NCERT Science Notes - Class 8
Chapter 2 - Microorganisms : Friend and Foe

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 2 - Microorganisms : Friend and Foe. The chapter covers the diverse world of microorganisms, including bacteria, fungi, protozoa, and viruses. It explores their habitats, beneficial roles in food production, medicine, and agriculture, as well as their harmful effects, such as causing diseases.The chapter also discusses food preservation methods, nitrogen fixation, and the nitrogen cycle, highlighting the dual nature of microorganisms as both friends and foes.

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NCERT Science Notes - Class 8
Chapter 2 - Microorganisms : Friend and Foe

Microorganisms and Microbes

• Microorganisms or microbes are living organisms that are usually not visible to the naked eye.
• These include organisms like fungi, bacteria, viruses, etc.
• Examples in daily life:
  • During the rainy season, moist bread often spoils and develops greyish-white patches on its surface.
  • When observed through a magnifying glass, these patches reveal tiny black, rounded structures.
  • These structures are likely fungal spores, which grow in moist conditions.

Key Concepts

• Microbes are present all around us but are often unseen.
• They thrive in moist environments, particularly during humid seasons like the rainy season.
• Spoiling of food, such as bread, is a common sign of microbial growth.

2.1 - Microorganisms

Activity 2.1

• Objective: To observe microorganisms in soil.
• Procedure:
  • Collect moist soil in a beaker and add water.
  • After soil settles, observe a drop of the water under a microscope.
  • Observation: Tiny organisms may be visible.
    
    

Activity 2.2

• Objective: To observe microorganisms in pond water.
• Procedure:
• Take a few drops of pond water and spread them on a glass slide.
• Observe under a microscope.
• Observation: Tiny organisms moving around can be seen.

Key Insights

• Water and soil are full of tiny organisms, although not all are classified as microbes.
• Microorganisms (microbes) are extremely small and invisible to the naked eye.
• Example: Fungi growing on bread can be seen with a magnifying glass, but most microbes require a microscope.

Types of Microorganisms

• Microorganisms are classified into four major groups:
1. Bacteria
2. Fungi
3. Protozoa
4. Algae

Viruses

• Viruses are microscopic but different from other microorganisms.
• They reproduce only inside the cells of a host organism (bacterium, plant, or animal).
• Common viral diseases:
• Cold, influenza (flu), and coughs.
• Polio and chickenpox are also caused by viruses.

Microbial Diseases

• Protozoa: Cause diseases like dysentery and malaria.
• Bacteria: Cause diseases like typhoid and tuberculosis (TB).

Review (Based on Earlier Classes)

• The classification and impact of microorganisms have been studied in Classes VI and VII.

2.2 - Where do Microorganisms Live?

Types of Microorganisms

• Single-celled organisms:
  • Examples: Bacteria, some algae, and protozoa.
• Multicellular organisms:
  • Examples: Many algae and fungi.
    
    

Habitats of Microorganisms

• Microorganisms are extremely versatile and can live in a variety of environments:
  • Cold climates: Found in ice or cold regions.
  • Hot environments: Thrive in hot springs.
  • Deserts: Survive in dry, harsh conditions.
  • Marshy lands: Live in wet, swampy areas.
• Inside living organisms: Microorganisms can be found inside animals, including humans.
  
  

Modes of Existence

• Some microorganisms:
  • Live freely in nature, independent of a host.
  • Grow on other organisms, depending on them for survival.

2.3 Microorganisms and Us

Making of Curd and Bread Formation

• Bacteria play a vital role in turning milk into curd.
  • Lactobacillus, a type of bacterium, promotes curd formation.
  • It multiplies in warm milk and converts it into curd.
    
    

• A small amount of curd is added to warm milk to introduce Lactobacillus, which speeds up the curd-setting process for the next day.
  
  

• Cheese, pickles, and other food items are made with the help of bacteria.
• Curd is an important ingredient in dishes like rava (sooji) idlis and bhaturas due to its fermentation properties.
  
  

• Bacteria and yeast are crucial for the fermentation of rice-based dishes like idlis and dosas.
  • They help in making the batter rise and give the food its soft texture.

Procedure

• Take ½ kg of flour (atta or maida).
• Add sugar and warm water, and mix well.
• Add a small amount of yeast powder and knead to make a soft dough.
• Let the dough rest for two hours.


Observation

• After two hours, the dough rises, increasing in volume.


