Class 8 Science Notes Chapter 2 (Microorganisms: Friend and Foe) – Science Book

This summary of NCERT Class 8 Science Chapter 2: Microorganisms: Friend and Foe is designed to help you in cracking your exams, memorising contents easily.

NCERT Class 8 Science: Chapter 2 - Microorganisms: Friend and Foe (Detailed Notes for Exam Preparation)

1. Introduction to Microorganisms (Microbes)

• Definition: Microorganisms or microbes are living organisms that are too small to be seen with the naked eye. They require a microscope to be observed.
• Ubiquity: They are found everywhere – in air, water (hot springs, ice-cold water, salt water), soil (deserts, marshy lands), inside the bodies of other organisms (including humans), and even in extreme environments.
• Types: Can be unicellular (single-celled) or multicellular (many-celled).
• Existence: They can exist alone (like Amoeba) or in colonies (like fungi and bacteria).

2. Major Groups of Microorganisms

Microorganisms are broadly classified into five major groups:

• (a) Bacteria:
  • Single-celled prokaryotic organisms (lack a true nucleus).
  • Shapes: Spherical (cocci), Rod-shaped (bacilli), Spiral (spirilla), Comma-shaped (vibrio).
  • Examples: Lactobacillus (curd formation), Rhizobium (nitrogen fixation), Escherichia coli (E. coli).
• (b) Fungi:
  • Eukaryotic organisms (have a true nucleus).
  • Mostly multicellular (except Yeast, which is unicellular).
  • Saprophytic (feed on dead organic matter) or parasitic.
  • Examples: Yeast (used in baking/brewing), Penicillium (source of penicillin), Mushrooms, Bread mould (Rhizopus), Aspergillus.
• (c) Protozoa:
  • Single-celled eukaryotic organisms.
  • Mostly aquatic.
  • Many are motile (can move).
  • Examples: Amoeba, Paramoecium, Plasmodium (causes malaria), Trypanosoma (causes sleeping sickness).
• (d) Algae:
  • Simple, plant-like eukaryotic organisms.
  • Mostly aquatic.
  • Contain chlorophyll and perform photosynthesis (autotrophic).
  • Can be unicellular or multicellular.
  • Examples: Chlamydomonas (unicellular), Spirogyra (multicellular, filamentous), Blue-green algae (Cyanobacteria - technically bacteria but perform photosynthesis like algae).
• (e) Viruses:
  • Unique Nature: Considered on the border line between living and non-living.
  • Acellular (not made of cells).
  • Microscopic, much smaller than bacteria.
  • Obligate Intracellular Parasites: Reproduce only inside the cells of a host organism (bacteria, plant, or animal). Outside the host cell, they are inert.
  • Examples: Influenza virus (flu), HIV (AIDS), Polio virus, Bacteriophage (virus that infects bacteria), Coronavirus.

3. Friendly Microorganisms (Beneficial Roles)

Microorganisms are useful in various ways:

• (a) Making Curd and Bread:
  • Curd: The bacterium Lactobacillus promotes the formation of curd from milk. It multiplies in milk and converts lactose sugar into lactic acid, causing milk proteins to coagulate.
  • Bread/Cake: The fungus Yeast (Saccharomyces cerevisiae) reproduces rapidly and produces carbon dioxide (CO₂) during respiration (fermentation). Bubbles of CO₂ gas fill the dough and increase its volume, making bread/cakes soft and spongy.
• (b) Commercial Use:
  • Alcohol & Wine: Yeast is used for large-scale production of alcohol, wine, and acetic acid (vinegar).
  • Fermentation: The process of conversion of sugar into alcohol by yeast is called fermentation. Louis Pasteur discovered fermentation in 1857.
• (c) Medicinal Use:
  • Antibiotics: Medicines produced from microorganisms (bacteria and fungi) that kill or stop the growth of disease-causing microorganisms.
    • Examples: Penicillin (from fungus Penicillium notatum - discovered by Alexander Fleming in 1929), Streptomycin, Tetracycline, Erythromycin (derived from bacteria and fungi).
    • Precaution: Antibiotics should be taken only on the advice of a qualified doctor and the course must be completed. They are ineffective against viral diseases (like cold or flu).
  • Vaccines:
    • Principle: Introducing dead or weakened microbes (or their parts) into a healthy body. The body produces antibodies to fight these invaders. These antibodies remain in the body, providing immunity against the actual disease if encountered later.
    • Diseases Preventable by Vaccines: Cholera, Tuberculosis (TB), Smallpox, Hepatitis, Polio.
    • Discovery: Edward Jenner discovered the vaccine for smallpox in 1798 using cowpox.
    • Pulse Polio Programme: A government initiative in India for polio eradication.
• (d) Increasing Soil Fertility:
  • Nitrogen Fixation: Some bacteria (Rhizobium - found in root nodules of leguminous plants) and blue-green algae (Cyanobacteria) can fix atmospheric nitrogen gas (N₂) into usable nitrogen compounds (like nitrates) in the soil, enriching it. These microbes are called biological nitrogen fixers.
• (e) Cleaning the Environment:
  • Decomposition: Microorganisms (bacteria and fungi) decompose dead organic waste (dead plants, animals, faeces) into simpler, harmless substances. This cleans up the environment and returns nutrients to the soil.

