NCERT Explained: Class VIII Science, Chapter 2 - Microorganisms: Friend and Foe

Introduction to the Topic

In our everyday surroundings, we are surrounded by a vast world of living organisms that we cannot see with the naked eye. These tiny organisms are known as microorganisms or microbes. While some of these organisms are so small that they can only be seen through a microscope, their impact on our lives, our health, and our environment is immense. The study of these organisms is called microbiology.

The NCERT Class VIII Science Chapter 'Microorganisms: Friend and Foe' takes us on an intriguing journey to understand these invisible neighbors. As the title suggests, microorganisms play a dual role in our lives. On one hand, they are essential for processes like making curd, baking bread, and even producing life-saving medicines. On the other hand, they can be 'foes' by causing diseases in humans, animals, and plants, and by spoiling our food. Understanding this balance is crucial for science students and anyone interested in how the biological world functions.

Key Concepts Explained

1. Classification of Microorganisms

Microorganisms are classified into four major groups based on their structure and characteristics:

• Bacteria: Single-celled organisms that can be found almost everywhere. Examples include Lactobacillus and E. coli.
• Fungi: These can be unicellular (like yeast) or multicellular (like bread mould or mushrooms). They often grow on moist, decaying organic matter.
• Protozoa: Mostly single-celled organisms that can cause diseases. Examples include Amoeba and Paramecium.
• Algae: These are plant-like organisms that contain chlorophyll and can perform photosynthesis. Examples include Chlamydomonas and Spirogyra.

A Note on Viruses: Viruses are also microscopic, but they are different from the four groups mentioned above. They reproduce only inside the cells of a host organism, such as a plant, animal, or bacterium. Outside a host cell, they behave like non-living things, which makes them a unique bridge between the living and non-living worlds.

2. Friendly Microorganisms (Our Friends)

Microorganisms are used for various beneficial purposes in our daily lives and industries:

• Making Curd and Bread: The bacterium Lactobacillus promotes the formation of curd by multiplying in milk and converting it into curd. In baking, yeast is used because it reproduces rapidly and produces carbon dioxide during respiration. This gas fills the dough, making it increase in volume and giving bread its soft, fluffy texture.
• Commercial Use: Microorganisms are used for the large-scale production of alcohol, wine, and acetic acid (vinegar). Yeast is used for the commercial production of alcohol and wine by growing it on natural sugars present in grains like barley, wheat, and rice. The process of conversion of sugar into alcohol is known as fermentation, discovered by Louis Pasteur in 1857.
• Medicinal Use (Antibiotics): When we fall ill, doctors may give us antibiotic tablets or injections. These medicines, like Penicillin, are made from fungi and bacteria. They kill or stop the growth of disease-causing microorganisms. Common antibiotics include Streptomycin and Tetracycline.
• Vaccines: Vaccines protect humans and animals from several diseases like cholera, tuberculosis, smallpox, and hepatitis. When a vaccine is introduced into a healthy body, the body produces antibodies to fight the weakened or dead microbe. These antibodies remain in the body to protect us from future infections.
• Increasing Soil Fertility: Some bacteria, such as Rhizobium and blue-green algae, are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility. These are called biological nitrogen fixers.
• Cleaning the Environment: Microorganisms decompose dead organic waste of plants and animals, converting them into simple substances (nutrients) that are returned to the soil. This helps in keeping the environment clean.

3. Harmful Microorganisms (Our Foes)

Some microorganisms cause diseases in living beings. Such disease-causing microorganisms are called pathogens.

• Human Diseases: Pathogens enter our body through the air we breathe, the water we drink, or the food we eat. Diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact are called communicable diseases. Examples include the common cold, chickenpox, and tuberculosis.
• Carriers of Diseases: Some insects and animals act as carriers of disease-causing microbes. The female Anopheles mosquito carries the parasite of malaria (Plasmodium), and the female Aedes mosquito acts as a carrier of the dengue virus.
• Diseases in Animals: Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.
• Diseases in Plants: Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, and apple. These diseases reduce the yield of crops. For example, Citrus Canker (bacterial) and Rust of Wheat (fungal).

4. Food Preservation

Microorganisms can grow on our food and produce toxic substances, leading to food poisoning. To prevent this, we use various preservation methods:

• Chemical Method: Salts and edible oils are common chemicals used to check the growth of microbes. Sodium benzoate and sodium metabisulphite are common preservatives used in jams and squashes.
• Preservation by Common Salt: Meat, fish, amla, raw mangoes, and tamarind are preserved using salt to prevent bacterial growth.
• Preservation by Sugar: Sugar reduces the moisture content which inhibits the growth of bacteria that spoil food.
• Heat and Cold Treatments: Boiling kills many microorganisms. Similarly, keeping food in the refrigerator inhibits the growth of microbes because of low temperatures.
• Pasteurization: This process involves heating milk to about 70°C for 15 to 30 seconds and then suddenly chilling and storing it. This prevents the growth of microbes. This method was discovered by Louis Pasteur.

5. Nitrogen Fixation and the Nitrogen Cycle

Our atmosphere has 78% nitrogen gas. Nitrogen is an essential constituent of all living organisms as part of proteins, chlorophyll, nucleic acids, and vitamins. However, plants and animals cannot take atmospheric nitrogen directly. Rhizobium bacteria (living in the root nodules of leguminous plants) and blue-green algae fix nitrogen from the air into nitrogenous compounds. Once nitrogen is converted into these usable compounds, plants can absorb them from the soil through their root system. When plants and animals die, bacteria and fungi in the soil convert the nitrogenous wastes back into nitrogenous compounds to be used by plants again. Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Summary & Key Takeaways

• Microorganisms are tiny organisms that can only be seen with a microscope.
• They are classified into bacteria, fungi, protozoa, and algae. Viruses are distinct and require a host to reproduce.
• Friendly microbes help in making curd, bread, medicines, and cleaning the environment.
• Pathogens are harmful microbes that cause communicable diseases in humans, animals, and plants.
• Food preservation techniques like salting, sugaring, and pasteurization help prevent food spoilage.
• The Nitrogen Cycle is a natural process that maintains the atmospheric nitrogen levels through fixation and decomposition.