NCERT Solutions and Notes for Class 8 Science Chapter 2: Microorganisms: Friend and Foe (Free PDF)

To great learning, notes are an important aspect. They also play a key part in revision hence eventually enabling a student to pass their exams. So, in NCERT Solutions and Notes Class 8 Science Chapter 2 Microorganisms: Friend and Foe we have provided summarised notes of the chapter. You also, get a free PDF of the solutions for you to download. These notes will help you recap the chapter within minutes. Let’s proceed!

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Explore all the Chapters of Class 8 Science:

Chapter 1	Chapter 2	Chapter 3
Chapter 4	Chapter 5	Chapter 6
Chapter 7	Chapter 8	Chapter 9
Chapter 10	Chapter 11	Chapter 12
Chapter 13

Class 8 Science Chapter 2 Microorganisms: Friend and Foe Notes

To begin with, below are the notes from Class 8 Chapter 2 Microorganisms: Friends and Foe. these notes have been prepared in such a way that it will be easy for you to understand, go through and revise the chapter. Let’s have a look at them.

Microorganisms

• The organisms that exist around us but we cannot see are known as microorganisms.
• The fungus can be seen with magnifying glass, other microbes can be seen under a microscope.
• Microbes are classified into 4 major groups:- Bacteria, fungi, protozoa and algae.
• Viruses are different from microorganisms.
• Viruses reproduce inside the host cells.
• Diseases like dysentery and malaria are caused by protozoa(protozoans) whereas typhoid and tuberculosis (TB) are bacterial diseases.

Where do Microorganisms live?

• Microbes may be single-celled like bacteria or may be multicellular like algae, or protozoa.
• They live in every environment ice cold climates to hot springs, and deserts to marshy lands.

Microorganisms and Us

• They are essential for us, some are advantageous, some harmful, and some are disease-causing.

Friendly Microorganisms

• They are used in making curd, bread, cakes, etc.
• Wastes like dead plant residues, vegetable peels, etc are broken down by them into simpler molecules.
• In agriculture, they are used to increase soil fertility by fixing nitrogen.

Making of Curd and Bread

• The making of curd involves the bacterium, Lactobacillus. It multiplies in milk and converts it into curd.
• Bacteria are also involved in the making of cheese, pickles and many other food items. An important ingredient of rava (sooji ) idlis and bhaturas is curd.
• Yeast is also helpful for fermentation of rice idlis and dosa batter.
• It is also used in the baking industry. It multiplies rapidly and produces carbon-dioxide.

Commercial use of Microorganisms

• Microorganisms are used for the large-scale production of alcohol, wine and acetic acid (vinegar).
• For this purpose, yeast is grown in sugars of grains such as wheat, barley, rice, etc.
• The process of conversion of sugar into alcohol is known as fermentation. 

Medicinal use of Microorganisms

• Microbes are the source of medicines.
• Medicines that kill or stop the growth of the disease-causing microorganisms are known as antibiotics.
• Several antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics that are made from fungi and bacteria.
• They are also used to control many plant diseases.
• Antibodies are produced to fight diseases causing microbes that enter the body.
• Weekend microbes are introduced inside the body of the individual, they lead to the creation of antibodies that fight the disease-causing microbes. These antibodies remain in the body forever hence protecting against disease. This is how vaccines work.
• Many diseases like cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

Increasing soil fertility

• Certain microbes that fix atmospheric nitrogen and enrich the soil with the same are known as biological nitrogen fixers, such as cyanobacteria.

Cleansing the Environment

• Biodegradable waste such as vegetable peels, dead plant residues, etc. are decomposed by the bacteria and get converted into manure.
• Microorganisms can be used to degrade harmful and smelly substances and thereby clean up the environment.

Harmful Microorganisms

• Some microbes are disease-causing and are known as pathogens.
• Some microorganisms spoil food, clothing and leather.