4. Harmful Microorganisms (Foes)

Some microorganisms cause diseases in humans, plants, and animals.

• Pathogens: Disease-causing microorganisms are called pathogens.
• Modes of Transmission: Pathogens enter our body through:
  • Air we breathe (e.g., common cold, TB)
  • Water we drink (e.g., cholera, typhoid)
  • Food we eat (e.g., food poisoning, typhoid)
  • Direct contact with an infected person or carrier (e.g., chickenpox, common cold)
  • Animals/Insects (Vectors): Animals that carry pathogens from an infected person to a healthy person.
    • Examples:
      • Female Anopheles mosquito: Carries Plasmodium (protozoan) causing Malaria.
      • Female Aedes mosquito: Carries Dengue virus causing Dengue fever.
      • Housefly: Carries pathogens on its body, contaminates food.
• Communicable Diseases: Microbial diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact. Examples: Cholera, common cold, chickenpox, tuberculosis.

5. Common Human Diseases Caused by Microorganisms

6. Diseases in Animals

• Anthrax: Dangerous human and cattle disease caused by a bacterium (Bacillus anthracis). Discovered by Robert Koch.
• Foot and mouth disease: Caused by a virus in cattle.

7. Diseases in Plants

Microbes cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple etc., reducing crop yield.

8. Food Poisoning

• Cause: Consumption of food spoiled by microorganisms. Some microbes grow on food and produce toxic substances.
• Symptoms: Vomiting, stomach ache, diarrhoea, etc. Can be serious, even leading to death.
• Prevention: Proper food preservation is crucial.

9. Food Preservation

Techniques used to prevent the spoilage of food by inhibiting the growth of microorganisms.

• (a) Chemical Methods:
  • Preservatives: Salts and edible oils are common chemicals.
  • Examples: Sodium benzoate and Sodium metabisulphite are common preservatives used in jams, squashes to check microbial growth.
• (b) Preservation by Common Salt:
  • Used to preserve meat, fish, amla, raw mangoes, tamarind. Salt draws out water, inhibiting bacterial growth.
• (c) Preservation by Sugar:
  • Used in jams, jellies, squashes. Sugar reduces moisture content, inhibiting bacterial growth.
• (d) Preservation by Oil and Vinegar:
  • Used in pickles, vegetables, fish, meat. Bacteria cannot live in such an acidic or oily environment.
• (e) Heat and Cold Treatments:
  • Boiling: Kills many microorganisms.
  • Refrigeration: Low temperature inhibits the growth of microbes.
  • Pasteurization: Heating milk to about 70°C for 15-30 seconds and then suddenly chilling and storing it. This prevents the growth of microbes. Discovered by Louis Pasteur. Pasteurized milk can be consumed without boiling.
• (f) Storage and Packing:
  • Dry fruits and vegetables are often sold in sealed, airtight packets to prevent attack by microbes.

10. Nitrogen Cycle

• Importance: Nitrogen is an essential constituent of proteins, chlorophyll, nucleic acids, and vitamins. Atmospheric nitrogen (N₂) gas (about 78% of air) cannot be directly used by plants and animals.

• Definition: The cyclical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems.

• Key Steps & Microbial Roles:

  1. Nitrogen Fixation: Conversion of atmospheric nitrogen (N₂) into usable nitrogen compounds (ammonia, nitrates).
     • Atmospheric Fixation: By lightning.
     • Biological Fixation: By nitrogen-fixing bacteria (Rhizobium in root nodules of leguminous plants) and blue-green algae (Cyanobacteria) in the soil.
  2. Nitrogen Assimilation: Plants absorb nitrogen compounds (nitrates, nitrites, ammonia) from the soil through their roots and convert them into proteins and other organic compounds. Animals get nitrogen by eating plants or other animals.
  3. Ammonification: When plants and animals die, decomposers (bacteria and fungi) break down the nitrogenous organic matter (proteins, nucleic acids) into ammonia (NH₃).
  4. Nitrification: Soil bacteria convert ammonia (NH₃) first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). Nitrifying bacteria are involved (e.g., Nitrosomonas, Nitrobacter). Plants can absorb nitrates.
  5. Denitrification: Certain soil bacteria (denitrifying bacteria) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), which returns to the atmosphere.

• Result: The nitrogen cycle ensures that the percentage of nitrogen in the atmosphere remains relatively constant.

Key Scientists Mentioned:

• Louis Pasteur: Discovered fermentation (1857) and pasteurization.
• Alexander Fleming: Discovered penicillin (1929).
• Edward Jenner: Discovered the vaccine for smallpox (1798).
• Robert Koch: Discovered the bacterium (Bacillus anthracis) that causes anthrax disease (1876).

This comprehensive overview covers the essential points from Chapter 2 for effective revision for government exams. Focus on definitions, examples, specific names of microbes, diseases they cause, modes of transmission, prevention methods, beneficial roles, food preservation techniques, and the steps/microbes involved in the Nitrogen Cycle.