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NCERT Solutions for Class 8 Science - Microorganisms- Friend and Foe

The CBSE Class 8 Science Chapter 2 - Microorganisms: Friend and Foe is an essential topic in the field of microbiology, which aims to teach students about the different types of microorganisms, their role in the ecosystem, and their impact on human life. Let's have a look at NCERT Solutions of the chapter.
NCERT Solutions for Class 8 Science - Microorganisms- Friend and Foe


Q1. Fill in the blanks:
(a) Microorganisms can be seen with the help of a microscope.

Microscope: A microscope is a scientific instrument used to magnify objects that are too small to be seen with the naked eye.


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

Blue-green algae: Blue-green algae are a type of microorganism known as cyanobacteria. They have the unique ability to convert atmospheric nitrogen gas into a usable form that plants can absorb, a process called nitrogen fixation.

Blue-green algaeBlue-green algae(c) Alcohol is produced with the help of yeast.

Yeast: Yeast is a type of microscopic fungus that plays a crucial role in the production of alcohol.

YeastYeast(d) Cholera is caused by bacteria.

Bacteria: Bacteria are single-celled microorganisms that can cause various diseases in humans, animals, and plants. Cholera is a severe diarrheal illness caused by a specific bacterium called Vibrio cholerae.


Q2. Tick the correct answer:
(a) Yeast is used in the production of 
(i) sugar
(ii) alcohol
(iii) hydrochloric acid
(iv) oxygen
Ans: (ii) alcohol

Yeast is used in the production of alcohol. Yeast has the ability to convert sugars into alcohol through a process called fermentation. This property of yeast is utilized in industries like brewing and winemaking.

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast
Ans: (ii) Streptomycin

Streptomycin is an antibiotic. Antibiotics are medications that can inhibit the growth of or destroy bacteria. Streptomycin is used to treat various bacterial infections and is one of the many antibiotics available to fight against bacterial diseases.

(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito
(ii) cockroach
(iii) housefly
(iv) butterfly
Ans: (i) female Anopheles mosquito

The female Anopheles mosquito is the carrier of the malaria-causing protozoan. When an infected mosquito bites a person, it injects the protozoan into their bloodstream, which can lead to malaria.

(d) The most common carrier of communicable diseases is
(i) ant
(ii) housefly
(iii) dragonfly
(iv) spider
Ans: (ii) housefly 

The housefly is the most common carrier of communicable diseases. Houseflies can come in contact with various sources of bacteria and pathogens, such as garbage, feces, and decaying organic matter. When they land on food or surfaces, they can transfer these pathogens, leading to the spread of diseases.

(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

The bread or idli dough rises because of the growth of yeast cells. When yeast is added to the dough, it ferments the sugars present in the dough, producing carbon dioxide gas. The trapped gas causes the dough to rise, making it light and fluffy.

(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation
(ii) molding
(iii) fermentation
(iv) infection
Ans: (iii) fermentation 

The process of conversion of sugar into alcohol is called fermentation. During fermentation, yeast or bacteria break down the sugar molecules into alcohol and carbon dioxide in the absence of oxygen.

Q3. Match the organisms in Column I with their action in Column II.

Column AColumn 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


Column AColumn 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
  • (i) Bacteria - (e) Causing Cholera
    Bacteria are tiny organisms that can cause diseases. Cholera is a severe illness that is caused by a specific bacterium called Vibrio cholerae. 
  • (ii) Rhizobium - (a) Fixing Nitrogen
    Rhizobium is a helpful type of bacteria that live in the roots of certain plants. It has a special ability to take nitrogen from the air and convert it into a form that plants can use. 
  • (iii) Lactobacillus - (b) Setting of curd
    Lactobacillus is a type of bacteria used in the process of making curd. When we add curd starter containing lactobacillus to milk, it helps in fermenting the milk. This fermentation process turns the milk into curd by converting the milk sugar into lactic acid.
  • (iv) Yeast - (c) Baking of bread
    Yeast is a type of microorganism that is used in baking bread. When we add yeast to dough, it produces tiny bubbles of gas called carbon dioxide through a process called fermentation. These bubbles make the dough rise and become fluffy when we bake it.
  • (v) A protozoan - (d) Causing Malaria
    A protozoan is a type of tiny organism. Some protozoans can cause diseases. Malaria is a disease that is caused by a protozoan parasite.
  • (vi) A Virus - (f) Causing AIDS
    A virus is a tiny organism that can cause diseases. The Human Immunodeficiency Virus (HIV) is a virus that can weaken a person's immune system. It can lead to a disease called Acquired Immunodeficiency Syndrome (AIDS), which makes it harder for the body to fight off infections and diseases.
  • (g) Producing antibodies
    This option does not have a corresponding organism in Column A. Antibodies are special substances produced by our immune system to fight against infections.

