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Science: Class 8 — Chapter 2: Microorganisms: Friend and Foe

2.1 Microorganisms
2.1.1 Introduction to Microorganisms
  • Microorganisms are living organisms around us that are so small in size that they cannot be seen with the unaided eye.
  • Visualizing Microbes requires a magnifying glass for larger structures like bread mould, or a microscope for tiny unicellular organisms.
2.1.2 Classification of Microbes
  • Major Groups of microorganisms are classified into four key categories: bacteria, fungi, protozoa, and some algae.
  • Common Examples include Chlamydomonas and Spirogyra (algae), Amoeba and Paramecium (protozoa), and Bread mould, Penicillium, and Aspergillus (fungi).
2.1.3 Viruses and Associated Diseases
  • Viruses are microscopic entities that reproduce only inside the host cells of bacteria, plants, or animals, remaining distinct from other microbes.
  • Viral Diseases include common ailments like cold, influenza (flu), and coughs, as well as serious conditions like polio and chicken pox.
  • Bacterial and Protozoan Illnesses range from dysentery and malaria (caused by protozoa) to typhoid and tuberculosis (caused by bacteria).
2.2 Where do Microorganisms Live?
2.2.1 Habitats and Cellular Structure
  • Cellular Forms of microorganisms can be unicellular (like bacteria, some algae, and protozoa) or multicellular (such as many algae and fungi).
  • Environmental Range of microbes spans across all extremes, from freezing ice-cold climates and hot springs to dry deserts and marshy lands.
  • Living Conditions include existing freely in the soil or water, growing on other organisms, and residing inside the bodies of animals and humans.
2.3 Microorganisms and Us
2.3.1 Friendly Microorganisms
  • Organic Waste Decomposition involves bacteria breaking down organic wastes like vegetable peels, animal remains, and faeces into harmless, usable substances.
  • Agricultural Application utilizes specific microbes in the soil to increase fertility by fixing atmospheric nitrogen.
2.3.2 Making of Curd and Bread
  • Lactobacillus is the specific bacterium that multiplies in milk to convert it into curd, promoting curd formation on a domestic scale.
  • Yeast Respiration produces carbon dioxide gas rapidly during respiration, which fills dough with bubbles and increases its volume.
  • Baking Industry utilizes the rapid multiplication and gas production of yeast to make breads, pastries, and cakes.
2.3.3 Commercial Use and Fermentation
  • Commercial Microbes are grown on natural sugars in grains like barley, wheat, rice, and fruit juices for the large-scale production of alcohol, wine, and acetic acid.
  • Fermentation is the chemical process of converting sugar into alcohol, discovered by Louis Pasteur in 1857.
2.3.4 Medicinal Use and Antibiotics
  • Antibiotics are medicines made from bacteria and fungi, such as streptomycin, tetracycline, and erythromycin, which kill or stop disease-causing microbes.
  • Penicillin was discovered in 1929 by Alexander Fleming after he observed green mould spores preventing bacterial growth in culture plates.
  • Medical Precautions require taking antibiotics only under qualified medical advice and finishing the full course to prevent drug resistance and protect helpful gut bacteria.
2.3.5 Vaccines and Immunity
  • Vaccination is the process of introducing dead or weakened microbes into a healthy body, prompting the immune system to produce antibodies to fight future infections.
  • Eradication Campaigns have successfully used vaccines to prevent diseases like cholera, tuberculosis, smallpox, and hepatitis, completely wiping out smallpox worldwide.
  • Smallpox Vaccine was discovered by Edward Jenner in 1798, paving the way for modern immunization.
2.3.6 Increasing Soil Fertility
  • Biological Nitrogen Fixers are specialized soil microbes, like cyanobacteria (blue-green algae), that capture and fix atmospheric nitrogen to enrich soil.
2.3.7 Cleaning the Environment
  • Decomposers are microorganisms that break down dead organic matter from plants and animals, converting them into simple nutrients recycled by other organisms.
2.4 Harmful Microorganisms
2.4.1 Pathogens and Communicable Diseases
  • Pathogens are disease-causing microorganisms that enter the body via air, water, food, or direct contact with an infected host.
  • Communicable Diseases are microbial infections that transmit from an infected person to a healthy one through environmental vectors, including common cold, cholera, and chicken pox.
2.4.2 Carriers of Disease (Insects and Animals)
  • Disease Carriers are vectors like the housefly, which collects pathogens from waste and transfers them to uncovered food, causing illness upon consumption.
  • Anopheles Mosquito acts as the carrier of the malaria parasite Plasmodium, while the female Aedes mosquito acts as the carrier of the dengue virus.
  • Breeding Control involves keeping surroundings dry and clean to prevent mosquitoes from breeding in stagnant water sources like coolers and flower pots.
2.4.3 Animal and Plant Diseases
  • Anthrax is a highly fatal human and cattle disease caused by the bacterium Bacillus anthracis, discovered by Robert Koch in 1876.
  • Plant Infections like Citrus Canker (bacterial), Rust of Wheat (fungal), and Yellow Vein Mosaic of Bhindi (viral) severely diminish crop yields.
2.4.4 Food Poisoning
  • Food Poisoning is a condition caused by consuming food containing toxic substances produced by the growth of spoilage microorganisms.
2.5 Food Preservation
2.5.1 Chemical Methods and Preservatives
  • Preservatives are chemicals like salts, edible oils, sodium benzoate, and sodium metabisulphite used to check microbial growth in food items like jams and squashes.
2.5.2 Preservation by Salt and Sugar
  • Salting dehydrates and preserves fish, meat, amla, raw mangoes, and tamarind by checking bacterial cell growth.
  • Sugar Preservation reduces the moisture content in sweet spreads like jams and jellies, inhibiting bacterial multiplication.
2.5.3 Preservation by Oil, Vinegar, and Temperature
  • Oil and Vinegar prevent microbial spoilage of pickles because food-spoiling bacteria cannot survive in such acidic environments.
  • Temperature Treatments utilize high heat (boiling) to destroy microbes, and low heat (refrigeration) to inhibit the growth of bacteria.
2.5.4 Pasteurisation and Storage
  • Pasteurisation is a process discovered by Louis Pasteur where milk is heated to 70oC for 15 to 30 seconds and then suddenly chilled to prevent micro-organic growth.
  • Sealed Packaging protects dry fruits and vegetables from air moisture and direct exposure to airborne microbes.
2.6 Nitrogen Fixation
2.6.1 Rhizobium and Leguminous Plants
  • Rhizobium Bacteria establish a symbiotic relationship in the root nodules of leguminous plants, converting atmospheric nitrogen into soluble forms.
2.7 Nitrogen Cycle
2.7.1 Atmospheric Nitrogen and the Cyclic Process
  • Atmospheric Nitrogen comprises 78% of the atmosphere and is a structural component of proteins, chlorophyll, nucleic acids, and vitamins.
  • Nitrogen Cycling involves continuous biological conversions where soil bacteria fix atmospheric nitrogen, plants absorb it, animals consume plants, and decomposers return nitrogen back to the soil and atmosphere.