Disease-causing Microorganisms in Humans

• Pathogens can enter our bodies through water, food, air etc.
• Diseases can spread from an infected person to a healthy person through water, air, etc. Such diseases are known as communicable diseases. Examples:- cholera, common cold, chicken pox and tuberculosis.
• Some insects and animals act as carriers of disease-causing microbes such as flies, mosquitoes, etc.
• Female Anopheles mosquito carries the parasite of malaria (Plasmodium).
• Female Aedes mosquito acts as a carrier of the dengue virus.
• All mosquitoes breed in water.

Human Disease	Causative Microorganism	Mode of Transmission	Preventive Measures (General)
Tuberculosis Measles Chicken Pox Polio	Bacteria Virus Virus Virus	Air Air Air/Contact Air Water	Keep the patient in complete isolation. Keep the personal belongings of the patient away from those of the others. Vaccination to be given at a suitable age
Cholera Typhoid	Bacteria Bacteria	Water/Food Water	Maintain personal hygiene and good sanitary habits. Consume properly cooked food and boiled drinking water. Vaccination.
Hepatitis A	Virus	Water	Drink boiled drinking water. Vaccination.
Malaria	Protozoa	Mosquito	Use mosquito net and repellents. Spray insecticides and control the breeding of mosquitoes by not allowing water to collect in the surroundings.

Disease-causing Microorganisms in Animals

• Anthrax is a dangerous human and cattle disease caused by a bacterium.
• Foot and mouth disease of cattle is caused by a virus.

Disease-causing Microorganisms in Plants

• Microbes cause diseases in plants as well such as apples, wheat, rice, etc.
• They can be controlled by the use of chemicals.

Food Poisoning

• Food spoilt by microorganisms can cause food poisoning.
• We must preserve food to prevent it from being spoilt.

Food Preservation

• There are many methods by which food can be prevented from being spoiled.

Chemical Method

• Salts and edible oils are the common chemicals generally used to check the growth of microorganisms, hence, these are known as preservatives.
• Sodium benzoate and sodium metabisulphite are common preservatives used in jams and squashes to prevent them from getting spoilt.
• Salt or acid are added as preservatives to pickles to prevent the attack of microbes.

Preservation by Common Salt

• Common salt is used to preserve fish, meat, amla, raw mangoes, tamarind, etc.

Preservation by Sugar

• Sugar reduces the moisture content and is used in the preservation of jams, jellies and squashes.

Preservation by Oil and Vinegar

• It makes the environment unsuitable for bacteria.
• Vegetables, fruits, fish and meat are often preserved by this method.

Heat and Cold Treatments

• Milk is boiled before it is stored. It is heated to about 70 C for 15 to 30 seconds and then suddenly chilled and stored. This kills many microbes in it.
• The process of heating food to high temperatures and then instantly cooling it is called pasteurisation. It was discovered by Louis Pasteur.

Storage and Packing

• Dry fruits and even vegetables are sold in sealed air-tight packets to prevent the attack of microbes.

 Nitrogen Fixation

• Rhizobium is involved in the fixation of nitrogen in leguminous plants (pulses).
• It lives in the root nodules of leguminous plants such as beans and peas in a symbiotic association.
• Sometimes nitrogen gets fixed through the action of lightning.

Nitrogen cycle

• Our atmosphere has 78% nitrogen gas. Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.
• Although it cannot be directly used by plants and animals. 
• Certain bacteria fix this atmospheric nitrogen into compounds of nitrogen that can be taken up by plants from the soil. 
• When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again.
• Some percentage is released into the atmosphere making the percentage of nitrogen in the atmosphere constant.

Important Definitions of NCERT Class 8 Science Chapter 2 

Here are important definitions in Class 8 Science Chapter 2:

• The organisms that exist around us but we cannot see are known as microorganisms.
• The process of conversion of sugar into alcohol is known as fermentation. 
• Medicines that kill or stop the growth of the disease-causing microorganisms are known as antibiotics.
• Some microbes are disease-causing and are known as pathogens.
• Diseases can spread from an infected person to a healthy person through water, air, etc. Such diseases are known as communicable diseases. Examples:- cholera, common cold, chicken pox and tuberculosis.
• Salts and edible oils are the common chemicals generally used to check the growth of microorganisms, hence, these are known as preservatives.
• The process of heating food to high temperatures and then instantly cooling it is called pasteurisation. It was discovered by Louis Pasteur.

