NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend And Foe are provided here with simple step-by-step explanations. These solutions for Microorganisms: Friend And Foe are extremely popular among Class 8 students for Science Microorganisms: Friend And Foe Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 8 Science Chapter 2 are provided here for you for free. You will also love the ad-free experience on Meritnation’s NCERT Solutions. All NCERT Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

Page No 7:

Question 1:

Which of the following reproduces only inside a host cell?
(a) Bacteria 
(b) Virus
(c) Amoeba
(d) Fungus. 

Answer:

Viruses reproduce only inside a host cell. That host may be a bacterium, a plant, or an animal cell. They act as non-living particles when present outside the host and acts as a living cell when they are present inside the host cell. So, they require a living host cell for their reproduction. 

Hence, the correct answer is option (b).

Page No 7:

Question 2:

A disease in human beings caused by viruses is _______.
(a) typhoid 
(b) influenza
(c) dysentry
(d) cholera

Answer:

Influenza is a disease in human beings that is caused by viruses. It affects the respiratory system of humans and causes flu. 

Hence, the correct answer is option (b).

Page No 7:

Question 3:

Pathogenic micro-organisms present in host cells are killed by medicines called
(a) pain killer 
(b) antibodies
(c) antibiotics
(d) vaccines

Answer:

Antibiotics are medicines that kill or inhibit the growth of disease-causing microorganisms in the host cells. The most commonly known antibiotics are tetracycline, erythromycin, and streptomycin that are made from bacteria and fungi. 

Hence, the correct answer is option (c).

Page No 7:

Question 4:

The two micro-organisms which live in symbiotic association in lichens are
(a) fungus and protozoa 
(b) alga and bacteria
(c) bacteria and protozoa
(d) alga and fungus

Answer:

Lichens are a combination of two microorganisms algae and fungus that live in symbiotic association with each other. In this, algae provide food to fungi and in turn, fungi gives water and shelter to the algae.

Hence, the correct answer is option (d).

Page No 7:

Question 5:

The gas released during the preparation of bread is
(a) oxygen 
(b) carbon dioxide
(c) nitrogen
(d) sulfur dioxide

Answer:

The gas released during the preparation of bread is carbon dioxide. Carbon dioxide gas is produced by yeast during respiration. During the process of bread making, the bubbles of carbon dioxide gas fill the dough and increase its volume.

Hence, the correct answer is option (b).

Page No 7:

Question 6:

The disease caused by a protozoan and spread by an insect is _________. 
(a) dengue 
(b) malaria
(c) polio
(d) measles

Answer:

Malaria is a protozoan disease that is spread by a bite of an insect. It is caused by the bite of the female Anopheles mosquito. Dysentry is also caused by a protozoan.

Hence, the correct answer is option (b).
 

Page No 7:

Question 7:

Paheli dug two pits, A and B, in her garden. In pit A, she put a polythene bag packed with some agricultural waste. In pit B, she dumped the same kind of a waste but without packing it in a polythene bag. She then covered both the pits with soil. What did she observe after a month?
(a) Waste in pit A degraded faster than that in pit B.
(b) Waste in pit B degraded faster than that in pit A. 
(c) Waste in both pits degraded almost equally.
(d) Waste in both pits did not degrade at all.

Answer:

Waste present in pit B degraded faster than waste present in pit A because waste in pit B was not covered by polythene bags. So, it traps more oxygen that is required growth microbes and promotes decomposition of organic matter.

Hence, the correct answer is option (b).



Page No 8:

Question 8:

Unscramble the jumbled words underlined in the following statements.
(a) Cells of our body produce santiidobe to fight pathogens.
(b) curbossulite is an air-borne disease caused by a bacterium.
(c) Xanrhat is a dangerous bacterial disease.
(d) Yeasts are used in the wine industry because of their property of meronettinaf. 

