Ch.2: Microorganisms: Friend Foe

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Microorganisms: Friend & Foe | Class 8 | Science | Chapter 2 | NCERT/CBSE Board

Measurement of Physical Quantitites | Class 7 | Science | Chapter 6 | Maharashtra State Board

Explore the fascinating world of microorganisms—tiny, invisible life forms that can both benefit and harm us. Learn how microbes contribute to food production, medicine, environmental cleanup, and how they can also cause diseases in humans, animals, and plants.

Questions & Answers

1. Fill in the blanks.

(a) Microorganisms can be seen with the help of a _____

Ans.

(a) microscope

(b) Blue-green algae fix ______ directly from the air to enhance the fertility of soil.

Ans.

(b) nitrogen

(c) Alcohol is produced with the help of _____

Ans. (c) yeast

(d) Cholera is caused by ______

Ans.

(d) bacteria

2. Tick the correct answer.

(a) Yeast is used in the production of

(i) sugar

(ii) alcohol

(iii) hydrochloric acid

(iv) oxygen

Ans.

(ii) alcohol

(b) The following is an antibiotic:

(i) sodium bicarbonate

(ii) streptomycin

(iii) alcohol

(iv) yeast

Ans.

(ii) streptomycin

(c) Carrier of malaria-causing protozoan is:

(i) female Anopheles mosquito

(ii) cockroach

(iii) housefly

(iv) butterfly

Ans.

(i) female Anopheles mosquito

(d) The most common carrier of communicable diseases is

(i) ant

(ii) housefly

(iii) dragonfly

(iv) spider

Ans.

(ii) housefly

(e) The bread or idli dough rises because of:

(i) heat

(ii) grinding

(iii) growth of yeast cells

(iv) kneading

Ans.

(iii) growth of yeast cell

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation

(ii) moulding

(iii) fermentation

(iv) infection

Ans.

(iii) fermentation

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

A	B
(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

Ans.

A	B
(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?

Ans.

The microorganisms cannot be seen with our naked eyes because they are very small in size. Some of these, such as fungus growing on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope.

5. What are the major groups of microorganisms?

Ans.

Microorganisms are classified on the basis of their size into four major groups. These groups are:

(a) Bacteria (b) Fungi (c) Protozoa (d) Some algae

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

Ans.

Rhizobium, Clostridium and Azotobacter.

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

Ans.

Microorganisms are useful to us in many ways. For example,

Bacteria like Lactobacillus convert milk into curd.
Bacteria are also involved in the making of cheese.
Acetobacter aceti is used for producing acetic acid from alcohol.
Yeast is used in the commercial production of alcohol, wine and bakery products.
Some specific microorganisms are helpful in manufacturing of antibiotics.
Microorganisms act as cleansing agents and decompose the waste products into manure.
Dead or weakened microbes are used in the preparation of vaccines.
Some bacteria fix atmospheric nitrogen and increase soil fertility.
Algae, yeast, fungi or bacteria may be used as an ingredient or a substitute for protein-rich foods that are suitable for human or animal consumption.
Some microorganisms are taken as probiotics, that are believed to provide health benefits when consumed.

8. Write a short paragraph on the harms caused by microorganisms.

Ans.

Microorganisms are harmful to us in many ways. For example, microorganisms, called pathogens cause disease in humans, plants and animals. Pathogens or germs enter a healthy body through air, water, contaminated food and infected person by direct or indirect contact or by the carrier. Common ailments like cold, influenza (flu), cough, polio, chicken pox are caused by viruses. Foot and mouth diseases in the cattle are also caused by viruses. Typhoid, tuberculosis (TB) are caused by bacteria. Anthrax a dangerous human and cattle diseases is also caused by bacteria.

Diseases like dysentery and malaria are caused by protozoa. Ringworm is caused by fungi. Several microbes causes diseases in plants and thus reduces the yield. Citrus canker, a bacterial disease, affects trees of citrus fruit and is spread by air. Bhendi yellow vein mosaic disease is caused by a virus and is spread by insects in lady fingers. Rust of wheat is a fungal disease spread through air. Microorganisms that grow on our food sometimes produce toxic substances. These make the food poisonous causing serious illness and even death. This food-borne illness is called food poisoning.

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

Ans.

Antibiotics are the medicines which kill or stop the growth of the disease-causing microbes. They are manufactured by growing specific microorganisms. They are used to cure a variety of diseases.