Explore all the Chapters of Class 8 Science:

Chapter 1	Chapter 2	Chapter 3
Chapter 4	Chapter 5	Chapter 6
Chapter 7	Chapter 8	Chapter 9
Chapter 10	Chapter 11	Chapter 12
Chapter 13

Download the NCERT Class 8 Science Chapter 2 Solutions PDF Here!

NCERT Solutions of Class 8 Science Chapter 2 Microorganisms: Friend and Foe

Now, let us explore NCERT Solutions of Class 8 Science Chapter 2 to get answers to all writing questions in the lesson microorganisms: friend and foe. 

Exercise Questions:

1. Fill in the blanks.

(a) Microorganisms can be seen with the help of a ____________.

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

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

Soln:

a) Microscope

b) Nitrogen

c) Yeast

d) Bacteria- Vibrio cholera

2. Tick the correct answer.

(a) Yeast is used in the production of

(i) sugar 

(ii) alcohol 

(iii) hydrochloric acid 

(iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate 

(ii) Streptomycin 

(iii) Alcohol 

(iv) Yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito 

(ii) cockroach 

(iii) housefly

iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant 

(ii) housefly 

(iii) dragonfly 

(iv) spider

(e) The bread or idli dough rises because of

(i) heat 

(ii) grinding 

(iii) growth of yeast cells 

(iv) kneading

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

(i) nitrogen fixation 

(ii) moulding 

(iii) fermentation 

(iv) infection

Soln:

a) (ii) alcohol

b) (ii) Streptomycin

c) (i) female Anopheles mosquito

d) (ii) housefly

e) (iii) growth of yeast cells

f) (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

Sol:-

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?

Soln:

No, microorganisms cannot be seen with the naked eye. They can be seen either with magnifying glass such as for brad mould, or they can be seen under a microscope.

5. What are the major groups of microorganisms?

Soln:

There are majorly 5 groups of microorganisms:-

1. Bacteria
2. Fungi
3. Protozoans
4. Algae
5. Viruses

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

Soln:

Rhizobium

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

Soln:

The uses of microorganisms in our lives are listed below.

• They help clean up the environment. For example, organic wastes (vegetable peels, dead plant residues, etc.) are broken down into harmless and usable substances by bacteria.
• Bacteria are also involved in the making of cheese, pickles and many other food items. An important ingredient of rava (sooji ) idlis and bhaturas is curd.
• Cheese, pickles, etc all involve the use of bacteria.
• Bacteria and yeast are also helpful for the fermentation of rice idlis and dosa batter.
• Yeast is used in the baking industry for making cakes.
• Microorganisms are used for the large-scale production of alcohol, wine and acetic acid (vinegar).
• Microbes are the source of medicines.
• Microorganisms help produce vaccines against disease-causing agents.
• Several antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics that are made from fungi and bacteria.
• Certain bacteria fix this atmospheric nitrogen into compounds of nitrogen that can be taken up by plants from the soil. 

8. Write a short paragraph on the harmful effects of microorganisms.

Soln:

Some microbes are disease-causing and are known as pathogens. Some microorganisms spoil food, clothing and leather. Pathogens can enter our bodies through water, food, air etc. and cause diseases that can spread from an infected person to a healthy person through water, air, etc. Such diseases are known as communicable diseases. Examples:- cholera, common cold, chicken pox and tuberculosis. Some insects and animals act as carriers of disease-causing microbes such as flies, mosquitoes, etc. Female Anopheles mosquito carries the parasite of malaria (Plasmodium). Female Aedes mosquito acts as a carrier of the dengue virus. Food spoilt by microorganisms can cause food poisoning.

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

Soln:

Medicines that kill or stop the growth of the disease-causing microorganisms are known as antibiotics. Several antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics that are made from fungi and bacteria.

Antibiotics should be taken only on the advice of a qualified doctor. Also, one must complete the course prescribed by the doctor.

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