Answer:

(a) Antibodies. Antibodies are produced when disease-causing microbe enters the body to protect against pathogens.
(b) Tuberculosis. Tuberculosis is a bacterial infection that affects the lungs.
(c) Anthrax. Anthrax is a bacterial infection that affects livestock and wild animals.
(d) Fermentation. In the wine industry, yeast is used to convert sugar into alcohol, which is called fermentation. 

Page No 8:

Question 9:

Suggest a suitable word for each of the following statements.
(a) Chemicals added to food to prevent the growth of microorganisms.
(b) Nitrogen-fixing microorganisms present in the root nodules of legumes.
(c) Agent which spreads pathogens from one place to another.
(d) Chemicals which kill or stop the growth of pathogens.

Answer:

(a) Preservatives- Salts and edible oils are commonly used chemicals preservatives to check the growth of microorganisms. Salts and acids are added to pickles to prevent them from microbes. Sodium metabisulphite and sodium benzoate are the common preservatives used in jams and squash to prevent spoilage.
(b) Rhizobium- It lives in the root nodules of the leguminous plant and is involved in the nitrogen fixation in these plants. For example, beans and pea.
(c) Carriers or vectors- Carriers such as insects and animals spread disease-causing microorganisms from one organism to another. 
(d) Antibiotics- Antibiotics are medicines that either kill or inhibit the growth of microorganisms. They are produced from fungi and bacteria. Some commonly known antibiotics are tetracycline, streptomycin, and erythromycin. 
 

Page No 8:

Question 10:

Match the names of scientists given in Column A with the discovery made by them given in Column B. 
Column A                                               Column B

(a) Louis Pasteur                                 (i) Penicillin
(b) Robert Koch                                 (ii) anthrax bacterium
(c) Edward Jenner                             (iii) Fermentation
(d) Alexander Fleming                     (iv) small pox vaccine
                                                          (v) Typhoid

Answer:

(a)-(iii). Louis Pasteur discovered the process of fermentation in 1857.
(b)-(ii). Robert Koch discovered the bacterium Bacillus anthracis that causes anthrax disease in livestock and wild animals in 1876.
(c)-(iv). Edward Jenner discovered the vaccine for smallpox in 1798.
(d)-(i). Alexander Flemming discovered Penicillin used for the treatment of bacterial infections.

Page No 8:

Question 11:

Name one commercial use of yeast.

Answer:

Yeast is used for the commercial production of alcohol, wine, and vinegar by the process of fermentation. In this process, it converts sugar into alcohol. It is also used for making bread, pastries, and cakes. During baking, it reproduces and produces carbon dioxide gas that increasess the volume of the dough.

Page No 8:

Question 12:

Name the process in yeast that converts sugars into alcohol.

Answer:

Yeast converts sugar into alcohol by the process of fermentation. In this process, yeast is allowed to grow on a medium containing natural sugar that is present in grains like barley, rice, wheat, and crushed fruit juices.

Page No 8:

Question 13:

In the soil, which nutrient is enriched by blue-green algae (cyanobacteria)?

Answer:

Cyanobacteria fix atmospheric nitrogen and convert them into compounds of nitrogen. These compounds of nitrogen are utilized by the roots of the plants from the soil. Nitrogen is used by plants for the process of protein synthesis, and other compounds.

Page No 8:

Question 14:

Why should we avoid standing close to a tuberculosis patient while he/she is coughing?

Answer:

Tuberculosis is an airborne communicable disease that can be spread from an infected person to a healthy person through air, water, food, or physical contact. 



Page No 9:

Question 15:

 Polio drops are not given to children suffering from diarrhea. Why?

Answer:

Polio drops are not given to children suffering from diarrhea because polio drops are given orally to children. During diarrhea, it can be excreted out of the body due to frequent motions. So, it is ineffective against polio virus in this condition. 

Page No 9:

Question 16:

Paheli watched her grandmother making mango pickle. After she bottled the pickle, her grand mother poured oil on top of the pickle before closing the lid. Paheli wanted to know why oil was poured? Can you help her understand why?

