption. Consumption of such food causes food poisoning. Microbes also spoil clothings and leather products. Microbes also cause diseases of plants like blights in potatoes, sugarcanes, organes etc. They also reduce the yield.
Q. 9. What are antibiotics? What precautions must be taken while taking antibiotics?
Ans. Medicines taken to kill or stop the growth of harmful or disease causing microbes in human body are called antibiotics. Antibiotics are very useful as only antibiotics can save us from many microbial infections and diseases. Antibiotics are made from fungi and bacteria. Alexander Fleming discovered the first antibiotic called Penicillin in 1929. Now-a-days a number of antibiotics are used to cure a variety of human and animal diseases. Streptomycin, Tetracycline, Erythromycin etc., are some commonly used antibiotics.
Antibiotics should only be taken when advised by a qualified physician. Antibiotics should only be taken when needed otherwise, they become less effective for future use.
