DAV Class 8 Science Chapter 2 Solutions

Class 8 DAV Science Solutions Chapter 2: Are you looking for DAV Books Solutions then you are in right place , we have discussed the solution of Science class 8 book Chapter 2 Microorganisms: Friends or Foes which is followed in all DAV School. Solutions are given below with proper Explanation please bookmark our website for further updates!! All the Best !!

DAV Class 8 Science Chapter 2 Solutions

Class- 8 DAV Science Microorganisms: Friends or Foes Question and Answer

A. Fill in the blanks.

1. The study of microorganisms is known as Microbiology.

2. The bread dough rises because of the production of Carbon dioxide.

3. Pathogens are the microorganisms that cause diseases.

4. Sodium Benzoate and Potassium Metabisulfite are two chemical preservatives.

5. Aspergillus is a fungus that causes food poisoning.

B. Match the following.

1. Ringworm————————- Fungus

2. Clostridium———————— Food poisoning

3. Anthrax—————————- Cattle

4. Lactobacillus———————- Curd

5. Methane————————— Biogas reactor

C. Tick the correct option.

C. Tick (✓) the correct option.

Q. The process, that helps the milk to last longer but does not kill all microbes present in it, is known as 1. vaccination 2.bioaugmentation 3.pasteurisation 4.fermentation


Q Rust of wheat is caused by— 1. fungus 2. bacteria 3. virus 4.protozoa


Q Medicines, containing killed or weakened pathogens, and used to prevent infectious diseases, are called—1.Disinfectants 2. Antibodies 3.Antibiotics 4. Vaccines


Q. Which of the following reproduces only inside a host cell?

1.virus 2. Alga 3.protozoan 4.bacteria

Ans) Virus

Q. A disease, caused by a virus, and spread by an insect, is— 1.Polio 2. Dengue 3. rabies 4. mumps


D. Answer the following questions in brief.

D. Answer the following questions in brief.

1. Why are viruses considered as being at the ‘borderline’ between living and non-living things?

Viruses are considered at the borderline of living and non-living because they show both the characteristics of a living and a non-living. As they react like non-living in the free atmosphere but when they enter the body of a living organism then they show the features of a living organism and start reproduction.

2.What is fermentation? How is this process useful in the food and beverage industries?

Ans. An anaerobic process through which sugars are converted into alcohol or acids and carbon dioxide by the action of microbes is called fermentation. Fermentation is useful in the food and beverages industries in various ways.

Some examples are as follows:

• Fermentation in batter helps in making tasty dosa.

• Fermentation in dough helps in raising the dough while making bread and cake.

• Fermentation of fruit juice helps in making wine.

3. ‘In the absence of microorganisms the earth would become a heap of dead plants and animals.’ Justify this statement.

Ans. When a plant or animal dies, its remains are decomposed by microorganisms. Decomposition is the process through which all constituent materials of a living being are sent back to the environment. Had there been no microorganisms, dead remains of plants and animals would be keeping on accumulating all around us. So, it can be said that in the absence of microorganisms the earth would become a heap of dead plants and animals.

4. Munna observes that the yield of wheat, growing in his field, has reduced in the current year. He gets the soil tested and the report confirms the deficiency of one particular nutrient. Accordingly, he is advised to grow peas after harvesting wheat.

(i) Name the nutrient found insufficient.


(ii) How do you think growing peas will help in replenishing the soil?

Ans:Pea Plants give shelter to Rhizobium bacteria. These bacteria carry out nitrogen fixation in soil. Thus, pea plants help in replenishing nitrogen in the soil.

5. Why does it take less time to prepare curd in summers as compared to preparing it in winters?

Microganisms reproduce at faster rate between 20 to 40°C. They reproduce at much slower rate when temperature is below this range. Milk is converted curd by multiplication of lactobacillus. This is why it takes less time to prepare curd in summers as compared to preparing in winters.

How are the following diseases transmitted?