It is important to take antibiotic only on the advice of a qualified doctor. One must finish the course prescribed by the doctor to make the drug more effective. Antibiotics must not be taken unnecessarily because it may kill beneficial bacteria also. Antibiotics are, however, not effective against cold and flu as they are caused by viruses.

E X T R A

Activities Solved

Activity 1 (NCERT Textbook, Page 17)

Collect some moist soil from the field in a beaker and add water to it. After the soil particles have settled down, observe a drop of water from the beaker under a microscope. What do you see?Solution:It is observed that some tiny organisms are moving around.

Activity 2 (NCERT Textbook, Page 17)

Take a few drops of water from a pond. Spread on a glass slide and observe through a microscope.Solution:It is observed that some tiny organisms are moving around.

Activity 3 (NCERT Textbook, Page 20)

Take 1/2 kg flour (atta or maida), add some sugar and mix with warm water. Add a small amount of yeast powder and knead to make a soft dough. What do you observe after two hours? Did you find the dough rising? Solution:It is observed that the dough begins to rise up in volume. Yeast reproduces rapidly and releases C02 during respiration. Bubble of this gas fill the dough and increase its volume.

Activity 4 (NCERT Textbook, Page 20)

Take 500 ml. beaker filled upto 3/4 with water. Dissolve 2-3 teaspoons of sugars in it. Add half a spoon of yeast powder to the sugar solution. Keep it covered in a warm place for 4-5 hours. Now smell the solution. Can you get a smell?Solution:It is observed that the solution smell like alcohol. This process of conversion of sugar in alcohol is known as fermentation.

Activity 5 (NCERT Textbook, Page 22)

Take two pots and fill each pot half with soil. Mark them A and B. Put plant waste in pot A and things like polythene bags, empty glass bottles and broken plastic toys in pot B. Put the pots aside. Observe them after 3-4 weeks.Solution:It is observed that plant waste in pot A has been decomposed, whereas the polythene bags, empty glass bottles and broken plastic toys in pot B did not undergo such changes.

NCERT Solutions for Class 8 Science Chapter 2 – (1 Mark)

6. Questions and Answers

1. Name the bacteria responsible for the disease typhoid.

Ans. Salmonella typhi.

2. Chicken pox is caused by ………. virus.

Ans.

Varicella zoster.

3. ………….. (disease) is caused by mycoplasma.

Ans.

Pleuropneumonia.

4. Name the toxin released by T.B. bacteria.

Ans.

Tuberculin toxin is released by T.B. bacteria.

5. Which microorganism is the cause of malaria?

Ans.

A Protozoan, Plasmodium.

6. Write two diseases caused by bacteria.

Ans.

Two diseases caused by bacteria are tuberculosis and diphtheria.

7. Name any two items that are prepared by using yeast.

Ans.

Yeast is used to prepare bread and alcohol.

7. Fill in the blanks :

1. Microorganisms can be seen with the help of a …………..

Ans.

Microscope

2. Blue-green algae fix ………… directly from air to enhance fertility of soil.

Ans.

Nitrogen

3. Alcohol is produced with the help of ………….

Ans.

Yeast

4. Cholera is caused by ……………

Ans.

Bacteria.

8. Tick the correct answer :

(a) Yeast is used in the production of

(i) sugar

(ii) alcohol

(iii) hydrochloric acid

(iv) oxygen.

Ans.

(ii) alcohol

(b) The following is an antibiotic

(i) Sodium bicarbonate

(ii) Streptomycin

(iii) Alcohol

(iv) Yeast.

Ans.

(ii) Streptomycin

(c) Carrier of malaria-causing protozoan is

(i) female anopheles mosquito

(ii) cockroach

(iii) housefly

(iv) Butterfly.

Ans.

(i) female anopheles mosquito

(d) The most common carrier of communicable diseases is

(i) ant

(ii) housefly

(iii) dragonfly

(iv) spider.

Ans.

(ii) housefly

(e) The bread or idli dough rises because of

(i) heat

(ii) grinding

(iii) growth of yeast cells

(iv) kneading.

Ans.

(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.

Ans.

(iii) fermentation

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

Column I	Column II
(a) Bacteria(b) Rhizobium(c) Lactobacillus(d) Yeast(e) A protozoan(f) A virus	(i) Fixing nitrogen(ii) Setting of curd(iii) Baking of bread(iv) Causing malaria(v) Causing cholera(vi) Causing AIDS

Ans.

