Microorganisms Friend and Foe for Class 8 Biology

Microorganisms, or microbes, are tiny living organisms that are usually seen only through a microscope. They are found in air, water, soil, food, and inside the bodies of plants and animals. Some can survive in extreme conditions such as hot springs, salty water, and very cold places.

The main groups studied in this chapter are bacteria, fungi, protozoa, algae, and viruses. Bacteria are usually unicellular. Fungi include yeast, moulds, and mushrooms. Protozoa are mostly unicellular and many live in water. Algae may be microscopic or larger and many can make food by photosynthesis.

Viruses are treated separately because they do not behave like complete living organisms outside a host cell. They multiply only inside living cells.

Curd formation happens because Lactobacillus bacteria multiply in warm milk and convert milk sugar into lactic acid. This acid changes the texture and taste of milk, forming curd.

Bread and cake become soft and spongy because yeast respires and releases carbon dioxide. The gas bubbles get trapped in the dough, making it rise. That is why dough kept with yeast becomes fluffy after some time.

Fermentation is the process by which microorganisms convert sugar into alcohol, acids, or gases. It is used in making items such as bread, idli batter, dosa batter, vinegar, and some beverages.

Antibiotics are medicines that kill or stop the growth of disease-causing microorganisms. Penicillin was the first widely used antibiotic, discovered from a fungus called Penicillium. Antibiotics should be taken only under medical advice because misuse can make them less effective.

Vaccines help the body develop immunity. A vaccine usually contains weakened, killed, or harmless parts of a disease-causing microbe. The body learns to recognise the invader and produces antibodies. Later, if the real pathogen enters, the body responds faster.

Common vaccine-preventable diseases include polio, measles, tuberculosis, diphtheria, whooping cough, and tetanus. The main idea is not that vaccines cure disease immediately; they prepare the immune system in advance.

Atmospheric nitrogen cannot be used directly by most plants. Nitrogen-fixing microorganisms convert atmospheric nitrogen into compounds that plants can absorb. Rhizobium bacteria live in root nodules of leguminous plants such as peas, beans, and pulses.

Blue-green algae also help fix nitrogen in soil, especially in paddy fields. This is why some microorganisms are called biofertilizers: they improve soil fertility naturally.

Decomposers such as bacteria and fungi break down dead plants, dead animals, and organic waste. This returns nutrients to the soil and keeps the environment clean.

Disease-causing microorganisms are called pathogens. Communicable diseases spread from an infected person to a healthy person through air, water, food, physical contact, or carriers. Examples include common cold, cholera, tuberculosis, malaria, measles, and chickenpox.

Malaria is caused by Plasmodium and spreads through the bite of female Anopheles mosquito. Dengue spreads through Aedes mosquito. Cholera commonly spreads through contaminated food and water. Common cold spreads through droplets released by coughing or sneezing.

Animals and plants can also suffer from microbial diseases. Anthrax affects animals and humans. Foot-and-mouth disease affects cattle. Plant diseases include citrus canker, rust of wheat, powdery mildew, and late blight of potato.

Food spoilage happens when microorganisms grow on food and change its smell, taste, colour, or texture. Spoiled food can cause food poisoning, so it should not be eaten.

Food preservation methods work by stopping or slowing microbial growth. Heating kills many microbes. Refrigeration slows their growth. Drying removes water. Salt and sugar draw water out of microbial cells. Oil and vinegar prevent microbial contact with air or create unfavourable conditions.

Common preservation methods include boiling, pasteurisation, refrigeration, deep freezing, drying, salting, sugaring, pickling in oil or vinegar, and using approved preservatives such as sodium benzoate.

For a 2-mark answer, define the term and give one example. For example: Antibiotics are medicines that kill or stop the growth of disease-causing microorganisms. Penicillin is an example.

For a 3-mark answer, write the idea, the process, and one example. For example, curd formation: Lactobacillus grows in warm milk, converts milk sugar into lactic acid, and changes milk into curd.

For a 5-mark answer, use headings. If the question asks why microorganisms are friends and foes, write useful roles under one heading and harmful roles under another. Give examples under each heading.