Class 8 Science Notes Chapter 2 (Chapter 2) – Examplar Problems Book
Alright class, let's focus on Chapter 2, 'Microorganisms: Friend and Foe' from your Science Exemplar book. This is a crucial chapter, not just for your class exams but also for various government exams where general science is a component. Pay close attention as we break down the key concepts.
Chapter 2: Microorganisms: Friend and Foe - Detailed Notes for Exam Preparation
1. Introduction to Microorganisms (Microbes)
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Definition: Living organisms that are extremely small and cannot be seen without the aid of a microscope.
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Major Groups:
- Bacteria: Unicellular prokaryotic organisms. Found everywhere. Can be beneficial (e.g., Lactobacillus in curd) or harmful (e.g., causing Tuberculosis, Cholera).
- Fungi: Eukaryotic organisms. Can be unicellular (Yeast) or multicellular (Moulds, Mushrooms). Saprophytic or parasitic. Used in making bread, antibiotics (Penicillium), but also cause diseases (Ringworm, Rust of wheat).
- Protozoa: Unicellular eukaryotic organisms. Mostly aquatic. Some are harmful parasites (e.g., Plasmodium causing Malaria, Amoeba causing Amoebic dysentery).
- Algae: Simple, plant-like eukaryotic organisms. Mostly aquatic. Can be unicellular (Chlamydomonas) or multicellular (Spirogyra). Photosynthetic. Some blue-green algae (Cyanobacteria) fix nitrogen.
- Viruses: Unique entities, considered on the border of living and non-living. They are microscopic but reproduce only inside the living cells of a host (bacteria, plant, animal). Cause diseases like Common Cold, Flu, Polio, Chicken Pox, COVID-19.
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Habitat: Microbes are ubiquitous – found in air, water, soil, hot springs, ice-cold regions, deserts, marshlands, and inside the bodies of other organisms.
2. Friendly Microorganisms: Our Allies
- Food Industry:
- Curd: Lactobacillus bacteria convert lactose sugar in milk into lactic acid, causing milk to coagulate into curd.
- Bread & Cakes: Yeast (fungus) undergoes anaerobic respiration (fermentation), producing Carbon Dioxide (CO2). This gas makes the dough rise (leavening).
- Alcoholic Beverages & Vinegar: Yeast ferments sugars (from grains, fruits) into alcohol (ethanol) – used in wine, beer production. Bacteria can further convert alcohol into acetic acid (vinegar).
- Fermentation: The process of conversion of sugar into alcohol or acids by the action of microorganisms (like yeast, bacteria). Discovered by Louis Pasteur (1857).
- Medicinal Uses:
- Antibiotics: Chemicals produced by certain bacteria and fungi that kill or inhibit the growth of other disease-causing microorganisms.
- Examples: Penicillin (from Penicillium fungus, discovered by Alexander Fleming, 1929), Streptomycin, Tetracycline, Erythromycin.
- Caution: Take antibiotics only on doctor's advice, complete the full course, avoid misuse to prevent antibiotic resistance.
- Vaccines: A preparation of dead or weakened pathogens (or their components) introduced into the body.
- Mechanism: Stimulates the immune system to produce antibodies against that specific pathogen. These antibodies remain and provide immunity against future infections by the same pathogen.
- Examples: Vaccines for Tuberculosis (BCG), Polio (OPV), Measles (MMR), Hepatitis, Smallpox (discovered by Edward Jenner, 1798).
- Antibiotics: Chemicals produced by certain bacteria and fungi that kill or inhibit the growth of other disease-causing microorganisms.
- Agriculture (Increasing Soil Fertility):
- Nitrogen Fixation: Certain bacteria (Rhizobium in root nodules of leguminous plants like peas, beans) and blue-green algae convert atmospheric nitrogen gas (N2), which plants cannot use directly, into usable nitrogen compounds (like nitrates) in the soil. These are called Biological Nitrogen Fixers.
- Environmental Cleaning:
- Decomposers: Bacteria and fungi break down complex organic matter (dead plants, animals, waste products) into simpler inorganic substances, returning nutrients to the soil and cleaning the environment.
3. Harmful Microorganisms: Our Foes
- Pathogens: Disease-causing microorganisms.
- Diseases in Humans:
- Communicable Diseases: Diseases that spread from an infected person to a healthy person through air, water, food, physical contact, or vectors.
- Modes of Transmission:
- Air: Common cold, Tuberculosis, Pneumonia, COVID-19
- Water/Food: Cholera, Typhoid, Hepatitis A, Amoebic Dysentery
- Direct Contact: Chicken Pox, Ringworm, Common Cold
- Vectors (Carriers): Insects or animals that transmit pathogens.
- Female Anopheles mosquito: Transmits Plasmodium (protozoan) causing Malaria.
- Female Aedes mosquito: Transmits Dengue virus.
- Housefly: Carries pathogens from garbage to food.
- Common Human Diseases (Know causative microbe, mode of transmission, prevention): Refer to Table 2.1 and 2.2 in NCERT Textbook.
- Bacterial: Tuberculosis, Cholera, Typhoid, Anthrax
- Viral: Common Cold, Flu, Measles, Chicken Pox, Polio, Hepatitis, Dengue, AIDS
- Protozoan: Malaria, Amoebic Dysentery
- Fungal: Ringworm, Athlete's foot
- Diseases in Animals:
- Anthrax: Serious bacterial disease (Bacillus anthracis) affecting cattle and humans.