Answer:

Edible oil and vinegar are considered as a commonly used chemical preservatives in pickles because it inhibits the growth of bacteria and prevent them from spoilage. It is also used for the preservation of fruits, fishes, meat and vegetables.

Page No 9:

Question 17:

Match the microorganisms given in Column A to the group to which they belong in Column B
Column A                           Column B

(a) Lactobacillus               (i) Algae
(b) Aspergillus                 (ii) Protozoa
(c) Spirogyra                   (iii) Fungi
(d) Paramecium               (iv) Bacteria

Answer:

(a)-(iv). Lactobacillus is a bacterium that promotes the formation of curd. This bacterium multiplies in curd and carried out the conversion of milk into curd.
(b)- (iii). Aspergillus is a fungus that is used for the process of fermentation. 
(c)-(i). Spirogyra is algae and is considered an important part of the ecosystem as provides oxygen to aquatic organisms. 
(d)-(ii). Paramecium is an single-celled protozoans.

Page No 9:

Question 18:

Classify the following into friendly and harmful microorganisms. Yeast, malarial parasite, Lactobacillus, bread mold, Rhizobium, Bacillus anthracis

Friendly                 Harmful
________             ________
________             ________
________             ________
 ________            ________

Answer:

Friendly- Yeast, Rhizobium, and Lactobacillus.
Yeast- It is used for the commercial production of alcohol, wine, and vinegar. It can be also used for making bread, cakes, and pastries. 
Rhizobium- It is found in the root nodules of leguminous plants and is involved in the process of nitrogen fixation in such plants. For example, Beans and peas.
Lactobacillus- It is a bacterium that multiplies and convert milk into curd.

Harmful- Malarial parasite, bread mould and Bacillus anthracis.
Malarial parasite- It causes malaria in humans by the bite of the female Anopheles mosquito. 
Bread mould- It is a fungus that grows on bread results in spoilage.
Bacillus anthracis- It is an bacterium that causes anthrax disease in humans and animals.
 

Page No 9:

Question 19:

While returning from school, Boojho ate chaat from a street hawker. When he reached home, he felt ill and complained of stomach ache and fell ill. What could be the reason?

Answer:

Boojho is suffering from food poisoning that is caused due to the consumption of food spoilt by microorganisms. They grow on food and produce toxins that make food poisonous results in serious illness and even death.

Page No 9:

Question 20:

What will happen to ‘pooris’ and ‘unused kneaded flour’ if they are left in the open for a day or two?

Answer:

Pooris if left open for a day will remain in good condition because they are deep-fried in heated oil that kills the microbes and prevent them from spoilage. Unused kneaded flour if left open a day or two will get contaminated by microbes results in spoilage and fermentation of the flour.

Page No 9:

Question 21:

 (a) Name two diseases that are caused by virus.
 (b) Write one important characteristic of virus.

Answer:

(a) Common colds, influenza, and coughs are caused by viruses.
(b) Viruses are small microscopic organisms that act as non-living particles when living outside the host. They only reproduce in living host that could be bacterium, plant and animals.



Page No 10:

Question 22:

Observe the given Figure and answer the questions that follows.

(a) Write the name of the disease.
(b) Name the causative agent of this disease?
(c) How does the disease spread from one plant to another?
(d) Name any two plant diseases and the microbes that cause them.

Answer:

(a) Yellow vein mosaic of bhindi (Okra).
(b) This disease is caused by viruses.
(c) This disease is transmitted from one plant to another by insects.
(d) Citrus canker - caused by bacteria
      Rust of wheat - caused by fungi.

Page No 10:

Question 23:

How do vaccines work?

Answer:

Vaccines are dead or weakened microbes that are introduced into the healthy body. The body fights and kills the diseases causing microbes by producing antibodies against them. These antibodies remain in the body and provide lifetime protection against disease-causing microbes. 

