CBSE Class 8 Science Revision Notes Chapter 2

CBSE Class 8 Science Revision Notes Chapter 2 – Microorganisms: Friend and Foe

Microorganisms are covered in Chapter 2 of the CBSE Class 8 Science syllabus. Through this chapter, students will learn which microorganisms are useful and also those that cause diseases. Therefore, they will learn about new microorganisms, their uses, and many new concepts to understand and remember. 

Class 8 Science Chapter 2 Notes have been created by the Extramarks’ subject matter experts to make the chapter simpler to understand. Each chapter is thoroughly explained in these notes, all written in an organised and understandable style. Students may refer to these notes before an exam to review the chapter.


There are many organisms that live around us that we cannot see with our naked eye. Some of them, like the fungus on bread, can be seen with a magnifying glass. Some can only be seen under a microscope because they are so minute. 

Microorganisms, often known as microbes, are organisms that are invisible to the naked eye.

Microorganisms are broadly classified as follows:

  • Bacteria are a type of prokaryotic organism (they lack a membrane-bound nucleus and organelles) that make up the majority of microorganisms. They live in soil, water, and any other habitat on Earth. They can be a few micrometres long and come in a variety of shapes, such as spheres, rods, or spirals. Examples: Escherichia coli, coliform bacteria, and so on.
  • Fungi are eukaryotic organisms (they have a membrane-bound nucleus and organelles) that are typically heterotrophic in nature. Examples: Penicillium, bread mould, mushroom, etc.
  • Protozoa are a class of unicellular eukaryotic organisms that can be parasitic or can live independently and are heterotrophic (cannot prepare their own food). Examples: Amoeba and paramecium.
  • Algae are a type of photosynthetic eukaryotic organism that is mostly found in water. Examples: Spirogyra, Chlamydomonas, and other bacteria.

In nature, these microorganisms can be either harmful or beneficial.

The virus is a microorganism like the others in this class, but it is unique in that it can only reproduce in the body of a host, whether that is an animal, a plant, or a person. AIDS, herpes, rubella, zika, and other diseases are examples of a virus.

Where do Microorganisms Live?

Microorganisms can be single-celled or unicellular, as in bacteria and protozoa, or multicellular, as in fungi and animals. They live in water, air, and a variety of environmental conditions ranging from extreme cold to hot springs, deserts, and marshy lands. Some even live inside the bodies of animals and humans. Some develop on their own, while others thrive on the bodies of other animals.

Microorganisms and Us:

These microorganisms are vital in our lives. Some of them may be beneficial, while others may be detrimental and harmful.

Friendly Use of Microorganisms:

Microorganisms can be used in a variety of ways, including in the production of alcohol, bread, cakes, and medicines, as well as in agriculture and environmental cleaning.

Making curd and bread

  • Lactobacillus bacterium multiplies in milk, converting it into curd. Bacteria are also used in the production of cheese, pickles, and other food products.
  • Bacteria and yeast aid in the fermentation by converting sugar to alcohol in the rice batter for making idlis and dosas.
  • When yeast is mixed in a dough, it quickly multiplies and produces carbon dioxide bubbles that raise the dough. Hence, yeast is utilised to make bread, cakes, and other baked foods.

Commercial Use of Microorganisms: 

  • Yeast is a common ingredient in the production of alcohol, wine, and vinegar. This is accomplished by growing yeast on the natural sugar found in fruit juices such as grapes, and grains such as barley, wheat, rice, and so on.
  • Fermentation converts sugars to alcohol.

Medicinal Use of Microorganisms: 

  • Microorganisms are the source of widely used medicines and antibiotics (medicines that stop or kill disease-causing agents).
  • These are grown specifically as a cure for various diseases.
  • Antibiotics include streptomycin, erythromycin, azithromycin, and others.


  • When a pathogen, a disease-causing microorganism, enters our bodies, antibodies are produced against it. These antibodies are retained in our bodies, and we are forever protected from the disease.
  • A vaccination is a biological agent that induces the body to generate antibodies against a particular illness. As a result, a vaccination protects the body against that specific disease.
  • Many diseases, such as polio, tuberculosis, chickenpox, hepatitis, and others, are routinely vaccinated in children. This method has been used to eradicate smallpox. The Covid vaccine is currently doing the same.

Increasing Soil Fertility: 

  • Some bacteria, like rhizobium, which are found in the soil, typically in the roots of leguminous plants, have the ability to fix atmospheric nitrogen to replenish the soil with nitrogen, hence enhancing soil fertility and crop output.

Cleaning the Environment: 

  • Some microorganisms can decompose dead organic matter and convert it into simpler substances or manure that can be reused for animals or plants, thereby keeping the environment clean.

Harmful Uses of Microorganisms

Some microorganisms can harm humans, plants, and animals in a variety of ways and cause diseases. Pathogens, as previously stated, are disease-causing microorganisms.

