Class 8 Science Chapter 2 notes pdf Microorganism Friend And Foe notes

In this post, we will discuss Class 8 Science Chapter 2 notes pdf Microorganism: Friend And Foe. Class 8th science ch 2 notes help students in revision. In Microorganism Friend And Foe notes class 8, we will learn about microorganisms, the Classification of Microorganisms and the Use of Microorganisms. You can also download Microorganism Friend And Foe note class 8 pdf for further study. NCERT notes for Class 8 Science Chapter 2 are designed by our subject expert team.

In this Class 8th Science Ch 2 Notes pdf, we will learn about Microorganism: Friend And Foe.

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Class 8 Science Chapter 2 notes

  • Recall some experience.
    1. During the rainy season, moist bread gets spoilt and its surface gets covered with greyish-white patches. Do you notice where these patches come across?. Observe these patches through a magnifying glass. You will see tiny, black rounded structures. This is because of microorganisms.
    2. Have you ever think why milk gets spoilt in the summer season? This is because of microorganisms.


Microorganism:-Microorganisms are living organisms around us that we cannot see with naked eyes. These are called microorganisms or microbes.

  • They can live in all kinds of environments like ice, cold, hot, spring, desert and marshy land.
  • They may be unicellular or multicellular.
  • They are too small and are not visible by naked eyes.
  • They can be seen with a magnifying glass.
  • Microorganisms are found in air, water and in the body of plants and animals.
  • Some Microorganisms are beneficial for us whereas some are harmful and cause diseases.

Classification of Microorganisms

  • Bacteria,
  • Fungi,
  • Protozoa and
  • Some algae.


  • Viruses are also microscopic but are different from other microorganisms.
  • Viruses reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
  • Common ailments like cold, influenza (flu) and most coughs are caused by viruses.
  • Serious diseases like polio and chicken pox are also caused by viruses.

Friendly Microorganisms

  • Friendly Microorganisms are Microorganisms which are beneficial to us.
  • Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake.
  • The bacterium, lactobacillus converts milk into curd.
  • Microorganisms have been used for the production of alcohol, wine and acetic acid(vinegar).
  • They are also used in cleaning up the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria.
  • Bacteria are used in the preparation of medicines.

Medicinal Use of Microorganisms

  • some antibiotic tablets, capsules or injections such as penicillin are prepared by microorganisms. These medicines kill or stop the growth of the disease-causing microorganisms. Such medicines are called antibiotics.
  • A number of antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of them.
  • The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases.
  • They are also used to control many plant diseases.

Increasing Soil Fertility

Some bacteria are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

Harmful Microorganisms

Harmful Microorganisms are microorganisms which are harmful to us in many ways.

  • microorganisms cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens.
  • Some microorganisms spoil food, clothing and leather.

Disease-causing Microorganisms in Humans

Pathogens enter our body through the air we breathe, the water we drink or the food we eat.

Human DiseaseCausative MicroorganismMode of TransmissionPreventive Measures
Chicken Pox
Keep the patient in complete isolation. Keep the personal
belongings of the patient away from those of the others.
Vaccination to be given at
suitable age.
Maintain personal hygiene
and good sanitary habits.
Consume properly cooked food
and boiled drinking water.
Hepatitis AVirusWaterDrink boiled drinking water.
MalariaProtozoaMosquitoUse a mosquito net and
repellents. Spray insecticides
and control breeding of
mosquitoes by not allowing
water to collect in the
Some Common Human Diseases caused by Microorganisms

communicable diseases:- Diseases which spread from direct contact like air, water, food and physical contact from an infected person to a healthy person are called communicable diseases.

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

Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others. The diseases reduce the yield of crops.

PlantMicroorganismsMode of Transmission
Rust of wheatFungiAir,
wheat seeds
Yellow vein mosaic of
bhindi (Okra)
Some Common Plant Diseases caused by Microorganisms

Food Poisoning

Some harmful microorganisms grow in food and make food toxic. These make the food poisonous causing serious illness and even death. So, it is very important that we preserve food to prevent it from being spoilt.

Food Preservation

Food preservation means store-cooked food for a long time. There are some common methods
of preserving food in our homes.

  • Chemical Method
  • Preservation by Common Salt
  • Preservation by Sugar
  • Preservation by Oil and Vinegar
  • Heat and Cold Treatments
  • Storage and Packing

Preservatives:- Salts and edible oils are the common chemicals generally used to check the growth of microorganisms they are called preservatives.

