Class 8 Science Chapter 2 | Microorganisms: Friend And Foe | Important Questions

1. What are microorganisms or microbes?

Microbes are living organisms that we cannot see with the naked eye.

2. What are the four groups of microorganisms?

Microorganisms are classified into four major groups: They are: bacteria, fungi, protozoa and algae.

3. How are viruses different from microorganisms?

Viruses are also microscopic but they are different from bacteria or fungi. Viruses can reproduce only inside the cell of the host organism which may be a plant, animal or bacterium.

4. Name some common diseases caused by virus?

Cold, cough, influenza, polio and chickenpox

5. Name some diseases caused by protozoa?

Dysentery and malaria

6. Typhoid and tuberculosis are caused by ……………………….


7. Name some uses of microorganisms?

Some microorganisms are beneficial to us. They are used in the preparation of curd, bread and cake. They also play a role in cleaning up the environment. For example, bacteria break down organic waste into useful substances. Some bacteria are used in the preparation of medicines. Some of them also help to increase soil fertility by fixing atmospheric nitrogen. Microorganisms are also used for the large scale production of alcohol, wine and acetic acid (vinegar)..

8. How does milk turn into curd?

Milk turns into curd due to the action of bacteria. Curd contains several microorganisms. Of these, the lactobacillus bacterium converts milk into curd by multiplying in milk.

9. Why is yeast used in the baking industry?

Yeast reproduces rapidly and produces carbon dioxide during respiration. When yeast is added to dough, bubbles of the gas fill the dough and increase its volume. Because of this reason yeast is used as a biological raising agent.

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10.  What is fermentation?

The process of conversion of sugar into alcohol is called fermentation.

11. Who discovered fermentation?

Louis Pasteur in 1857

12. What are antibiotics?

These are medicines used to kill or stop the growth of disease causing bacteria. Antibiotics are produced from bacteria and fungi.

13. Which was the first antibiotic?


14. Who invented penicillin?

Alexander Fleming

15. Why are antibiotics not effective against cold and flu?

Antibiotics are not effective against cold and flu because they are caused by viruses. Antibiotics can only kill bacteria.

16. Why is it important to avoid the misuse of antibiotics?

Antibiotics are used to cure diseases caused by bacteria. Antibiotics should be taken only on the advice of a qualified doctor. Also, their course must be completed. If antibiotics are taken in the wrong doses or when they are not needed, it will lead to a condition called drug resistance. This makes the antibiotic less effective against bacteria the next time you take it. Antibiotics may also kill friendly bacteria in the body. For these reasons it is important to use antibiotics only when it is absolutely necessary.

17. What are antibodies?

These are substances produced by the body to fight disease carrying microbes that enter our body.

18. How does a vaccine work?

When disease carrying microbes enter our body, the body produces antibodies to fight the pathogens. Once the body defeats a microbe it will remember how to fight that microbe if it enters the body again. Vaccines contain dead or weakened microbes. When they are introduced to a healthy body, the body will fight and kill the invading germs by producing suitable antibodies. These antibodies will remain in our body forever giving us life-long protection from that microbe. This is how vaccines work.

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19. Who discovered vaccine for small pox?

Edward Jenner in 1798

20. How can bacteria be used to increase soil fertility?

Rhizobium can fix atmospheric nitrogen and enrich the soil with this nutrient.

21. Briefly explain the role microbes play in cleaning the environment.

Microbes cause dead organic matter such as decaying plants or dead animals to decompose. This process converts it into simpler substances that other plants and animals can use for their growth. Thus, microbes degrade dead and smelly substances in the atmosphere and convert them into manure. In this way, they also help to clean up the environment.

22. Name some diseases that can be prevented through vaccination?

Small pox, hepatitis, tuberculosis, cholera and polio

23. What are pathogens?

Pathogens are disease causing microorganisms.

24. What are communicable diseases?

Communicable diseases are microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact. Examples are: chickenpox, common cold, tuberculosis, cholera

25. What are carriers?

Carriers are insects and animals that carry disease causing microbes. Example: housefly

26. Malaria is caused by the parasite …………………………


27. Which mosquito carries the parasite causing malaria?

Female Anopheles mosquito

28. Dengue virus is transmitted through the bite of the …………………………. mosquito

Female Aedes

29. How can we control the spread of malaria and dengue?

Malaria and dengue are transmitted through the bite of mosquitos. Since mosquitos breed only in water by not allowing water to collect anywhere in our surroundings, we can prevent them from breeding.

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30. Name some diseases caused by bacteria

Tuberculosis, cholera and typhoid

31. Malaria is caused by …………………………..


32. Name some common diseases caused by virus?

Measles, chicken pox, polio, hepatitis A

33. Name some airborne diseases

Tuberculosis, chicken pox, polio and measles

34. Name some water borne diseases

Cholera, typhoid, hepatitis A, polio

35. Which bacteria cause anthrax disease?

Bacillus anthracis

36. Who discovered Bacillus Anthracis?

Robert Koch

37. What are preservatives?

Preservatives are chemicals used to increase the shelf life of food products by checking the growth of microorganisms in them.

38. Sodium benzoate is a ………………………….


39. Name some preservatives used in jams and squashes

Sodium benzoate and sodium metabisulphite

40. How can jams and squashes be preserved by using sugar?

Sugar reduces the moisture content in jams and squashes and thus inhibit the growth of bacteria which spoil food.

41. What is pasteurisation?

Pasteurisation is the process of destroying microbes in milk. To pasterise milk, it is heated to 70 degree Celsius for 15 to 30 seconds and then it is suddenly chilled and stored. This prevents the growth of microbes in milk and make it edible for consumption before boiling it.

Say true or false

1. Microorganisms can be seen with the naked eye.

2. All microbes are unicellular.

3. Viruses can reproduce only inside the host organism.

4. Dysentery and malaria are caused by bacteria.

5. Tuberculosis is caused by bacteria.


1. False; 2. False; 3. True; 4. False; 5. True

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