Microorganisms Class 8 NCERT: Friend and Foe

What are Microorganisms?

Microorganisms, or microbes, are extremely tiny organisms that cannot be seen with the naked eye. We need a microscope to observe them. They exist everywhere around us—in air, water, soil, inside the bodies of plants and animals, and even in extreme environments like hot springs and ice-cold polar regions. Some microbes are unicellular (single-celled like bacteria), while others are multicellular (like many fungi and algae).

In the Class 8 NCERT syllabus, microorganisms are classified into five major groups:

1. Bacteria: Single-celled prokaryotic organisms (e.g., Lactobacillus, E. coli).
2. Fungi: Saprophytic or parasitic multicellular/unicellular organisms (e.g., Yeast, Bread Mould).
3. Protozoa: Unicellular eukaryotic organisms, often aquatic (e.g., Amoeba, Paramecium).
4. Algae: Autotrophic, photosynthetic organisms (e.g., Chlamydomonas, Spirogyra).
5. Viruses: Unique agents that are non-living outside a host cell but reproduce rapidly inside host organisms (e.g., Influenza virus, Bacteriophage).

Key Scientific Terms to Memorize

Pathogens

Disease-causing microorganisms that enter our body through air, water, food, or direct contact.

Fermentation

The chemical process of conversion of sugar into alcohol by yeast in the absence of oxygen.

Pasteurization

A food preservation process where milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled to prevent microbial growth.

Nitrogen Fixation

The biological process where atmospheric nitrogen is converted into usable nitrogenous compounds by microbes like Rhizobium.

Step-by-Step: How Vaccines Protect Our Body

1. Introduction of Weakened Pathogen — A dead or weakened form of the pathogen is introduced into a healthy body through vaccination.
2. Antibody Production — The body's immune system recognizes these harmless invaders and produces specific proteins called antibodies to fight them.
3. Memory Cell Formation — The antibodies and memory cells remain in the body, remembering how to fight the actual active microbe if it ever attacks again.
4. Active Defense — If the real active pathogen enters the body in the future, the immune system destroys it immediately using the prepared antibodies.

Crucial Distinction: Antibiotics vs Antibodies

Many students confuse antibiotics with antibodies during exams. Antibiotics are chemical medicines (like penicillin or streptomycin) produced by certain microorganisms to kill or stop the growth of disease-causing bacteria. Antibodies are protective proteins naturally produced by our body's own immune system to target and neutralize foreign pathogens. Remember: Antibiotics are effective ONLY against bacteria and fungi, not viral diseases like the common cold or flu!

Practice Questions with Solutions

• Q: Explain how yeast helps in the baking industry.
  A: Step 1: Yeast is added to dough consisting of flour, sugar, and warm water.
  Step 2: Yeast reproduces rapidly and respires anaerobically, releasing carbon dioxide (CO2) gas.
  Step 3: The bubbles of CO2 gas fill the dough and increase its volume, making it rise.
  Final answer: Yeast aids the baking industry by producing carbon dioxide gas during respiration, which causes the dough to rise and makes bread, cakes, and pastries soft and fluffy.

• Q: Why are blue-green algae and Rhizobium bacteria highly important for agriculture?
  A: Step 1: Plants cannot directly absorb gaseous nitrogen from the atmosphere.
  Step 2: Rhizobium bacteria (living in root nodules of leguminous plants) and blue-green algae fix atmospheric nitrogen into usable nitrogenous compounds.
  Step 3: These compounds mix with the soil and are absorbed by plants to synthesize proteins.
  Final answer: They are crucial because they perform biological nitrogen fixation, enriching the soil with nitrogen and increasing its fertility naturally without chemical fertilizers.

• Q: Mention three methods of food preservation and explain how they prevent food spoilage.
  A: Step 1: Chemical method: Adding preservatives like sodium benzoate or salt to create an environment where microbes cannot survive.
  Step 2: Heat and Cold treatments: Boiling kills existing microbes, and storing food in refrigerators keeps the temperature low, which inhibits microbe growth.
  Step 3: Drying (Dehydration): Removing moisture from food items (like grains or sun-dried vegetables) prevents fungi and bacteria from growing.
  Final answer: Food preservation methods like adding chemicals, using heat/cold treatments, and dehydration prevent food spoilage by eliminating moisture or creating unfavorable temperatures and conditions for microbial growth.

• Q: What are communicable diseases? Give two examples and describe how they spread.
  A: Step 1: Communicable diseases are microbial diseases that can spread from an infected person to a healthy person through air, water, food, physical contact, or vectors.
  Step 2: Examples include Common Cold, Tuberculosis, and Cholera.
  Step 3: When an infected person sneezes, fine droplets carrying pathogens are released into the air, which a healthy person might breathe in. Vectors like female Anopheles mosquitoes also transmit disease-causing protozoans (Plasmodium) by biting hosts.
  Final answer: Communicable diseases are infectious diseases spread via air, water, food, physical contact, or vectors. Examples include the common cold and malaria.

Frequently Asked Questions