CBSE Class 8 Science Microorganisms: Friend and Foe Worksheets

Microorganisms play a crucial role in various industries. From an exam perspective, knowing these is important for 3-mark questions. Key commercial uses include:

• Production of Alcohol and Wine: Yeast is used for the large-scale production of alcohol, wine, and beer through a process called fermentation. Yeast converts natural sugars present in grains like barley, wheat, and rice into alcohol.
• Baking Industry: Yeast is also a key ingredient in the baking industry for making bread, cakes, and pastries. It reproduces rapidly and produces carbon dioxide during respiration, which makes the dough rise.
• Production of Acetic Acid: Certain bacteria are used to produce acetic acid (vinegar) from alcohol.

This is a very important question for exams. Antibiotics are medicines produced by microorganisms (like certain bacteria and fungi) that kill or stop the growth of other disease-causing microorganisms. Examples include Streptomycin and Penicillin.

Important precautions to be followed are:

• Antibiotics should only be taken on the advice of a qualified doctor.
• One must complete the full course prescribed by the doctor.
• Antibiotics should not be taken unnecessarily, as they are ineffective against viral infections like the common cold or flu and can make beneficial bacteria in the body resistant.

This is a Higher Order Thinking Skills (HOTS) question. Viruses exhibit characteristics of both living and non-living entities, which is why they are considered unique.

• Non-living characteristics: Outside a host cell, a virus is an inert particle. It does not respire, metabolise, or grow. It can be crystallised and stored like a non-living chemical.
• Living characteristics: Once a virus enters a living host cell (like a plant, animal, or bacterium), it takes over the host's cellular machinery to reproduce and create thousands of new viruses. This ability to replicate is a key characteristic of living organisms.

Food preservation techniques prevent the growth of microorganisms and spoilage of food. This is a common 5-mark question. Five key methods are:

• Chemical Method: Preservatives like sodium benzoate and sodium metabisulphite are added to foods like jams and squashes to check the growth of microbes.
• Preservation by Common Salt: Salting is used to preserve meat, fish, amla, and raw mangoes. It draws out water from food items, preventing microbial growth.
• Preservation by Sugar: Sugar reduces the moisture content in foods like jams, jellies, and squashes, which inhibits the growth of spoilage bacteria.
• Heat and Cold Treatments: Boiling milk kills many microorganisms. Similarly, keeping food at low temperatures in a refrigerator inhibits the growth of microbes.
• Pasteurisation: This method is used for preserving milk. The milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored. This process, discovered by Louis Pasteur, prevents the growth of microbes.

A vaccine works by stimulating the body's own immune system to protect against a specific disease. When a vaccine, which contains weakened or dead microbes, is introduced into a healthy body, the body's immune system is triggered. It produces special proteins called antibodies to fight these invaders. These antibodies remain in the body, providing a 'memory' of the microbe. If the actual disease-causing microbes enter the body later, the immune system immediately recognises them and fights them off with the pre-existing antibodies, thus preventing the disease.

The Nitrogen Cycle is a critical topic, often asked as a 5-mark question. The key steps are:

• Nitrogen Fixation: Atmospheric nitrogen is converted into usable nitrogen compounds (like nitrates) by lightning or by nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of leguminous plants) and blue-green algae.
• Nitrogen Assimilation: Plants absorb these nitrogen compounds from the soil through their roots and use them to synthesise proteins and other essential compounds.
• Ammonification: When plants and animals die, bacteria and fungi in the soil decompose the nitrogenous waste and convert it into ammonia.
• Nitrification: Nitrifying bacteria in the soil convert this ammonia into nitrites and then into nitrates, which can be used by plants again.
• Denitrification: Denitrifying bacteria convert the nitrates back into gaseous nitrogen, which is released into the atmosphere, completing the cycle.

Communicable diseases are microbial diseases that can spread from an infected person to a healthy person through air, water, food, or physical contact. Examples include cholera, chickenpox, and tuberculosis.

Insects can act as carriers or vectors for these diseases. For example, the housefly is a common carrier. When a housefly sits on garbage or animal excreta, harmful pathogens stick to its body. When this same fly sits on uncovered food, it transfers these pathogens. A healthy person who consumes this contaminated food can get sick.

This is a frequently asked 1 or 2-mark question to test knowledge of pathogens.

• (a) Diseases caused by Bacteria: Tuberculosis (TB) and Cholera.
• (b) Diseases caused by Protozoa: Malaria (caused by Plasmodium) and Dysentery (caused by Amoeba).

This is a crucial concept-based question. It's important to know because the common cold and flu are caused by viruses, not bacteria. Antibiotics are specifically designed to target and destroy bacteria by disrupting their cell walls or metabolic processes. Viruses have a different structure and replicate inside host cells, making them immune to the effects of antibiotics. Taking antibiotics for a viral infection is not only useless but can also harm the beneficial bacteria in your gut and contribute to the serious problem of antibiotic resistance.