Quick Revision Notes & Formula Sheet

Class 8 Science - Chapter: Microorganisms: Friend and Foe

Madhya Pradesh Board (MP Board)

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### 1. Introduction to Microorganisms

• Microorganisms (सूक्ष्मजीव): Organisms that are too small to be seen with the naked eye. They can be seen only with the help of a microscope.
• Branch of Science: The study of microorganisms is called Microbiology.
• Major Groups: Microorganisms are broadly divided into four major groups:
  1. Bacteria (जीवाणु): Single-celled disease-causing or helpful organisms (e.g., Lactobacillus, Rhizobium).
  2. Fungi (कवक): Mostly multicellular or single-celled plant-like organisms that lack chlorophyll (e.g., Bread mould, Penicillium, Yeast).
  3. Protozoa (प्रोटोजोआ): Single-celled microscopic animals (e.g., Amoeba, Paramecium, Plasmodium).
  4. Algae (शैवाल): Simple, plant-like, chlorophyll-containing organisms (e.g., Spirogyra, Chlamydomonas).
• Viruses (विषाাণु): Different from other microbes because they reproduce only inside the cells of the host organism (bacterium, plant, or animal). They act as a bridge between living and non-living things.

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### 2. Where do Microorganisms Live?

• Microorganisms can be unicellular (single-celled like bacteria, some algae, and protozoa) or multicellular (many fungi and algae).
• They survive in all types of environments: ranging from ice-cold climate to hot springs, and deserts to marshy lands.
• They are also found inside the bodies of animals and humans.

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### 3. Friendly Microorganisms (मित्र सूक्ष्मजीव)

Microorganisms are used for various purposes in our daily life:

1. Making Curd and Bread:
   • Lactobacillus bacterium promotes the formation of curd from milk.
   • Yeast reproduces rapidly and produces carbon dioxide during respiration, which puffs up dough to make bread, pastries, and cakes.
2. Commercial Use:
   • Fermentation (किण्वन): The process of conversion of sugar into alcohol by yeast is known as fermentation (discovered by Louis Pasteur in 1857).
3. Medicinal Use:
   • Antibiotics (प्रतिजैविक): Medicines produced by microorganisms (like bacteria and fungi) that kill or stop the growth of disease-causing microorganisms (e.g., Streptomycin, Erythromycin, Tetracycline). Discovered by Alexander Fleming (Penicillin).
   • Vaccines (टीका): Dead or weakened microbes introduced into a healthy body to produce antibodies and build immunity against diseases (e.g., polio, cholera, smallpox, hepatitis).

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### 4. Harmful Microorganisms (शत्रु सूक्ष्मजीव)

• Pathogens (रोगाणु): Disease-causing microorganisms.
• Communicable Diseases (संक्रामक रोग): Microbial diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact (e.g., Cholera, Common cold, Chickenpox, Tuberculosis, COVID-19).
• Vectors (वाहक): Insects and animals that carry disease-causing microbes.
  • Example 1: Female Anopheles mosquito carries the parasite Plasmodium (causes Malaria).
  • Example 2: Female Aedes mosquito acts as a carrier of Dengue virus.
  • Example 3: Houseflies sit on garbage and animal excreta, transferring pathogens to uncovered food.

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### 5. Common Human Diseases Caused by Microorganisms

Human Disease	Causative Microorganism	Mode of Transmission
Tuberculosis (TB)	Bacteria	Air
Measles	Virus	Air
Chickenpox	Virus	Air / Contact
Polio	Virus	Air / Water
Cholera	Bacteria	Water / Food
Typhoid	Bacteria	Water
Hepatitis B	Virus	Water
Malaria	Protozoa	Mosquito (Female Anopheles)

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### 6. Diseases in Plants Caused by Microorganisms

Plant Disease	Microorganism	Mode of Transmission
Citrus Canker	Bacteria	Air
Rust of Wheat	Fungi	Air, Seeds
Yellow Vein Mosaic of Bhindi (Okra)	Virus	Insect

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### 7. Food Preservation (खाद्य परीक्षण)

Methods to prevent food from getting spoiled by microbes:

1. Chemical Method: Using salts and edible oils as preservatives (e.g., Sodium benzoate, Sodium metabisulphite).
2. Preservation by Common Salt: Used to preserve meat, fish, amla, raw mangoes, and tamarind.
3. Preservation by Sugar: Reduces moisture content which inhibits bacterial growth (used in jams, jellies, and squashes).
4. Preservation by Oil and Vinegar: Prevents spoilage because bacteria cannot live in such an environment (used in pickles).
5. Heat and Cold Treatments: Boiling milk kills microbes; storing food in refrigerators inhibits microbe growth.
6. Pasteurization: Heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling and storing it to prevent microbe growth (discovered by Louis Pasteur).
7. Storage and Packing: Keeping dry fruits and vegetables in air-tight sealed packets.

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### 8. Nitrogen Fixation (नाइट्रोजन स्थिरीकरण)

• Atmospheric Nitrogen: Air contains 78% nitrogen gas, which plants cannot use directly.
• Rhizobium Bacteria: Lives in the root nodules of leguminous plants (like peas, beans) and fixes atmospheric nitrogen into usable compounds in the soil.
• Lightning: High temperature during lightning also fixes nitrogen in the atmosphere.
• Nitrogen Cycle (नाइट्रोजन चक्र): The continuous circulation of nitrogen through air, plants, animals, and soil to maintain a constant percentage of nitrogen in the atmosphere.