Chapter 2: Microorganisms: friend and foe

These NCERT Class 8 Science Chapter 2 notes explain microorganisms and their role in our daily life. The chapter describes different types of microbes such as bacteria, viruses, fungi, protozoa and algae.

It highlights both useful and harmful roles of microorganisms. Students learn about food production, antibiotics, nitrogen fixation, food preservation, diseases, vaccination and the nitrogen cycle.

Chapter 2 – Microorganisms: Friend and Foe is important for understanding health, hygiene, agriculture and environment. It builds a strong foundation for school examinations and basic competitive exam preparation.

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1. Introduction: what are microorganisms?

• Microorganisms or microbes are very small living organisms.
• They cannot be seen with the naked eye.
• They can be observed only with the help of a microscope.
• Microorganisms may be useful or harmful to humans.
• They play an important role in nature and daily life.

2. Where are microorganisms found?

• Microorganisms are found in air, water and soil.
• They live inside the bodies of plants and animals.
• Some are found in hot springs, deep oceans and ice-cold regions.
• They can survive in extreme conditions of temperature and pressure.
• The place where microorganisms live is called their habitat.

3. Types of microorganisms

• Bacteria are single-celled organisms found everywhere, and some are useful while others are harmful.
• Examples of bacteria include Lactobacillus and Rhizobium.
• Fungi can be unicellular or multicellular and grow on dead and decaying matter, such as yeast and mould.
• Protozoa are single-celled organisms mostly found in water, such as Amoeba and Paramecium.
• Algae prepare their own food by photosynthesis, and viruses reproduce only inside a living host cell.

4. Microorganisms as friends in daily life

• Lactobacillus converts milk into curd by producing lactic acid.
• Yeast produces carbon dioxide during bread making, making it soft and spongy.
• Fermentation is the process of converting sugar into alcohol and carbon dioxide.
• Antibiotics like penicillin are medicines made from microorganisms to kill harmful bacteria.
• Some bacteria help in nitrogen fixation, improving soil fertility.

5. Microorganisms as decomposers

• Microorganisms break down dead plants and animals.
• They convert complex substances into simpler ones.
• This process helps in recycling nutrients in nature.
• Such microorganisms are called decomposers.
• Decomposition keeps the environment clean.

6. Microorganisms as foes

• Disease-causing microorganisms are called pathogens.
• Diseases in humans include tuberculosis, cholera, malaria and polio.
• Diseases in animals include anthrax and foot and mouth disease.
• Diseases in plants include citrus canker, rust of wheat and yellow vein mosaic.
• Many of these diseases are communicable diseases, which can spread from one person to another.

7. Mode of transmission of diseases

• Diseases can spread through air and water.
• They may spread through contaminated food.
• Direct physical contact can also spread infection.
• Some diseases spread through carriers like mosquitoes and houseflies.
• Maintaining hygiene reduces transmission.

8. Food spoilage and preservation

• Food spoilage is the damage of food due to growth of microorganisms.
• Signs include bad smell, change in taste, colour change and formation of mould.
• Food preservation prevents spoilage and increases shelf life.
• Methods include chemical preservatives like sodium benzoate and heat treatment like pasteurisation.
• Other methods include refrigeration, drying, salting and use of oil or vinegar.

9. Vaccination and immunity

• Vaccination is the process of introducing weakened or dead microorganisms into the body.
• It helps the body develop immunity against diseases.
• Examples include polio and smallpox vaccines.
• A vaccine prepares the body to fight infection.
• Vaccination prevents the spread of communicable diseases.

10. Nitrogen cycle in nature

• The nitrogen cycle is the circulation of nitrogen between atmosphere, soil, plants and animals.
• Nitrogen fixation converts atmospheric nitrogen into usable form.
• Nitrification and assimilation help plants absorb nitrogen compounds.
• Ammonification and denitrification return nitrogen to the atmosphere.
• This cycle maintains balance of atmospheric nitrogen in nature.

11. Key exam-oriented keywords explained

• Microorganisms: Very small living organisms visible only under a microscope.
• Bacteria: Single-celled microorganisms found everywhere; may be useful or harmful.
• Virus: A microorganism that reproduces only inside a living host cell.
• Fungi: Microorganisms that grow on dead and decaying matter.
• Protozoa: Single-celled microorganisms mostly found in water.
• Algae: Simple plant-like organisms that perform photosynthesis.
• Antibiotics: Medicines made from microorganisms to kill disease-causing bacteria.
• Pathogens: Microorganisms that cause diseases.
• Vaccination: Method of preventing disease by developing immunity.
• Nitrogen fixation: Conversion of atmospheric nitrogen into usable form by bacteria.
• Food preservation: Methods used to prevent spoilage of food.
• Nitrogen cycle: Movement of nitrogen between atmosphere, soil and living organisms.

12. One-page summary for quick revision

• Microorganisms are microscopic organisms found everywhere.
• They are useful in food production, medicine, agriculture and environmental cleaning.
• Some microorganisms act as pathogens and cause diseases.
• Vaccination and food preservation protect humans from harmful microbes.
• Microorganisms play a vital role in maintaining the nitrogen cycle.

Understanding NCERT Class 8 Science Chapter 2 is essential to learn how microorganisms affect human life, plants and animals. The chapter clearly explains both beneficial and harmful effects of microbes.

The concepts help students understand disease prevention, vaccination, food safety and environmental balance. It is highly useful for school exams and foundation level competitive preparation.

Continue reading NCERT Class 8 Science Chapter 3 – Synthetic Fibres and Plastics to understand man-made materials and their impact on environment.

FAQs

Q1. What are microorganisms?

Microorganisms are very small living organisms that can be seen only with a microscope.

Q2. Name the main types of microorganisms.

The main types are bacteria, viruses, fungi, protozoa and algae.

Q3. What is nitrogen fixation?

Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen into a usable form for plants.

Q4. What is vaccination?

Vaccination is the process of introducing weakened or dead microbes to develop immunity against diseases.

Q5. Why is this chapter important for exams?

It explains diseases, prevention, food preservation and environmental cycles, which are important for school exams and foundation preparation.

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