(a) Malaria

Ans. By female Anopheles mosquito

(b) Common cold

Ans. Through cough droplets in air

(c) Tetanus

Ans. Through cut by a rusted iron article

(d Typhoid

Ans. Through contaminated food and water

7. What are antibiotics? How are they produced? Give two examples of these.

A substance that prevents the multiplication of bacteria or kills the bacteria is called an antibiotic. Antibiotics are prepared from microorganisms. Example Penicillin, Streptomycin.

E. Answer the following questions.

1. ‘Microorganisms are very useful in manufacturing different food items’. Explain the above statement.

Answer: Microorganisms are very useful in manufacturing different food items in the following ways: a). Lactobacillus is a bacterium that helps in the formation of curd Some bacteria and funds are also involved in the making of cheese. b). Fungi, like yeast, reproduce rapidly and produce carbon dioxide and helps to make some food items fluffy and soft. c) Many microorganisms are used in the manufacture of alcohol, wine and acetic acid by the process of fermentation.

2. Give reasons for the following.

(a) Yeast is added to the batter used to make ‘dosas’.

Answer: Yeast is added to the batter so that fermentation take place. It causes it to increase in volume and makes it fluffy and soft.

(b) It is important to brush one’s teeth before going to bed.

Answer: It is important to brush one’s teeth before going to bed because bacteria can grow on food particles that get lodged in between our teeth. Acid produced by these bacteria, corrode the tooth enamel and cause cavities. Hence it is important to brush one’s teeth before going to bed.

(c) Reducing the quantity of salt in pickle can cause it to go bad soon.

Answer: Salt reduce the water content of pickle and make it unavailable for the growth of microorganisms. As the result pickle gets preserved. If the quantity of salt is reduced then it increases the growth of microorganisms and can it to go bad soon.

(d) Foods, from puffed or enlarged cans, should not be consumed.

Answer: Enlarged or puffed can represent that there had been some bacterial action on the item which might lead to formation of toxic substance.

(e) When using frozen food items, one should take out only the required quantities; thawed food should never be refrozen.

Answer: One should take out only required quantity of frozen food as it might spoil the rest of food item and refrozing the frozen item destroys its nutrients.

3. How do communicable diseases spread? Suggest ways to prevent the following diseases:

(a) Tuberculosis (b) Athlete’s foot (c) Cholera

Answer: Communicable diseases spread from one person to another through air, water, food, physical contact or insects.

(a) Tuberculosis- It can be prevented by vaccination.

(b) Athlete’s foot- It can be prevented by maintaining good personal hygiene.

(c) Cholera- Cholera can be prevented by proper disposal of waste and using disinfected water.

4. What are pathogens? Name two pathogens each that cause diseases in (i) plants and (ii) animals.

Answer: Some microorganisms cause diseases in human beings, plant and animals. Such diseases causing organisms are known as pathogens.

Bacteria and Fungi are the examples of pathogens which cause diseases in plants and animals.

5. Mohan bought samosas from a road-side vendor and ate it. After sometime he felt nausea and started vomitting. He had severe pain in the abdominal region and suffered from diarrhoea. What could be the reason for his condition? What is it called? How could it be prevented?

Answer: This could happen due to consumption of contaminated food. This disease is called food poisoning. Food must be properly prepared and stored to prevent food poisoning.

6. Which diseases the following children are most likely to suffer from?

(a) Ramu drinks water from a nearby lake. This water is neither boiled nor disinfected.

Answer: Cholera

(b) Ashu does not take a bath every day. He maintains poor personal hygiene.

Answer: Ringworm

(c) Pulkit has been bitten by an infected street dog.

Answer: Rabies

(d) Sohan is living in an area where the population is very high. He does not use a mosquito net (while sleeping), or insect-repellent creams (while playing in the open).

Answer: Malaria

(e) Mohan was with two of his friends who were coughing and sneezing (they were suffering from common cold). One of these friends did not keep a handkerchief in front of his mouth while coughing and sneezing.

Answer: Common cold.

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