NCERT Revision Notes Chapter 2 Microorganisms: Friend and Foe Class 8 Science
CBSE NCERT Revision Notes1
Answer
• 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
2
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→ 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.
3
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→ Single-celled organisms
→ Found in wide range of habitats ranging from glaciers to deserts and hot springs
→ For example: curd bacteria (Lactobacillus)
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→ Multicellular, heterotrophic organisms
→ Lack chlorophyll and are generally found in colonies
→ For example: Penicillium, Aspergillus
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→ Unicellular or multicellular microorganisms
→ Usually found in water
→ For example: Amoeba and Paramecium
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→ Unicellular or multicellular autotrophic organisms
→ Contain chlorophyll pigment and carry out photosynthesis
→ For example: Chlamydomonas and Spirogyra
8
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→ 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.
9
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→ 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.
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→ 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.
11
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→ 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.