- Foot and mouth disease: Viral disease in cattle.
- Diseases in Plants:
- Reduce crop yield and quality.
- Examples: Citrus canker (Bacteria; transmitted by air), Rust of wheat (Fungi; air/seeds), Yellow vein mosaic of Bhindi/Okra (Virus; insect vector).
- Food Spoilage: Microbes grow on food items, producing bad smell, taste, and texture changes.
- Food Poisoning: Caused by consuming food contaminated with pathogenic microbes or toxins produced by them. Symptoms include vomiting, diarrhoea, abdominal pain. Can be severe.
4. Food Preservation
- Purpose: To prevent microbial growth and spoilage, increasing the shelf life of food.
- Methods:
- Chemical Method: Adding preservatives like Salt, Sugar, Edible Oils, Vinegar. Sodium benzoate and Sodium metabisulphite are common chemical preservatives used in jams, squashes, etc.
- Common Salt: Draws out water (osmosis), inhibiting microbial growth. Used for fish, meat, amla, pickles.
- Sugar: Reduces moisture content. Used for jams, jellies, squashes.
- Oil and Vinegar: Create an environment where bacteria cannot survive. Used for pickles.
- Heat Treatment (Boiling/Heating): Kills most microbes.
- Cold Treatment (Refrigeration/Freezing): Inhibits microbial growth (does not kill).
- Pasteurization: Heating milk to ~70°C for 15-30 seconds, followed by rapid cooling and storage. Kills most harmful bacteria without significantly affecting taste/nutrients. Discovered by Louis Pasteur.
- Drying (Dehydration): Removing water content (e.g., sun-drying grains, vegetables).
- Vacuum Packing/Air-tight packing: Prevents contact with air and microbes. Used for nuts, chips, etc.
5. Nitrogen Cycle
- Importance: Nitrogen is essential for life (proteins, nucleic acids, chlorophyll, vitamins). Atmospheric nitrogen (N2, ~78%) cannot be used directly by most organisms. The nitrogen cycle ensures its continuous circulation and availability.
- Key Steps:
- Nitrogen Fixation: Conversion of atmospheric N2 into usable nitrogen compounds (ammonia, nitrates).
- Biological: By nitrogen-fixing bacteria (Rhizobium, Azotobacter) and blue-green algae.
- Atmospheric: By lightning.
- Nitrogen Assimilation: Plants absorb nitrogen compounds from the soil; animals obtain nitrogen by eating plants or other animals.
- Ammonification: Decomposers (bacteria, fungi) break down dead organisms and waste products (urea, excreta) into ammonia (NH3).
- Nitrification: Certain soil bacteria convert ammonia first into nitrites (NO2-) and then into nitrates (NO3-). Plants can absorb nitrates.
- Denitrification: Other soil bacteria (e.g., Pseudomonas) convert nitrates back into nitrogen gas (N2), which returns to the atmosphere.
- Nitrogen Fixation: Conversion of atmospheric N2 into usable nitrogen compounds (ammonia, nitrates).
Multiple Choice Questions (MCQs)
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Which of the following microorganisms contains chlorophyll and performs photosynthesis?
(a) Fungus
(b) Virus
(c) Alga
(d) Protozoan -
The process of converting sugar into alcohol by yeast is called:
(a) Pasteurization
(b) Fermentation
(c) Nitrogen Fixation
(d) Vaccination -
Which scientist discovered the vaccine for smallpox?
(a) Louis Pasteur
(b) Alexander Fleming
(c) Edward Jenner
(d) Robert Koch -
Rhizobium bacteria, found in the root nodules of leguminous plants, are examples of:
(a) Pathogens
(b) Decomposers
(c) Biological Nitrogen Fixers
(d) Fermenting agents -
Malaria is caused by a _________ and transmitted by the _________ mosquito.
(a) Bacterium, Female Aedes
(b) Virus, Male Anopheles
(c) Protozoan, Female Anopheles
(d) Fungus, Female Culex -
Which of the following is NOT a method of food preservation?
(a) Adding salt
(b) Adding sugar
(c) Boiling
(d) Leaving food open at room temperature -
Antibiotics are effective against diseases caused by:
(a) Viruses only
(b) Bacteria and Fungi
(c) Protozoa only
(d) All types of microbes -
Citrus canker is a plant disease caused by a:
(a) Fungus
(b) Virus
(c) Bacterium
(d) Protozoan -
In the nitrogen cycle, the process where bacteria convert nitrates back into nitrogen gas is called:
(a) Nitrification
(b) Denitrification
(c) Ammonification
(d) Nitrogen Fixation -
Pasteurization involves heating milk to about 70°C for 15-30 seconds and then:
(a) Boiling it further
(b) Adding preservatives
(c) Suddenly chilling and storing it
(d) Filtering it
Answer Key for MCQs:
- (c)
- (b)
- (c)
- (c)
- (c)
- (d)
- (b)
- (c)
- (b)
- (c)
Remember to revise these notes thoroughly and practice more questions from the Exemplar book itself. Good luck with your preparation!