Column I	Column II
(a) Bacteria(b) Rhizobium(c) Lactobacillus(d) Yeast(e) A protozoan(f) A virus	(i) Causing cholera(ii) Fixing nitrogen(iii) Setting of curd(iv) Baking of bread(v) Causing malaria(vi) Causing AIDS

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

Ans.

Rhizobium, Clostridium md Azotobacter fix atmospheric nitrogen.

11. 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. They can only be seen with the help of microscope.

12. What are the major groups of microorganisms?

Ans.

Bacteria
Fungi
Protozoa
Algae.

13. What are microorganisms?

Ans.

Microorganisms are organisms that are so small that they can only be seen through a microscope.

14. Name two diseases caused by viruses.

Ans.

Influenza and cough.

15. Name two diseases caused by protozoans.

Ans.

Dysentery and malaria.

16. Which microorganisms are unicellular?

Ans.

Bacteria and protozoa are unicellular.

17. Name two multicellular microorganisms.

Ans.

Algae and fungi are multicellular.

18. Name the bacterium used for production of acetic acid from alcohol.

Ans.

Acetobacter aceti.

19. Define fermentation.

Ans.

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

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

Ans.

Cold and flu are caused by viruses, so the antibiotics are not effective against them.

21. What are antibodies?

Ans.

When a disease carrying microbe enters our body, the body produces antibodies to fight the invader.

22. Name two diseases which can be prevented by vaccination.

Ans.

Cholera and tuberculosis can be prevented by vaccination.

23. Name the diseases for which oral drops are given as vaccination.

Ans.

Polio.

24. What are biological nitrogen fixers?

Ans.

Some bacteria and blue-green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase fertility, are known as biological nitrogen fixers.

25. What are pathogens?

Ans.

Diseases-causing microorganisms are called pathogens.

26. What are communicable diseases?

Ans.

Microbial diseases which can spread from an infected person to a healthy person through air, water, food or physical contact are known as communicable diseases.

27. Name two communicable diseases.

Ans.

Chicken pox and tuberculosis.

28. Name two carriers of disease-causing microbes.

Ans.

Housefly and female Anopheles mosquito.

29. Name the causative microorganism of tuberculosis and its mode of transmission.

Ans.

Causative microorganisms – Bacteria.Mode of transmission – Air.

30. How are cholera and typhoid transmitted?

Ans.

Cholera is transmitted through water or food.Typhoid is transmitted through water.

31. Which microorganism causes foot and mouth disease of cattle?

Ans.

Virus.

32. Name one plant disease caused by bacteria and its mode of transmission.

Ans.

Citrus canker is caused by bacteria and transmitted through air

33. Name one plant disease transmitted by insects and caused by virus.

Ans.

Yellow vein mosaic of bhindi.

34. Name one plant disease caused by fungi and its mode of transmission.

Ans.

Rust of wheat. It is transmitted through air and seeds.

35. What is meant by food poisoning?

Ans.

Food poisoning is caused by consuming food that has bean spoilt by some microorganisms.

36. What are preservatives?

Ans.

Chemicals (salts, etc.) that check the growth of microorganisms are called preservatives.

37. How are pickles preserved?

Ans.

Pickles are preserved by adding salt or acid.

38. Name the chemicals used to preserve jams and squashes.

Ans.

Sodium benzoate and sodium metabisulphite are added to jams and squashes to preserve them.

39. How are meat and fish preserved?

Ans.

Meat and fish are preserved by covering them with dry salt to check the growth of bacteria.

NCERT Solutions for Class 8 Science Chapter 2 – (2 Mark)

Questions and Answers

1. Describe the role of blue-green algae in fertility of soil.

Ans.

Blue-green, algae, also called cyanobacteria, can fix atmospheric nitrogen into usable , compounds. These are then used as fertilizers.

2. Name three habitats of microorganisms.

Ans.

Microorganisms are present in soil, water, outer space and inside the body of animals.

3. Name the bacterium found in the roots of pea plant. How is this bacterium useful for human beings?

Ans.

Rhizobium is the bacterium found in the roots of pea plant. The. bacterium absorbs the atmospheric nitrogen and converts it to nitrates.

4. What is a vaccine? Why is it important to vaccinate small children?

Ans.

Vaccine is prepared from weak or dead disease-causing microbe. Vaccine is given to healthy persons to prevent occurrence of disease. It is important to vaccinate small children because it creates antibodies in blood.

