The Invisible Living World — Class 8 Science (Curiosity)

"There are more microorganisms in a single drop of pond water than there are people on Earth."

1. About the Chapter

This chapter opens the world of microorganisms (microbes) — living things too small to see with the naked eye, yet shaping every aspect of life on Earth.

What You Learn

• 5 major types of microorganisms
• Their beneficial uses (food, medicine, environment)
• Harmful effects (diseases)
• How we protect ourselves (immunity, vaccination, antibiotics)
• The role of microbes in food preservation and decomposition

Key Idea

Microorganisms are EVERYWHERE — air, water, soil, our bodies. Most are HARMLESS or BENEFICIAL. A few cause diseases.

2. What Are Microorganisms?

Definition

Microorganisms (or microbes) are living things so small they can only be seen using a microscope.

Discovery

Antonie van Leeuwenhoek (Dutch, 1670s) — built the first simple microscopes and observed 'animalcules' (microorganisms) in pond water. He is called the Father of Microbiology.

Where Found

• Air (e.g., during a sneeze)
• Water (ponds, rivers, oceans)
• Soil (especially fertile soil)
• Inside our bodies (gut, skin, mouth)
• Even in extreme places (hot springs, glaciers, deep sea)

3. The Five Major Types

1. Bacteria

• Single-celled
• Some have flagella for movement
• Found everywhere
• Shapes: rod (bacilli), sphere (cocci), spiral (spirilla), comma (vibrios)
• Examples: Lactobacillus (in curd), E. coli (in gut), Mycobacterium tuberculosis (TB)

2. Viruses

• NOT truly alive — need host cells to reproduce
• Smaller than bacteria
• Made of genetic material (DNA or RNA) inside protein coat
• Examples: influenza virus, coronavirus, HIV, polio virus

3. Fungi (singular: fungus)

• Some single-celled (yeast), some multicellular (moulds, mushrooms)
• Cannot photosynthesise — feed on dead matter or living organisms
• Examples: bread mould, mushrooms, yeast (Saccharomyces cerevisiae)

4. Protozoa

• Single-celled animal-like organisms
• Mostly in water
• Examples: Amoeba, Paramecium, Plasmodium (causes malaria)

5. Algae

• Plant-like, can photosynthesise
• Found in water (pond scum, seaweed)
• Examples: Spirogyra, Chlamydomonas, kelp

4. Beneficial Microorganisms

In Food Production

Curd / Yogurt:

• Lactobacillus bacteria convert milk sugar (lactose) into lactic acid
• Lactic acid curdles milk into curd
• Process: warm milk + a spoonful of curd → leave for 6-8 hours

Bread:

• Yeast (a fungus) ferments sugars in dough, releasing CO₂
• CO₂ bubbles make bread rise and become fluffy

Cheese, Idli, Dosa, Wine, Beer:

• All use fermentation by yeast or bacteria

Vinegar:

• Bacteria convert alcohol to acetic acid

In Medicine

Antibiotics:

• Penicillin (from Penicillium mould) — discovered by Alexander Fleming (1928)
• Streptomycin, tetracycline, erythromycin, etc.
• Used to kill harmful bacteria
• DO NOT work on viruses (no point taking antibiotics for colds!)

Vaccines:

• Weakened or killed microorganisms
• Trigger our immune system to develop defence
• Eradicated smallpox; nearly eradicated polio in India
• COVID-19 vaccines protected millions in 2020-2022

In Agriculture

Nitrogen fixation:

• Rhizobium bacteria in root nodules of leguminous plants (pea, beans, lentils)
• Convert atmospheric nitrogen to a form plants can use
• That's why farmers rotate crops — peas/beans REPLENISH soil nitrogen

Decomposition:

• Microbes break down dead matter into soil nutrients
• Without them, dead leaves and animals would pile up indefinitely

In Industry

• Production of citric acid, vitamins, ethanol
• Bioremediation (cleaning up pollutants)

5. Harmful Microorganisms (Diseases)

Bacterial Diseases

Viral Diseases

Fungal Diseases

• Ringworm (skin)
• Athlete's foot
• Dandruff

Protozoan Diseases

• Malaria (Plasmodium, spread by Anopheles mosquito)
• Amoebic dysentery (contaminated food/water)
• Sleeping sickness (tsetse fly in Africa)

6. How Diseases Spread

Modes of Transmission

1. Air — coughing, sneezing (TB, flu, COVID, common cold)
2. Water — drinking contaminated water (cholera, typhoid, hepatitis A)
3. Food — eating contaminated food (food poisoning, typhoid)
4. Contact — direct touch (ringworm, chickenpox, HIV)
5. Vectors — carriers like mosquitoes (malaria, dengue, chikungunya)
6. Soil — through wounds (tetanus)

Important Note

Communicable diseases spread from person to person. Non-communicable diseases (diabetes, hypertension) do NOT spread.