Q4. Can microorganisms be seen with the naked eye? If not, how can they be seen?
Ans: The microorganism can not be seen with naked eyes. Microorganisms are too small and are not visible to the unaided eye. Some of these, such as the fungus that grows on bread can be seen with a magnifying glass.  However, most require a microscope for visibility. 

Q5. What are the major groups of microorganisms?
Ans: Microorganisms are classified into four major groups:

  1. Bacteria
  2. Fungi
  3. Protozoa
  4. Algae

Q6. Name the microorganisms which can fix atmospheric nitrogen in the soil.
Ans: Bacteria like Rhizobium and blue-green algae are microorganisms which can fix atmospheric nitrogen in the soil thus increase soil fertility. These microbes are commonly called biological nitrogen fixers.

Blue-green algaeBlue-green algae

Q7. Write 10 lines on the usefulness of microorganisms in our lives.
Ans: Microorganisms are quite useful in our lives in many ways :

  • Yeast, a microorganism, is used for the large scale production of alcohol, wine, and acetic acid (vinegar).
  • Lactobacillus, a bacterial microorganism, helps us turn milk into curd.
  • Yeast, a microorganism, is used in the baking industry for making bread, pastries, and cakes.
  • Bacteria are also used for the commercial production of antibiotic medicines like streptomycin, tetracycline, and erythromycin.
  • They help increase soil fertility. Microorganisms like rhizobium and blue-green algae are of great importance in agriculture.
  • They increase soil fertility by fixing atmospheric nitrogen and converting it into nitrogenous compounds.
  • Certain bacteria convert compounds of nitrogen present in the soil into nitrogen gas, which is released into the atmosphere.
  • Some microorganisms decompose organic waste and dead plants and animals into simple substances and clean up the environment.
  • Some microorganisms inside our bodies help with food digestion.
  • They contribute to scientific research and discoveries in fields like genetics and biotechnology, helping us understand life and develop new technologies.

Q8. Write a short paragraph on the harms caused by microorganisms.
Ans: Microorganisms are harmful in many ways. Some of the microorganisms cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens. Pathogens spread a number of Microbial diseases or communicable diseases like cholera, common cold, chickenpox and tuberculosis from an infected person to a healthy person through air, water, food or physical contact.
Some microorganisms spoil food, clothing and leather. Food poisoning is caused due to the consumption of food spoilt by some microorganisms.

Q9. What are antibiotics? What precautions must be taken while taking antibiotics?
Ans: Antibiotic is a medicine that kills or stops the growth of disease-causing microorganisms in humans and animals. The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics medicines like Streptomycin, tetracycline and erythromycin are commercially produced from bacteria and fungi.


Precautions in taking Antibiotics:
(i) Antibiotics should be taken only on the advice of a qualified doctor. And one must finish the course prescribed by the doctor.
(ii) Antibiotics must be avoided when not needed or required.
(iii) Antibiotics taken unnecessarily may kill the beneficial bacteria in the body.

The document NCERT Solutions for Class 8 Science - Microorganisms- Friend and Foe is a part of the Class 8 Course Science Class 8.
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FAQs on NCERT Solutions for Class 8 Science - Microorganisms- Friend and Foe

1. What are micro-organisms?
Ans. Micro-organisms are tiny living organisms that can only be seen through a microscope. They include bacteria, viruses, fungi, protozoa, algae, and archaea. Some of these micro-organisms are beneficial to humans, while others can cause diseases.
2. How do micro-organisms help us in our daily lives?
Ans. Micro-organisms play a crucial role in our daily lives. They are used in the production of food, medicines, and vaccines. They help in the breakdown of organic matter, which is essential for the recycling of nutrients in the environment. They are also used for cleaning up oil spills and other environmental pollutants.
3. What are the harmful effects of micro-organisms?
Ans. Micro-organisms can cause diseases in humans, animals, and plants. They can cause infections, food spoilage, and decay. Some micro-organisms can produce toxins that can be harmful to humans and animals. They can also damage crops and cause economic losses.
4. How do we prevent the spread of harmful micro-organisms?
Ans. The spread of harmful micro-organisms can be prevented by practicing good hygiene, such as washing hands regularly, covering the mouth and nose while coughing or sneezing, and avoiding close contact with sick people. Food should be cooked properly, and surfaces should be cleaned and disinfected regularly.
5. Can all micro-organisms be classified as either friend or foe?
Ans. No, not all micro-organisms can be classified as either friend or foe. Some micro-organisms can be both beneficial and harmful, depending on the circumstances. For example, some bacteria are used in the production of yogurt and cheese, but they can also cause food poisoning if not handled properly. It is important to understand the nature of micro-organisms and how they interact with their environment to determine whether they are friend or foe.
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