Notes of Ch 2 Microorganisms: Friend and Foe| Class 8th Science

Study Material and Notes of Ch 2 Microorganisms: Friend and Foe Class 8th Science

Topics in the Chapter

 • Introduction
• Microorganisms
• Classifications of microorganisms
• Food Preservation
• Methods of food preservation
→ Chemical methods
→ Heat and cold Treatments
→ Storage and Packing
• Importance of microorganisms
→ In food industry
→ In beverage industry
→ In medicine production
→ In vaccine production
→ In increasing soil fertility
→ In cleaning the environment
• Nitrogen cycle
• Harmful microrganisms
• Examples of carriers
• Diseases in humans, animals and plants by microorganisms


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

Classification of microorganisms

(i) Bacteria

→ Single-celled organisms
→ Found in 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 are 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
→ 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

→ 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

→ 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.

• Examples of human diseases caused by bacteria


• Examples of human diseases caused by virus

Chicken pox

• Examples of human diseases caused by protozoa

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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