Pasteurisation:- Pasteurised milk can be consumed without boiling as it is free from harmful microbes. The milk is heated to about 70 degrees C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis
Pasteur. It is called pasteurisation.

Nitrogen Fixation

Rhizobium is involved in the fixation of nitrogen in leguminous plants (pulses). Rhizobium lives in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship. Sometimes nitrogen gets fixed through the action of lightning. The atmospheric nitrogen is fixed by bacteria and blue-green algae present in the soil and converted into compounds of nitrogen called Nitrogen Fixation.

Nitrogen cycle

Our atmosphere has 78% nitrogen gas. Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll etc. The atmospheric nitrogen cannot be taken directly by plants and animals Certain bacteria and blue-green algae present in the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system. 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.

Class 8 Science Chapter 2 notes Nitrogen cycle
Nitrogen cycle

Nitrogen from the atmosphere is converted into nitrogen compounds by some bacteria like Rhizobium and blue-green algae present in soil and also by lighting. on another hand, certain bacteria present in soil nitrogen compounds nitrogen gas that is released into the atmosphere.

What we learn Microorganism Friend And Foe notes

In Class 8th Science Ch 2 Notes pdf / Microorganism: Friend And Foe Notes

  • Microorganisms are too small and are not visible to the unaided eye.
  • They can live in all kinds of environments, ranging from ice-cold climates to hot springs
  • and deserts to marshy lands.
  • Microorganisms are found in air, water and in the bodies of plants and animals.
  • They may be unicellular or multicellular.
  • Microorganisms include bacteria, fungi, protozoa and some algae. Viruses, though different from the above-mentioned living organisms, are considered microbes.
  • Viruses are quite different from other microorganisms. They reproduce only inside the host organism: bacterium, plant or animal cell.
  • Some microorganisms are useful for the commercial production of medicines and alcohol.
  • Some microorganisms decompose the organic waste and dead plants and animals into simple substances and clean up the environment.
  • Protozoans cause serious diseases like dysentery and malaria.
  • Some of the microorganisms grow in our food and cause food poisoning.
  • Some microorganisms reside in the root nodules of leguminous plants. They can fix nitrogen from the air into the soil and increase soil fertility.
  • Some bacteria present in the soil fix nitrogen from the atmosphere and convert it into nitrogenous compounds.
  • Certain bacteria convert compounds of nitrogen present in the soil into nitrogen gas which is released into the atmosphere.

What are microorganisms, and where can they be found?

Microorganisms are living organisms that are too small to be seen with the naked eye. They can be unicellular or multicellular and live in various environments like ice, hot springs, deserts, and marshy lands. They are found in air, water, and in the bodies of plants and animals.

Name and describe the types of microorganisms.

Bacteria: Single-celled organisms that can be found in various environments.
Fungi: Include organisms like molds and yeasts that decompose organic matter.
Protozoa: Single-celled organisms that can cause diseases.
Algae: Simple, plant-like organisms that can perform photosynthesis.
Viruses: Microscopic agents that reproduce only inside the cells of host organisms and cause diseases like the common cold and flu.

How are viruses different from other microorganisms?

Unlike other microorganisms, viruses cannot reproduce outside the cells of a host organism. They need to infect a bacterium, plant, or animal cell to replicate. They are responsible for diseases like polio and chickenpox.

List some beneficial uses of microorganisms.

Food Production: Used in making curd, bread, and alcoholic beverages.
Environmental Cleanup: Decompose organic waste into harmless substances.
Medicine: Production of antibiotics like penicillin.
Agriculture: Certain bacteria fix nitrogen, enriching soil fertility.

What are communicable diseases and how do they spread?

Communicable diseases are illnesses that can spread from an infected person to a healthy person through direct contact, air, water, or food. Examples include tuberculosis and influenza.

List and explain some common methods of food preservation.

Chemical Method: Using preservatives like salts and edible oils to inhibit microbial growth.
Preservation by Common Salt: Used for preserving fish and meat.
Preservation by Sugar: Used for making jams and jellies.
Preservation by Oil and Vinegar: Used for pickles and vegetables.
Heat and Cold Treatments: Sterilizing food by heating or preserving by freezing.
Storage and Packing: Using airtight containers to prevent microbial contamination

Explain the nitrogen cycle and its importance in the ecosystem.

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This conversion is necessary because atmospheric nitrogen is not directly usable by most organisms. Certain bacteria and algae convert nitrogen gas into compounds usable by plants, which are then consumed by animals. When plants and animals die, decomposers return nitrogen to the soil, which can be reused by plants or returned to the atmosphere by denitrifying bacteria.

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