Page No 10:

Question 24:

Observe the set up given in the given Figure and answer the following questions.
(a) What happens to the sugar solution in A?
(b) Which gas is released in A?
(c) What changes will you observe in B when the released gas passes through it?

Answer:

(a) In flask A, the sugar solution is converted into alcohol and carbon dioxide by the action of yeast. This process is called fermentation.
(b) Carbon dioxide gas is released in flask A.
(c) When carbon dioxide gas is passed through flask B, it turns lime water milky. 



Page No 11:

Question 25:

Observe the given Figure and answer the following questions. 

(a) Name the microorganism and the group to which it belongs.
(b) Name the food item on which the organism grows.
(c) Does it grow well in dry or in moist conditions?
(d) Is it safe to eat infected bread?

Answer:

(a) Bread mould and belongs to a group of fungi.
(b) It grows on bread result in spoilage.
(c) Bread mould requires optimum temperature and moisture for growth.
(d) No, it is not safe to eat infected bread because it causes food poisoning results in serious illness and even death.

Page No 11:

Question 26:

Give reasons for the following.
(a) Fresh milk is boiled before consumption while processed milk stored in packets can be consumed without boiling.
(b) Raw vegetables and fruits are kept in refrigerators whereas jams and pickles can be kept outside.
(c) Farmers prefer to grow beans and peas in nitrogen-deficient soils.
(d) Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?

Answer:

(a) Processed milk is pasteurised milk that can be consumed without boiling as it is free from harmful microbes. In this process, milk is heated at 70°C for 15-30 seconds. After that, they are suddenly chilled and stored. It prevents the growth of microbes. This method was discovered by Louis Pasteur and is called pasteurisation.  Fresh milk is boiled before consumption to kill microbes that are present in it. 

(b) Jams are preserved by adding sugars that reduce the moisture content and inhibit the growth of bacteria that causes spoilage. Pickles are preserved by adding oil and vinegar to prevent them from spoilage by inhibiting the growth of bacteria. Raw vegetables and fruits are kept in refrigerators because low temperatures inhibit the growth of microorganisms present in the environment.

(c) Beans and peas are leguminous plants. Rhizobium is present in the root nodules of these plants and is involved in nitrogen fixation in such plants. This increases the nitrogen content in the soil results in increasing fertility of the soil.

(d) Collection of water in a small area is called stagnation of water that promotes the growth of mosquito larvae. As mosquitoes lay their eggs in stagnant water. 

Page No 11:

Question 27:

How can we prevent the following diseases?
(a) Cholera
(b) Typhoid
(c) Hepatitis A

Answer:

(a) Cholera is a bacterial infection that can be transmitted by the consumption of contaminated food and water. This disease is prevented by maintaining personal hygiene and good sanitary habits.
(b) Typhoid is a bacterial infection caused by the consumption of contaminated water. This disease is prevented by consuming properly cooked food and boiled water as well as by vaccination.
(c) Hepatitis A is a viral infection that affects the liver and is caused by the consumption of contaminated water. This disease is prevented by consuming boiling during water and proper vaccination.



Page No 12:

Question 28:

Complete the following cycle given as Figure by filling the blanks (a), (b), (c) (d)

Answer:

(a) Lightning: It breaks strong bonds of nitrogen that are present in the atmosphere.
(b) Nitrogen-fixing bacteria fix atmospheric nitrogen - Cyanobacteria and other bacteria present in the soil fix atmospheric nitrogen and convert it into compounds of nitrogen.
(c) Nitrogenous waste from excretion and death- Bacteria and fungi present in the soil convert the nitrogenous waste from dead remains of plants and animals into a nitrogenous compound that can be again used by plants. 
(d) Bacteria turns nitrogenous compounds into gaseous nitrogen - Some bacteria convert nitrogenous compounds into nitrogen gas that goes back into the atmosphere.



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