Disease-Causing Microorganisms in Humans: 

  • Pathogens that are harmful to humans enter the body through the air, water, or food. They can then be transmitted to others through contact with an infected person or through an animal or insect.
  • Communicable diseases are those that can be transmitted through contact, water, or the air from an infected person to a healthy person. Examples include the plague, chickenpox, tuberculosis, the common cold, and ongoing COVID-19 infections.
  • When disease-causing microorganisms are carried by insects or animals, they are referred to as disease carriers. The female anopheles mosquito, for example, is a carrier of Plasmodium, which causes malaria.
  • The mode of transmission, microorganism type, the disease caused by these microorganisms, and prevention differ as shown below:
  • The general approach to preventing these illnesses involves isolating the afflicted person, and their possessions, maintaining personal hygiene and good habits, drinking boiled water, eliminating mosquito breeding grounds, eating food that has been properly cooked, and, most importantly, getting routine vaccinations against all of these illnesses.

Food Poisoning:

When microorganisms contaminate food, they can produce toxic substances that cause illness in the person who consumes it, such as vomiting, and necessitates hospitalisation. Food poisoning is the result of consuming microorganism-contaminated food.

Fast Preparation With Science Class 8 Chapter 2 Notes

Authentic and reliable study materials are easily accessible on  Extramarks’ website. CBSE Revision Notes for Class 8 Science Chapter 2 provides summarised points regarding all concepts in simple language with diagrams where necessary to help students understand the concepts clearly which is particularly useful while answering tricky questions. Furthermore, the notes are concise, allowing them to recall and reinforce concepts in less time.

Revising a chapter before an exam usually takes time, as important concepts need to be covered in every chapter. Extramarks Revision Notes for CBSE Class 8 Science Chapter 2 summarises key points in an easy-to-understand format. They help students to prepare for chapters quickly, which will boost students’ confidence before exams.

Microorganisms Friend and Foe: Class 8 Chapter 2 Revision Notes Summary

Microorganisms can be found everywhere. They are completely invisible to the naked eye. Some are beneficial to health, while others are harmful. Students will learn more about microorganisms that benefit human life and those that cause diseases. This chapter will cover four different types of microbes: protozoa, fungi, bacteria, and algae. 

The chapter begins with an in-depth examination of the world of microbes. These organisms are present in the environment, but can only be seen with the help of a microscope. An extensive discussion of bacteria follows. These are primitive unicellular organisms with prokaryotic nuclei. There are various types of bacteria that either help human life or cause diseases. One example can be probiotics. It is a medicated solution of living yeasts and bacteria that can be consumed directly to restore digestive balance.

The next section covers fungi, their types, and their uses. Fungi are visible to the naked eye and more advanced than bacteria. They belong to the plant kingdom and are parasitic or saprophytic. Though fungi are usually multicellular, some fungal species exist as unicellular organisms. Students will understand various applications of fungi covered in Extramarks Class 8 Chapter 2 Science Notes. One such use is in the fermentation process to make alcohol and curd. All other applications will be explained in simpler terms in these revision notes.

This section will explain protozoa and their harmful effects on human life. Protozoa are microbes with a single cell. They have a eukaryotic nucleus and are classified as animals. Protozoa can move around using their cilia, flagella, and other structures. 

The last section in Extramarks Science Class 8 Chapter 2 Notes describes viruses. These organisms are natural wonders. These organisms exhibit living characteristics within a host cell but exhibit no signs of life outside the cell. The viruses cause harm by causing diseases such as rabies, HIV, and polio.

Why Should You Follow Chapter 2 Science Class 8 Notes?

Extramarks’ notes will be a student’s last-minute revision guide providing all the necessary information in a nutshell, to revise the entire syllabus quickly and easily. Chapter 2 Science Class 8 Notes provides a simple explanation of all the concepts discussed in this chapter. It will help students understand the beneficial and maleficial effects of microbes, from their role in everyday life to the diseases they cause. Extramarks Class 8 Science Chapter 2 Notes can be used as a reference to revise important concepts from the chapter.

FAQs (Frequently Asked Questions)

1. What is the nitrogen cycle according to Chapter 2 of Class 8 Science?

The nitrogen cycle is the conversion of nitrogen in the environment into chemical forms. Plants require nitrogen to grow, but they cannot absorb it directly from the air. Bacteria and green algae are microorganisms that assist the process to break down nitrogen and replenish the soil with it. Plants then use these nitrogen compounds, and through plants, they reach other living organisms. It is a natural process which does not require any inference.

2. What are the uses and harmful effects of microorganisms as covered in Chapter 2 of Class 8 Science?

Microorganisms play a variety of roles in nature because they are as much a part of the environment as other living things. We use several microorganisms in cooking, commercial and medical applications, soil fertilisation, and other fields. On the other hand, some microorganisms cause disease in humans, plants, and animals. Viruses are also responsible for certain fatal diseases such as AIDS, influenza, etc. Fungi are also harmful and can lead to certain skin infections and allergies.