Microorganisms - Friend & Foe Summary Class 8 NCERT Summary

Introduction

• Those living organisms around us that cannot see with the naked eye are called microorganisms or microbes.
• Some of these can be seen with a magnifying glass, while some cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes.

Microorganisms

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa, and some algae.

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Classification of microorganisms

(i) Bacteria

• Single-celled organisms
• Found in a wide range of habitats ranging from glaciers to deserts and hot springs
• For example, curd bacteria (Lactobacillus)

(ii) Fungi

• Multicellular, heterotrophic organisms
• Lack chlorophyll and is generally found in colonies
• For example: Penicillium, Aspergillus

(iii) Protozoa

• Unicellular or multicellular microorganisms
• Usually found in water
• For example, Amoeba and Paramecium

(iv) Algae

• Unicellular or multicellular autotrophic organisms
• Contains chlorophyll pigment and carries out photosynthesis
• For example, Chlamydomonas and Spirogyra

(v) Viruses

• Viruses are also microscopic and reproduce only inside the cells of the host organism, which may be a bacterium, plant, or animal.
• Ultramicroscopic organisms
• Require host cells to reproduce and complete their life cycle.
• For example, the Influenza virus and poliovirus.

Importance of microorganisms

(i) In the food industry

• Lactobacillus bacteria promote the conversion of milk into curd.
• Yeast is used in the preparation of breads, pastries, and cakes.

(ii) In the beverage industry

• Yeast is used for the commercial production of alcohol, wine, and vinegar (acetic acid).
• Yeast acts on sugar and converts it into alcohol by the process of fermentation. Louis Pasteur discovered fermentation.

(iii) In medicine production

• Medicines produced by certain microorganisms to kill or stop the growth of other disease-causing microorganisms are called antibiotics.
• Antibiotics are obtained from bacteria and fungi.
• Commonly used antibiotics are streptomycin, tetracycline, and erythromycin.
• The first antibiotic, penicillin, was prepared by Alexander Fleming

(iv) In vaccine production

• Protection of the body from the attack of various disease-causing microorganisms through vaccines is known as vaccination.
• Vaccines include dead or weakened microbes that trigger the production of antibodies in the body.

• These antibodies help in preventing the attack from disease-causing microorganisms.
• Vaccination helps in controlling diseases such as cholera, polio, smallpox, hepatitis, etc.
• A vaccine for smallpox was discovered by Edward Jenner.

(v) In increasing soil fertility

• Blue-green algae and Rhizobium bacteria are called biological nitrogen fixers.
• They fix free atmospheric nitrogen to enhance soil fertility.

(vi) In cleaning the environment

• Microorganisms (decomposers) help convert the dead waste of plants and animals into simpler substances through the process of decomposition.

Harmful microorganisms

• Disease-causing microorganisms are called pathogens.
• Diseases in humans caused by microorganisms

• Diseases caused by microorganisms that spread from an infected person to a healthy person through air, water, or food are called communicable diseases.
• Examples include cholera, chicken pox, and tuberculosis.
• The organisms that transmit diseases from one place to the other are called carriers.

Example of carriers

• Housefly spreads diseases such as cholera, dysentery, and typhoid.
• Female Anopheles mosquito spreads malarial parasites.
• Female Aedes mosquito spreads dengue virus.

Examples of human diseases caused by bacteria

• Tuberculosis
• Cholera
• Typhoid

Examples of human diseases caused by virus

• Measles
• Chickenpox
• Polio
• Hepatitis-B

Chickenpox

Examples of human diseases caused by protozoa

• Malaria
• Sleeping Sickness

Diseases in animals caused by microorganisms

• Anthrax is caused by bacteria
• Foot and mouth disease in cattle is caused by a virus

Diseases in plants caused by microorganisms

• Citrus canker disease is caused by bacteria
• Rust of wheat is caused by fungi
• Yellow vein mosaic of Bhindi (Okra) is caused by virus

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Food Preservation

• The process of preventing the spoilage of food items by the action of microbes is called food preservation.
• Microorganisms act on food items and spoil them.

Methods of food preservation

(i) Chemical methods

• The chemicals that control the growth of microorganisms in food are called preservatives.
• For example, sodium benzoate, sodium metabisulphite, salts, and edible oil.
• Common salt is used as a preservative in pickles. It is also used to preserve meat and fish.
• Sugar is used as a preservative in jams and jellies.
• Oil and vinegar are used as preservatives in pickles and vegetables.

(ii) Heat and cold treatments

• Boiling the milk helps kill the microorganisms present in it.
• Pasteurization is a technique of preserving milk in which it is boiled to about 70°C for 15 to 30 seconds and then suddenly chilled and stored.

(iii) Storage and packing

• Dry fruits and vegetables are stored in sealed air-tight packets to prevent microbial attack.

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Nitrogen cycle

• It involves the circulation of nitrogen through living and non-living components of nature.
• Nitrogen gas comprises 78% of the atmosphere.
• The first process of the nitrogen cycle is the fixation of nitrogen gas into nitrogenous compounds caused by the bacterium Rhizobium and lightning.
• Nitrogen compounds in soil are taken up by the plants through roots and used up in the synthesis of plant proteins. Animals obtain nitrogen by feeding on plants.
• Waste of plants and animals is converted to nitrogenous compounds by the action of bacteria and fungi in the soil.
• Some bacteria convert nitrogenous compounds back to nitrogen to maintain atmospheric levels of nitrogen.