NCERT Class 8 Science Chapter 2 - Microorganisms: Friends and Foe Notes

Chapter 2 - Microorganisms: Friends and Foe Notes

1. Microorganisms (Microbes):
• Microorganisms are very small organisms which cannot be seen with the unaided eye. They can be seen only with a magnifying glass or microscope.
• Microorganisms may be unicellular or multicellular
• Microorganisms may exist alone or in colonies.
• Microorganisms are found in all kinds of environment like ice cold climate, hot springs deserts, marshy lands etc. They are also found inside the bodies of other organisms.

2. Where do Microorganisms Live?
Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi. They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely. Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.

3. Types of microorganisms:
There are four main types of microorganisms. They are bacteria, fungi, algae and protozoans. Viruses are also considered as microorganisms. They are different from other microorganisms because they reproduce only in the body of host organisms like bacteria, plants or animals.
Example -
• Algae - Chlamydomonas, Spirogyra etc.
• Fungi - Bread mould, Penicillium, Aspergilles etc.
• Protozoans - Amoeba, Paramaecium etc.

4. Viruses which affect bacteria are called bacteriophages.

5. The bacteria which are in rod shape are called bacilli.

6. Friendly microorganisms (useful microorganisms)
• Microorganisms like bacteria decomposes organic waste into manure and increases the fertility of the soil.
• Microorganisms help in fixing atmospheric nitrogen into nitrogen.
• Microorganisms are used for making curd from milk for making cheese, pickles, bread, cakes, pastries, alcohol, wine, vinegar etc.
• Microorganisms are used to make medicines and vaccines.

7. Making curd from the milk
The bacterium called Lactobacillus reproduces in milk and helps to convert milk into curd.

8. Making alcohol
The fungus called yeast reproduces in sugar solution and converts it into alcohol. This process is called fermentation.

9. Making bread
The fungus called yeast reproduces in flour dough and produces carbon dioxide during respiration which makes dough soft and helps in making bread, cakes, biscuits, pastries etc.

10. Making medicines
Some bacteria and fungi are used to make medicines which kill or stop the growth of disease causing microorganisms. Such medicines are called antibiotics. Example - Streptomycin, Tetracycline, Erythromycin etc.
Vaccines are dead or weakened microbes introduced into the body to produce antibodies. These antibodies protect the body from disease causing microbes. Disease like polio, cholera, typhoid, small pox, hepatitis etc can be prevented by taking vaccines.

11. Harmful microorganisms
• Some microorganisms cause diseases in plants and animals. Such disease causing microorganisms are called pathogens.
• Some micro organisms grow in food substances and produce toxic substances and makes the food poisonous. Food poisoning causes illness and even death.
• Some microorganisms spoil materials like clothing, leather, wood etc.

12. Disease causing microorganisms in Humans
• Disease causing microorganisms enter our body through air, water, food, contact and insects. These microorganisms are called pathogens.
• Disease which can spread from an infected person to a healthy person are called communicable diseases. Example: Cholera, common cold, chicken pox, tuberculosis etc.
• Some insects and animals act as carriers of disease causing microbes. Example: house fly, female anopheles mosquito (malaria), female aedes mosquitos (dengue fever) etc.

13. Disease causing microorganisms in plants
Miroorganisms cause diseases in plants like rice, wheat, potato, sugarcane, apple, orange etc. These diseases reduce crop yield. They can be controlled by using chemicals which kill the microbes.

14. Some Common Human diseases caused by Microorganisms

15. Some common plant diseases caused by Microorganisms

16. Food poisoning
• Could be due to consumption of food spoilt by microbes.
• So it is important to preserve food.

17. Food preservatives
• Chemical methods
→ Pickles can be preserved by using chemicals like salts and edible oils.
→ Jams and squashes can be preserved by using sodium benzoate, sodium meta bisuphate.
→ These chemicals are preservatives.

• Salting
→ Food items like fish, meat, amla, raw mangoes, tamarind etc can be preserved by using common salt.
→ It prevents the growth of microbes.

• By using sugar
→ Food items like jams, jellies, squashes etc can be prevented by using sugar solution.
→ It reduces moisture content which inhibits the growth of bacteria.

• By using Oil and Vinegar
→ Prevents the spoilage of pickles because bacteria cannot survive in such an environment.
→ Vegetables, fruits, fish, meat are often preserved by this method.

• Heat and Cold treatments
→ Heating food items kills microbes.
→ Storing food items at low temperature prevents the growth of microbes.

• Storage and package
→ Many food items are stored in air tight containers to protect them from microbes.

• Pasteurization
→ The process of heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling it to prevent the growth of microbes is called Pasteurization.
→ This process was discovered by Louis Pasteur.

18. Nitrogen Cycle


19. Process of Nitrogen Cycle
The nitrogen in the atmosphere is converted into nitrogen compounds in the soil by nitrogen fixing bacteria and blue green algae. Lightning also converted into nitrogen compounds in the soil. The nitrogen compounds in the soil is used by plants for the synthesis of proteins and other compounds. Animals feeding on plants get these proteins and other compounds. When plants and animals die, bacteria and fungi in the soil converts the nitrogenous waste into nitrogen compounds in the soil which are again use by plants. Some other bacteria converts some nitrogen compounds in the soil into nitrogen gas which goes back into the atmosphere. Hence the nitrogen in the atmosphere almost remains constant

20. Louis Pasteur discovered fermentation in 1857.

21. In 1929, Alexander Fleming was working on a culture of disease causing bacteria. Suddenly he found the spores of a little green mould in one of his culture plates. He observed that the presence of mould prevented the growth of bacteria. In fact, it also killed many of these bacteria. From this the mould penicillin was prepared.

22. Edward Jenner discovered the vaccine for small pox in 1798.

23. Robert Köch (1876) discovered the bacterium (Bacillus anthracis) which causes anthrax disease.

24. Louis Pasteur discovered the vaccine for rabies in 1885.

25. Antony van Leeuwenhoek discovered the first microscope.

26. Use in energy
Microbes are used in fermentation to produce ethanol, and in biogas reactors to produce methane. Scientists are researching the use of algae to produce liquid fuels, and bacteria to convert various forms of agricultural and urban waste into usable fuels.

27.  Importance of microbes
Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms dead remains and waste products through decomposition. Microorganisms also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of biosphere to on an improper balance of microorganisms.


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