Explanation

• Yeast reproduces rapidly and, during respiration, produces carbon dioxide (CO₂).
  • The bubbles of CO₂ get trapped in the dough, causing it to expand and rise (Fig. 2.6).


Application

• This process is the basis for using yeast in the baking industry to make:
  • Breads
  • Pastries
  • Cakes

Commercial Use of Microorganisms

Large Scale Production

• Microorganisms are used in the commercial production of:
  • Alcohol
  • Wine
  • Acetic acid (vinegar)

Role of Yeast

• Yeast is specifically used for the commercial production of alcohol and wine.
  • Yeast is grown on natural sugars found in grains such as barley, wheat, rice, and crushed fruit juices.
  • During fermentation, yeast breaks down these sugars to produce alcohol.

Activity 2.4 - Fermentation Experiment

Procedure

• Take a 500 mL beaker and fill it up to ¾ with water.
• Dissolve 2-3 teaspoons of sugar in the water.
• Add half a spoon of yeast powder to the sugar solution.
• Cover the beaker and place it in a warm place for 4-5 hours.

Observation

• After 4-5 hours, the solution gives off a distinct smell.

Explanation

• The smell is that of alcohol, which is produced as the yeast converts sugar into alcohol.
• This process is called fermentation.

Discovery of Fermentation

• Louis Pasteur discovered the process of fermentation in 1857.

Medicinal Use of Microorganisms

• Antibiotics are medicines that either kill or stop the growth of disease-causing microorganisms.
  • Common forms: Tablets, capsules, injections (e.g., penicillin).
    
    

• Microorganisms (bacteria and fungi) are the main sources of antibiotics.
  • Examples of antibiotics:
    • Streptomycin
    • Tetracycline
    • Erythromycin
      
      

• Antibiotics are produced by growing specific microorganisms.
• They are used to treat a variety of human diseases.
  
  

• Antibiotics are added to the feed of livestock and poultry to prevent microbial infections.
• They are also used to control plant diseases.

Alexander Fleming's Experiment

• In 1929, Alexander Fleming was working on a culture of disease-causing bacteria.
• He noticed that green mould spores had accidentally grown on one of his culture plates.
  
  

Key Observation

• The presence of the mould prevented the growth of bacteria.
• The mould also killed many bacteria.

Penicillin

• The mould was identified as Penicillium.
• This led to the discovery of the first antibiotic, penicillin, which became a revolutionary treatment for bacterial infections.

Proper Use of Antibiotics

Important Guidelines

• Antibiotics should only be taken on the advice of a qualified doctor.
• It is essential to complete the prescribed course for the antibiotic to be effective.
  
  

Risks of Incorrect Use

• Taking antibiotics when not needed or in wrong doses can:
  • Make the drug less effective in the future (antibiotic resistance).
  • Harm the body by killing beneficial bacteria.
    
    

Limitations of Antibiotics

• Antibiotics are not effective against illnesses caused by viruses, such as cold and flu.

Vaccine

How Vaccines Work

• When a disease-causing microbe enters the body, the body produces antibodies to fight it.
• The body remembers how to fight the microbe if it enters again in the future.
• Vaccines work by introducing dead or weakened microbes into a healthy body.
  • The body fights and kills these microbes by producing antibodies.
  • These antibodies remain in the body, providing long-term protection from the disease.
    
    

Diseases Prevented by Vaccination

• Vaccines can prevent several diseases, including:
  • Cholera
  • Tuberculosis
  • Smallpox
  • Hepatitis
    
    

Historical Discovery

• Edward Jenner discovered the vaccine for smallpox in 1798.

Vaccinations in Childhood

Common Diseases Prevented by Vaccines

• In childhood, injections and vaccines are given to protect against various diseases, such as:
  • Polio
  • Diphtheria
  • Tetanus
  • Measles
  • Mumps
  • Rubella
  • Hepatitis
  • Tuberculosis
  • Whooping cough

Public Health Campaigns

• Pulse Polio Programme:
  • A public health initiative to provide polio drops (a vaccine) to children.
  • Advertised through TV and newspapers to raise awareness about polio prevention.
• Smallpox Eradication:
  • A global vaccination campaign successfully led to the eradication of smallpox in most parts of the world.

• Vaccines are now produced on a large scale from microorganisms to protect humans and animals from numerous diseases.

Increased Soil Fertility

• Certain bacteria can fix nitrogen from the atmosphere and enrich the soil.
• By increasing the nitrogen content, these bacteria help to enhance soil fertility.