7. The Body's Defence — Immunity

Innate Immunity (born with it)

• Skin — physical barrier
• Mucus — traps microbes
• Stomach acid — kills swallowed microbes
• Sweat, saliva, tears — contain antimicrobial substances

Adaptive Immunity (learned)

• White blood cells (WBCs) — detect and destroy invading microbes
• Antibodies — proteins made by WBCs to neutralise specific microbes
• Memory cells — remember past infections so we recover faster next time

Vaccination

• Introduces a weakened/killed pathogen or its parts
• Body learns to recognise and defeat it
• When real disease appears, body is ALREADY prepared
• Edward Jenner (1796) — first vaccine (smallpox, using cowpox)
• Universal Immunisation Programme in India: polio, measles, hepatitis B, etc.

8. Food Preservation

Why Food Spoils

Microbes (bacteria, fungi) grow on food, producing toxins and decomposing it.

Methods of Preservation

1. Refrigeration — slows microbial growth (5°C)
2. Freezing — stops microbial growth (−18°C)
3. Boiling — kills most microbes
4. Pasteurisation — heating milk to 60-70°C, then cooling rapidly (preserves nutrition)
5. Salt — pulls water out of microbes (pickles, dried meat)
6. Sugar — high sugar prevents microbial growth (jams, jellies)
7. Oil and vinegar — pickles
8. Drying — removes water needed by microbes
9. Smoking — kills microbes, adds flavour
10. Canning — boil and seal (no air, no contamination)
11. Vacuum packing — removes oxygen
12. Chemical preservatives — sodium benzoate, etc.

9. Nitrogen Cycle (Brief)

Microbes drive the entire nitrogen cycle:

1. Nitrogen fixation: Rhizobium and azotobacter convert N₂ → NH₃ (ammonia)
2. Nitrification: Bacteria convert ammonia → nitrites → nitrates
3. Assimilation: Plants absorb nitrates, make proteins
4. Animals eat plants, use proteins
5. Decomposition: When dead, bacteria release nitrogen back
6. Denitrification: Some bacteria convert nitrates → N₂ (back to atmosphere)

Without microbes, all nitrogen would stay locked in atmosphere as N₂, useless to plants.

10. Worked Examples

Example 1: Identifying microbes

• A drop of curd contains Lactobacillus — what type? Bacteria
• Mushroom growing on dead wood — what type? Fungi
• Pond scum (green slimy mass) — what type? Algae

Example 2: Why don't antibiotics work on colds?

• Antibiotics work on BACTERIA, not VIRUSES.
• Cold is caused by a rhinovirus (a VIRUS).
• Therefore antibiotics are useless for colds.
• Cold is treated by rest, fluids, and the body's own immune system.

Example 3: Food preservation

A jar of jam stays good for months even at room temperature. Why?

• High sugar concentration pulls water out of any microbes by osmosis, killing them.
• This is why jams, jellies, and honey rarely spoil.

11. Common Mistakes

1. Antibiotics for viral infections

   • Antibiotics only work on BACTERIA, not VIRUSES.

2. All microbes are bad

   • Most microbes are HARMLESS or BENEFICIAL.

3. Yeast is a bacterium

   • Yeast is a FUNGUS.

4. Viruses are alive

   • Viruses are on the BOUNDARY of living and non-living.

5. Boiling kills all microbes instantly

   • Some bacterial spores survive boiling. Need pressure cooking for full sterilisation.

12. Conclusion

The invisible living world is a hidden universe that runs our planet:

• Bacteria make our food (curd, cheese, bread)
• Yeast raises our bread
• Antibiotics save lives from bacterial infections
• Vaccines protect us from killer diseases
• Soil bacteria keep our crops growing
• Decomposers recycle nutrients

But this world also brings danger — TB, cholera, COVID, malaria. Understanding microorganisms helps us:

• Cure diseases
• Prevent epidemics (washing hands, vaccination)
• Improve food production and preservation
• Protect the environment (bioremediation)

The next time you eat curd, take a vaccine, or recover from a cold — remember: it's the invisible world at work.