5. Give full form of ORS. What is vaccination?

Ans.

ORS — Oral Rehydration Solution.
Vaccination is a method to immunise the body against diseases by making the body’s immune system produce antibodies against the disease-causing microbe in the vaccine.

6. Mention two important uses of fungi.

Ans.

Uses of fungi:

They convert dead organic matter into simple soluble minerals and gases, which can be used again by plants.
Fungi like yeast is used in bread, beer and wine.

7. How is pasteurised milk obtained?

Ans.

In pasteurisation, the milk is heated to 62.5°C for 30 minutes or to 71.5°C for 15 seconds. It is then rapidly cooled to 10°C and packed in airtight containers.

8. How are viruses different from other microorganisms?

Ans.

Viruses reproduce only inside the host organisms that is, bacterial, plant or animal cell.

9. Give two examples where microorganisms are useful at home.

Ans.

At home microorganisms are used for preparation of curd and cakes.

10. How are microorganisms useful commercially?

Ans.

Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar).

11. How do microorganisms clean the environment?

Ans.

Microorganisms degrade the harmful substances and clean the environment.

12. How can we prevent a person from getting Hepatitis B?

Ans.

By giving boiled water for drinking.
By vaccination.

13. Name one disease caused by bacteria and one disease caused by virus in cattle.

Ans.

Bacteria – Anthrax.
Virus – Foot and mouth disease.

14. How can we control plant diseases?

Ans.

Plant diseases can be controlled by chemicals which kill the microbes.

15. Why do we use oil and vinegar to preserve vegetables and fruits?

Ans.

Use of oil and vinegar prevents spoilage of vegetables and fruits because bacteria cannot live in such an environment.

16. Why do we keep milk in the refrigerator during summers?

Ans.

The low temperature in the refrigerator inhibits the growth of microbes.

17. Why do we boil milk before storing?

Ans.

Boiling kills the microorganisms.

18. How are dry fruits preserved?

Ans.

Dry fruits are preserved by keeping them sealed in airtight packets to prevent the attack of microbes.

19. Name the scientist who discovered the vaccine for smallpox. Who discovered the bacterium which causes anthrax disease?

Ans.

Edward Jenner
Robert Koch

NCERT Solutions for Class 8 Science Chapter 2 – (3 Mark)

Questions and Answers

1. Tuberculosis is a highly infectious disease. Justify the statement.

Which vitamin helps in the prevention of common cold?

Ans.

Tuberculosis is a highly infectious disease. TB is transmitted through minute droplets of infected sputum on phelgm, by drinking milk of an infected animal.
Common cold is prevented by taking vitamin C.

2. Can you store pickles in iron containers? Why?

Ans.

We can not store pickles in iron containers because the acid present in the pickles reacts with iron. This can cause food poisoning, if consumed.

3 What precautions must be taken while taking antibiotics?

Ans.

Antibiotics should be taken after consulting a doctor. The complete dose of the antibiotics should be taken as prescribed. They should not be taken unnecessarly because they will kill the useful bacteria.

4 Write the causal organism of cholera. Write any two symptoms of this disease. Why ORS should be given to the patients suffering from cholera?

Ans.

Cholera is caused by Vibrio cholerae.

Two symptoms of cholera are :

profuse and painless watery diarrhoea.
muscular cramps.

ORS should be given to the patients suffering from cholera to avoid excessive loss of body fluids.

5. Why are viruses considered to be on the borderline between living organisms and non-living things?

Ans.

Viruses cannot reproduce, respond to changes or use energy to grow. Since viruses reproduce in the host cell, scientist regard viruses as a link between living and non-living.

6. How are bacteria beneficial for us?

Ans.

Bacteria are useful to us as :

Nitrogen fixing bacteria increase the fertility of the soil.
They can be used to form curd, alcohol, etc.
They can decompose organic matter.

7. Write three types of bacteria on the basis of their shape. Give one example of each.

Ans.

Three types of bacteria are :

Bacillus or rod-shaped, e.g., Lactobacillus.
Coccus or spherical, e.g., Streptococcus.
Spirillum or spiral, e.g., Vibrio.

8. (a) Are bacteria plants or animals? Give reasons in support of your answer.

(b) Write two differences between autotrophic and heterotrophic bacteria.

Ans.

(a) Bacteria is considered as plant due to the presence of a rigid cell wall in it.

