Class 8th Science Chapter 2
Chapter 2 Microorganisms – Friend and Foe
🔬 1. Important 1–2 Line Notes
Microorganisms (microbes) are very tiny living organisms seen only with a microscope.
They are found in air, water, soil, and inside living bodies.
Four major groups: Bacteria, Fungi, Protozoa, Algae.
Viruses reproduce only inside a host cell.
Some microbes are useful (curd, medicines), some are harmful (diseases).
Antibiotics kill bacteria but do not work against viruses.
Vaccines protect us from diseases by forming antibodies.
Nitrogen fixing bacteria increase soil fertility.
Microbes help in decomposition and cleaning the environment.
🧫 2. Classification of Microorganisms
| Group | Example | Disease/Use |
|---|---|---|
| Bacteria | Lactobacillus | Curd formation |
| Fungi | Yeast | Bread making |
| Protozoa | Plasmodium | Malaria |
| Algae | Blue-green algae | Nitrogen fixation |
| Virus | Polio virus | Polio |
📌 3. Important Definitions
Microorganism: Tiny living organism seen only under microscope.
Pathogen: Disease-causing microorganism.
Antibiotic: Medicine that kills bacteria.
Vaccine: Substance that protects from disease.
Fermentation: Conversion of sugar into alcohol by yeast. (The process of milk into curd)
Pasteurisation: Heating milk at 70°C for 15–30 sec then cooling rapidly.
Nitrogen Fixation: Conversion of atmospheric nitrogen into usable form.
👨🔬 4. Important Personalities & Dates
Louis Pasteur (1857) – Discovered fermentation & pasteurisation.
Alexander Fleming (1929) – Discovered penicillin.
Edward Jenner (1798) – Discovered smallpox vaccine.
Robert Koch (1876) – Discovered anthrax bacterium.
🦠 5. Important Diseases (Repeated Board Question)
| Disease | Caused By | Spread By |
|---|---|---|
| Tuberculosis | Bacteria | Air |
| Cholera | Bacteria | Water |
| Malaria | Protozoa | Female Anopheles mosquito |
| Polio | Virus | Water |
| Dengue | Virus | Female Aedes mosquito |
📝 6. Repeated Questions with Solutions
Q1. What are the major groups of microorganisms?
Answer: Bacteria, Fungi, Protozoa, Algae and Viruses.
Q2. Why antibiotics are not effective against cold?
Answer: Cold is caused by viruses. Antibiotics work only on bacteria.
Q3. Why does bread rise?
Answer: Yeast produces CO₂ gas during respiration which increases dough volume.
Q4. What is nitrogen cycle?
Answer: Circulation of nitrogen between atmosphere, soil, plants and animals.
Q5. Why food gets spoiled?
Answer: Due to growth of microorganisms producing toxic substances.
🧪 7. Important Formulas & Rules
🌡 Pasteurisation Rule
Heat milk at 70°C for 15–30 seconds → Cool quickly → Prevent microbial growth.
🧪 Fermentation Formula
Sugar → (Yeast) → Alcohol + CO₂
🌾 Nitrogen Fixation Rule
Atmospheric N₂ → (Rhizobium) → Nitrogen compounds → Plants absorb
8. Important Locations
Root nodules → Found in leguminous plants (peas, beans)
Nitrogen cycle → Atmosphere ↔ Soil ↔ Plants ↔ Animals
Mosquito breeding → Stagnant water (coolers, tyres)
🔑 9. Important Keywords
Microorganism
Bacteria
Fungi
Protozoa
Virus
Pathogen
Vaccine
Antibiotic
Fermentation
Pasteurisation
Nitrogen
Fixation
Carrier
Communicable Disease.
⚡ 10. One-Line Facts (Very Important)
78% of atmosphere is nitrogen.
Yeast is a fungus.
Lactobacillus helps in curd formation.
Female Anopheles mosquito spreads malaria.
Smallpox has been eradicated globally.
Antibiotics should be taken only on doctor’s advice.
🧠 5 Marker Answer (Most Important)
Usefulness of Microorganisms (Write 5–7 points)
Making curd, bread, cheese.
Production of alcohol.
Making antibiotics.
Nitrogen fixation.
Decomposition of waste.
Cleaning environment.
Vaccine production.
🎯 Exam Strategy for This Chapter
Prepare Nitrogen Cycle diagram (very important).
Learn disease table properly.
Remember scientists with years.
Focus on difference between bacteria & virus.
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