Biological Nitrogen Fixers

• These nitrogen-fixing bacteria are referred to as biological nitrogen fixers.

CLEANING THE ENVIRONMENT

Composting: Manure Formation

• Boojho and Paheli observed that plant, vegetable, and fruit waste collected from nearby houses and gardens was placed in a pit for waste disposal.
• After some time, the waste decomposed and was converted into manure.

• Objective: To understand the process of decomposition.
• Materials:
  • Two pots (labeled A and B)
  • Soil (to fill half of each pot)
  • Plant waste (for Pot A)
  • Non-biodegradable items like polythene bags, glass bottles, plastic toys (for Pot B)
• Procedure:
  • Fill each pot halfway with soil.
  • Place plant waste in Pot A and non-biodegradable waste in Pot B.
  • Set both pots aside for 3-4 weeks.
• Observation:
  • After 3-4 weeks, observe how the contents of each pot have changed.
    
    

• Pot A: Plant waste will decompose, turning into manure.
• Pot B: Non-biodegradable items (plastic, glass) will not decompose.

Decomposition and Environmental Cleaning

Observation from Activity 2.5

• Pot A: The plant waste has decomposed and turned into manure.
  • This occurs due to the action of microbes that break down organic material.
  • The nutrients released during decomposition can be reused by plants.
• Pot B: The non-biodegradable items (polythene bags, bottles, plastic toys) did not decompose.
  • Microbes cannot act on these materials, so they do not break down or convert into manure.

• Microorganisms decompose dead organic waste from plants and animals, converting it into simple substances.
  • These simple substances are then recycled by other plants and animals.
• This natural process helps to clean up the environment by degrading harmful and smelly substances, like decaying plants and dead animals.

2.4 - Harmful Microorganisms

Harmful Microorganisms 

• Microorganisms can be harmful in various ways.
• Some microorganisms cause diseases in:
  • Humans
  • Plants
  • Animals
• These disease-causing microorganisms are known as pathogens.
  
  

• Certain microorganisms can spoil:
  • Food
  • Clothing
  • Leather

Disease-Causing Microorganisms in Humans

• Pathogens can enter the body through:
  • Air: Inhalation of pathogens present in the air.
  • Water: Consumption of contaminated water.
  • Food: Ingestion of food contaminated with pathogens.
  • Direct Contact: Touching an infected person or surfaces they have contaminated.
  • Animal Carriers: Pathogens can also be transmitted by animals.
    
    

Communicable Diseases

• Definition: Microbial diseases that can spread from an infected person to a healthy person through:
  • Air
  • Water
  • Food
  • Physical Contact
• Examples of communicable diseases include:
  • Cholera
  • Common Cold
  • Chicken Pox
  • Tuberculosis
    
    

Transmission of Common Cold

• When a person suffering from a common cold sneezes, fine droplets containing thousands of viruses are released into the air.
• These droplets can be inhaled by healthy individuals, leading to infection.
  
  

Prevention Measures

• Preventing the Spread of Communicable Diseases:
  • Use a Handkerchief: Cover the nose and mouth while sneezing to prevent droplets from spreading.
  • Maintain Distance: Keeping a safe distance from infected persons to minimize contact.

Disease-Carrying Insects and Animals

Insects as Carriers of Pathogens

• Some insects and animals act as carriers of disease-causing microorganisms.
  • Example: Housefly
    • Houseflies sit on garbage and animal excreta.
    • Pathogens adhere to their bodies.
    • When flies land on uncovered food, they can transfer pathogens, leading to illness.
    • Prevention: Always keep food covered and avoid consuming uncovered food.
      
      

Mosquitoes as Carriers

• Female Anopheles Mosquito
  • Carries the Plasmodium parasite, responsible for malaria.
• Female Aedes Mosquito
  • Acts as the carrier for the dengue virus.
    
    

Control Measures for Malaria and Dengue

• Preventing Mosquito Breeding:
  • Avoid Water Accumulation:
    • Do not let water collect in areas like:
      • Coolers
      • Tires
      • Flower pots
  • Keeping surroundings clean and dry helps prevent mosquito breeding.
• List of Measures to Avoid the Spread of Malaria:
  • Regularly empty and clean containers that can hold water.
  • Use mosquito nets while sleeping.
  • Apply mosquito repellent on exposed skin.
  • Install screens on windows and doors.
  • Seek medical help immediately if symptoms of malaria appear.

