NCERT Solutions for Class 8 Science Chapter 2 - Microorganisms: Friend And Foe

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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).

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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).

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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).

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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).

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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).
 

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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).

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(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. 

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(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. 
 

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(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.

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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.

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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.

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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.

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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. 

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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. 

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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.

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(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.

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.
 

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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.

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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.

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(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.

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(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.

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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. 

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(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. 

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(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.

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(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 $70°\mathrm{C}$ 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. 

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(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.

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(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.