(b)

Autotrophic bacteria	Heterotrophic bacteria
(i) Chlorophyll is present.(ii) Synthesize their own food.	(i) Chlorophyll is absent.(ii) Depend on readymade food from other sources.

9. How does a housefly transmit diseases?

Ans.

A housefly gets attracted towards garbage and excreta. The harmful microorganisms present in excreta and garbage easily stick to its fine array of body hair and are thus, transferred to food stuffs whenever they sit on them and a result food gets poisoned.

10. What are antibiotics? How are antibiotics manufactured? Name two important antibiotics.

Ans.

Medicines which kill or stop the growth of the disease causing microorganisms are called antibiotics.
The antibiotics are manufactured by growing specific microorganisms.
Two important antibiotics are Tetracycline and Bacitracin.

11. What will happen if you take antibiotics when not needed?

Ans.

If you take antibiotics when not needed, you help bacteria in your body to develop resistance to them. Next time when you fall ill and need these antibiotics, they would be less effective.

12. How does a vaccine act?

Ans.

The vaccine acts by making the body’s immune system to produce antibodies against the disease-causing microbe in the vaccine. The antibodies attack and destroy the weakened microbe as it enters the body.

13. How is common cold spread?

Ans.

When a person suffering from common cold sneezes, fine droplets of moisture carrying thousands of viruses causing common cold are spread in the air. The viruses may enter the body of a healthy person while breathing.

14. How can you prevent the spread of communicable diseases?

Ans.

It is better to avoid contact with the infected person. We should keep distance from infected persons.

15. Give the methods of prevention of cholera.

Ans.

Cholera can be prevented by :

Vaccination.
Maintaining good personal hygiene.
Consuming properly cooked food and drinking boiled water.

16. How does food become ‘poison’?

Ans.

Microorganisms that grow on our food sometimes produce toxic substances. These make the food poisonous causing serious illness and even death.

17. What will happen if yeast powder is put in sugar solution and kept for few hours? Name the process.

Ans.

When yeast powder is put in sugar solution and kept for few hours, fermentation takes place. The sugar is converted into alcohol by yeast and this process is known as Fermentation.

NCERT Solutions for Class 8 Science Chapter 2 – (5 Mark)

Questions and Answers

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

Ans.

Microorganisms are useful in our lives in the following ways :

Bacteria cause the decay of dead plants and animals.
Nitrogen fixing bacteria convert the atmospheric nitrogen into nitrates.
Bacteria are used in the production of vinegar, curd, cheese, etc.
Bacteria are used in sewage disposal plants.
Scientists have discovered ways to change bacteria so that they produce valuable medical, agricultural and industrial products.

2. Write a short paragraph on the harms caused by microorganisms.

Ans.

Microorganisms cause diseases in human beings, plants and animals.
Microorganisms spoil food and cause food poisoning.

3. How can we control the spread of malaria or dengue?

Ans.

All mosquitoes breed in water. Hence, by keeping the surroundings clean and dry, we can prevent mosquitoes from breeding. We should not allow water to collect in coolers, flower pots, etc. We should wear full sleeves shirts in the evenings and at night. We can also use mosquito repellant creams and mosquito nets.

NCERT Solutions for Class 8 Science Chapter 2 MCQs

1. Microorganisms exhibiting the characteristics of living and non-living organisms are known as

(a) bacteria

(b) virus

(d) fungi

Ans.

(b)

2. A person bitten by dog gets

(a) malaria

(b) tetanus

(d) rabies

Ans.

(d)

3. Amoebic dysentery is transmitted by

(a) sneezing

(b) using contaminated water

(d) wound

Ans.

(b)

4. Anthrax in cattle is caused by

(a) fungi

(b) virus

(d) algae

Ans.

(c)

5. Smut of rice is caused by

(a) virus

(b) protozoa

(d) bacteria

Ans.

(c)

6. Wheat, maize and oats are preserved by

(a) using sugar and salt

(b) pasteurisation

(d) using chemical preservatives

Ans.

(c)

7. Jam and jellies are preserved by

(a) drying

(b) using chemical preservatives

(d) using heat and cold treatments

Ans.

(b)

8. Pasteurisation is the method used for preserving

(a) milk

(b) jams

(d) grains

Ans.

(a)

9. Fermentation is the process used for making

(a) curd

(b) cheese

(d) chappatis

Ans.

(c)

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