Table 2.1: Some Common Human Diseases caused by Microorganisms

Disease-Causing Microorganisms in Animals

• Microorganisms and Animals:
  • Certain microorganisms are pathogenic not only to humans and plants but also to various animals.
• Examples of Animal Diseases:
  • Anthrax:
    • Caused by the bacterium Bacillus anthracis.
    • Recognized as a dangerous disease affecting both humans and cattle.
    • Discovered by Robert Koch in 1876.
  • Foot and Mouth Disease:
    • Caused by a virus.
    • Primarily affects cattle and other livestock.

Key Points

• Pathogenic microorganisms can affect a range of living organisms, including animals.
• Understanding these diseases is important for both animal health and public health due to the potential for zoonotic transmission (diseases that can be transmitted from animals to humans).

Disease-Causing Microorganisms in Plants

• Impact on Agriculture:
• Various microorganisms are responsible for diseases in plants, leading to reduced crop yields.
  
  
• Examples of Plant Diseases:
• Citrus Canker:
• Causative Microorganism: Bacteria
• Mode of Transmission: Air
• Rust of Wheat:
• Causative Microorganism: Fungi
• Mode of Transmission: Air, seeds
• Yellow Vein Mosaic of Bhindi (Okra):
• Causative Microorganism: Virus
• Mode of Transmission: Insect
  
  
• Key Points
• Plant diseases can significantly affect food production and agriculture.
• Control measures often involve the use of specific chemicals to eliminate the pathogenic microorganisms.

Food Poisoning

2.5 - Food Preservation

Food preservation is crucial for maintaining the quality and safety of food, especially in the home environment. Microorganisms can spoil food, causing bad smells, altered taste, and color changes. To prevent these issues, various methods can be employed to preserve cooked food and other perishables.

Common Methods of Food Preservation

1. Chemical Method
   • Preservatives:
     • Common chemicals like salts and edible oils are used to inhibit the growth of microorganisms.
     • Examples include sodium benzoate and sodium metabisulphite, which are used in products like pickles, jams, and squashes to prevent spoilage.
2. Preservation by Common Salt
   • Usage:
     • Common salt has been a traditional preservative for meat and fish, where the items are covered with dry salt to stop bacterial growth.
     • Salting is also applied to preserve fruits like amla, raw mangoes, and tamarind.
3. Preservation by Sugar
   • Usage:
     • Sugar is used to preserve jams, jellies, and squashes. It reduces moisture content, inhibiting the growth of spoilage-causing bacteria.
4. Preservation by Oil and Vinegar
   • Usage:
     • Oil and vinegar are used in pickles to create an environment hostile to bacteria.
     • This method is also effective for preserving vegetables, fruits, fish, and meat.
5. Heat and Cold Treatments
   • Boiling:
     • Heating foods, such as boiling milk, kills harmful microorganisms. This is a common practice before storing or consuming milk.
   • Refrigeration:
     • Keeping food in the refrigerator at low temperatures slows down microbial growth, thereby extending its shelf life.
       
       

Pasteurization

• Definition:
  • Pasteurization is a process that makes milk safe for consumption without boiling. The milk is heated to about 70°C for 15 to 30 seconds and then quickly chilled and stored.
• Purpose:
  • This method effectively kills harmful microbes, ensuring the milk remains safe to drink. The process was discovered by Louis Pasteur.
    
    

Storage and Packing

• Modern Practices:
  • Dry fruits and vegetables are now sold in sealed, airtight packets. This packaging prevents the attack of microbes, extending shelf life and maintaining quality.

By employing these preservation techniques, food can be kept safe from spoilage, ensuring its quality and edibility over time.

2.6 - Nitrogen Fixation

• Bacterium Involved:
  • Rhizobium is the key bacterium associated with nitrogen fixation in leguminous plants, such as beans and peas.
• Location of Rhizobium:
  • This bacterium resides in the root nodules of leguminous plants, forming a symbiotic relationship where both the plant and the bacterium benefit.
• Nitrogen Fixation Process:
  • Rhizobium helps convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can utilize for growth.
• Other Methods of Nitrogen Fixation:
  • Nitrogen can also be fixed through natural processes, such as lightning, which causes atmospheric nitrogen to react with oxygen and form nitrogen oxides.
• Constant Nitrogen Levels:
  • The amount of nitrogen in the atmosphere remains constant despite these processes due to the balance between nitrogen fixation and denitrification (the conversion of nitrogen compounds back into nitrogen gas).

2.7 - Nitrogen cycle

NCERT Science Notes - Class 8 | Chapter 2 - Microorganisms : Friend and Foe