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NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

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Introduction
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Introduction

  • Those living organisms around us which we cannot be seen with naked eyes 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.

Classification of microorganisms

(i) Bacteria

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

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

(ii) Fungi

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

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

(iii) Protozoa

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

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

(iv) Algae

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

  • Unicellular or multicellular autotrophic organisms
  • Contain chlorophyll pigment and carry out photosynthesis
  • For example: Chlamydomonas and Spirogyra

(v) Viruses

  • Viruses are also microscopic which 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: Influenza virus, polio virus.

Food Preservation

  • 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 controls the growth of microorganisms on food are called preservatives.
  • For example: sodium benzoate, sodium metabisulphite, salts and edible oil.
  • Common salt is used as preservative in pickles. It is also used to preserve meat and fish.
  • Sugar is used as 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 in killing 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.

Importance of microorganisms

(i) In food industry

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

(ii) In beverage industry

  • Yeast is used for 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.
  • 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.
  • Vaccine includes 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, small pox, hepatitis etc.
  • Vaccine for small pox was discovered by Edward Jenner.

(v) In increasing soil fertility

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

  • 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 in converting dead waste of plants and animals into simpler substances by the process of decomposition.

Nitrogen cycle

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

  • It involves circulation of nitrogen through living and non-living components of nature.
  • Nitrogen gas comprises 78% of the atmosphere.
  • First process of nitrogen cycle is fixation of nitrogen gas into nitrogenous compounds caused by bacterium Rhizobium and lightning.
  • Nitrogen compounds in soil are taken up by the plants through roots and used up in synthesis of plant proteins. Animals obtain nitrogen by feeding on plants.
  • Waste of plants and animals are 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.

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.
  • The example includes 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.

NCERT Summary: Microorganisms - Friend & Foe Notes | Study Science Class 8 - Class 8

Examples of human diseases caused by bacteria

  • Tuberculosis
  • Cholera
  • Typhoid

Examples of human diseases caused by virus

  • Measles
  • Chicken pox
  • Polio
  • Hepatitis-B

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 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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