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NCERT Notes Class 8 Science Chapter 2: Microorganisms: Friend and Foe

Last Updated : 07 Jun, 2023
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NCERT Notes for Class 8 Science Chapter-2: Microorganisms: Friend and Foe: Organisms that can be seen only through a microscope are known as Microorganisms or Microbes. These microorganisms are so small in size that they cannot be seen with the unaided eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes. They exist in vast numbers and play vital roles in our lives.

Friends and Foe explain that Microorganisms can be both our friends, providing numerous benefits, or our foes, causing harm and disease. Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa, and some algae. Viruses are also microscopic but are different from other microorganisms. Common ailments like colds, influenza (flu) and most coughs are caused by viruses. Serious diseases like polio and chicken pox are also caused by viruses. Diseases like dysentery and malaria are caused by protozoa(protozoans) whereas typhoid and tuberculosis (TB) are bacterial diseases.

Where do Microorganisms Live

Microorganisms are found in air, water, and in the bodies of plants and animals. They can live in all kinds of environments, ranging from ice-cold climates to hot springs and deserts to marshy lands.

Microorganisms and Us

Microorganisms play an important role in our lives. Let’s study them in detail:

  • Used in the making of curd and bread: Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. Bacteria and yeast are also involved in the making of cheese, and pickles and are helpful for fermentation of rice idlis and dosa batter.
  • Commercial Use of Microorganisms: Yeast is used for the commercial production of alcohol, wine, and vinegar by the process of Fermentation.
  • Medicinal Use of Microorganisms: Antibiotics are produced from bacteria and fungi. Streptomycin, tetracycline, and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria. Antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of 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.
  • Cleaning the Environment: Microorganisms decompose dead organic waste of plants and animals converting them into simple substances. These substances are again used by other plants and animals. Thus, microorganisms can be used to degrade harmful and smelly substances and thereby clean up the environment.

Harmful Microorganisms

Microorganisms are harmful in many ways. Let’s study their harmful activities:

  • Disease-causing Microorganisms in Humans: Disease-causing microorganisms are called pathogens. Pathogens enter our bodies through the air we breathe, the water we drink, or the food we eat. Microbial diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact are called communicable diseases. Examples of such diseases include cholera, the common cold, chicken pox, and tuberculosis. There are some insects and animals which act as carriers of disease-causing microbes. Housefly is one such carrier. Another example of a carrier is the female Anopheles mosquito, which carries the parasite of malaria (Plasmodium).
  • Disease-causing Microorganisms in Animals: Several microorganisms not only cause diseases in humans but also in other animals. For example, anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease in 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. Some common plant diseases are Citrus canker, Rust of wheat, and Yellow vein Virus Insect mosaic of Bhindi.
  • Food Poisoning: Microorganisms that grow on our food sometimes produce toxic substances. 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

Common methods of preserving food in our homes to save it from the attack of microorganisms are:

  • Chemical Method: Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore, they are called preservatives. Salt or acid preservatives are added to pickles to prevent the attack of microbes.
  • Preservation by Common Salt: Meat and fish are covered with common salt to check the growth of bacteria. Salting is also used to preserve amla, raw mangoes, tamarind, etc.
  • Preservation by Sugar: Jams, jellies, and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria that spoil food.
  • Preservation by Oil and Vinegar: The use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish, and meat are often preserved by this method.
  • Heat and Cold Treatments: The process of heating milk to about 700C for 15 to 30 seconds and then suddenly chilling and storing it is called Pasteurization. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur.
  • Storage and Packing: These days, dry fruits and even vegetables are sold in sealed air-tight packets to prevent the attack of microbes.

Nitrogen Fixation

Nitrogen fixation is a chemical process by which molecular nitrogen is converted into ammonia. Example: Bacterium Rhizobium is involved in the fixation of nitrogen in leguminous plants (Peas). Rhizobium lives in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship.

Nitrogen Cycle

Our atmosphere has 78% nitrogen gas. 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. Now, it can be utilized by plants from the soil through their root system.

Nitrogen is then used for the synthesis of plant proteins and other compounds. Animals feeding on plants get these proteins and nitrogen compounds. 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. Certain other bacteria convert some part of them to nitrogen which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Nitrogen Cycle

FAQs on Microorganisms: Friends and Foe

Q1: Name the microorganisms which can fix atmospheric nitrogen in the soil.


Bacteria such as Rhizobium, Azotobacter, and certain blue-green algae present in the soil fix atmospheric nitrogen in the soil.

Q2: Can microorganisms be seen with the naked eye? If not, how can they be seen?


Microorganisms are so tiny in size that they cannot be seen with the naked eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are also called microbes.

Q3: What are antibiotics?


Antibiotics are medicines that fight infections caused by bacteria in humans and animals by either killing the bacteria or making it difficult for the bacteria to grow and multiply.

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