Biology — Microorganisms
Overview
Microorganisms form a foundational topic in upper-primary science, bridging basic biology with real-world applications in health, agriculture, and industry. For KAR TET Paper II, this topic tests your understanding of the diversity of microscopic life, their characteristics, and their roles—both beneficial and harmful—in daily life.
Questions typically focus on classification of microorganisms, distinguishing features of bacteria, viruses, fungi, and other groups, as well as their practical applications and diseases they cause. You must also understand the five-kingdom classification system proposed by Whittaker, which provides the conceptual framework for understanding where microorganisms fit in the living world.
Mastering this topic requires memorising key differences between groups, understanding life processes at the microscopic level, and connecting theoretical knowledge to observable phenomena like food spoilage, fermentation, and infectious diseases.
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Key Concepts
- **Microorganisms** are living organisms too small to be seen with the naked eye; they include bacteria, viruses, fungi, protozoa, and some algae.
- **Whittaker's Five-Kingdom Classification** divides all living organisms into Monera (bacteria), Protista (protozoa, some algae), Fungi, Plantae, and Animalia based on cell structure, mode of nutrition, and body organisation.
- **Bacteria** are unicellular prokaryotes (no true nucleus) with cell walls; they reproduce by binary fission and can be autotrophic or heterotrophic.
- **Viruses** are acellular—they lack cellular structure and are considered at the borderline of living and non-living; they reproduce only inside a host cell.
- **Fungi** are eukaryotic organisms with cell walls made of chitin; they are heterotrophic and obtain nutrition through saprophytic or parasitic means.
- **Protozoa** are unicellular eukaryotes, mostly found in aquatic environments, and include pathogens like Plasmodium (malaria) and Amoeba.
- **Useful microorganisms** help in food production (curd, bread, alcohol), medicine (antibiotics), agriculture (nitrogen fixation), and waste decomposition.
- **Harmful microorganisms** cause diseases in humans, animals, and plants; disease-causing microorganisms are called **pathogens**.
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Formulas / Key Facts
| Feature | Bacteria | Virus | Fungi | |---------|----------|-------|-------| | Cell type | Prokaryotic | Acellular | Eukaryotic | | Cell wall | Present (peptidoglycan) | Absent | Present (chitin) | | Reproduction | Binary fission | Inside host only | Spores, budding, fragmentation | | Nutrition | Autotrophic or heterotrophic | Parasitic only | Saprophytic or parasitic | | Example | Lactobacillus, Rhizobium | HIV, TMV, Bacteriophage | Yeast, Penicillium, Mushroom |
**Important diseases and causative agents:**
- Tuberculosis — Mycobacterium tuberculosis (bacteria)
- Cholera — Vibrio cholerae (bacteria)
- Malaria — Plasmodium (protozoa)
- Common cold, COVID-19 — Viruses
- Ringworm, athlete's foot — Fungi
**Nitrogen cycle:** Rhizobium bacteria in root nodules of legumes fix atmospheric nitrogen into ammonia, enriching soil fertility.
**Antibiotics:** Penicillin was the first antibiotic, discovered by Alexander Fleming, produced by the fungus Penicillium. Antibiotics work against bacteria, not viruses.
**Pasteurisation:** Heating milk to about 70°C for 15–30 seconds kills harmful microorganisms without affecting taste—named after Louis Pasteur.
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Worked Examples
**Example 1:** Why are viruses considered to be at the borderline of living and non-living?
*Solution:*
- Living characteristics: Viruses contain genetic material (DNA or RNA) and can reproduce, though only inside a host cell.
- Non-living characteristics: Outside a host, viruses show no signs of life—no respiration, no metabolism, no growth. They can even be crystallised like chemicals.
- Conclusion: Because viruses show properties of both living and non-living matter depending on their environment, they are placed at the borderline.
**Example 2:** A farmer notices that his legume crops (like peas) improve soil fertility. Explain why.
*Solution:*
- Leguminous plants have root nodules containing Rhizobium bacteria.
- These bacteria are nitrogen-fixing—they convert atmospheric nitrogen (N₂) into ammonia and nitrates.
- Plants absorb these nitrogen compounds, and when the crop residue decomposes, nitrogen is released into the soil.
- This natural process enriches the soil, reducing the need for chemical fertilisers.
**Example 3:** Classify the following into bacteria, virus, fungus, or protozoa: (a) Amoeba, (b) Yeast, (c) TMV, (d) Lactobacillus
*Solution:*
- (a) Amoeba — Protozoa (unicellular eukaryote)
- (b) Yeast — Fungus (unicellular fungus used in baking and brewing)
- (c) TMV (Tobacco Mosaic Virus) — Virus
- (d) Lactobacillus — Bacteria (used in curd formation)
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Common Mistakes
- **Thinking viruses are bacteria:** Students often confuse the two. Remember—viruses are acellular and cannot survive independently; bacteria are complete cells. Antibiotics kill bacteria, not viruses.
- **Confusing prokaryotic and eukaryotic:** Bacteria are prokaryotes (no membrane-bound nucleus). Fungi, protozoa, plants, and animals are eukaryotes. This distinction is fundamental in the five-kingdom system.
- **Assuming all microorganisms are harmful:** Many students focus only on diseases. In exams, questions often test beneficial roles—curd making (Lactobacillus), bread rising (yeast), nitrogen fixation (Rhizobium), antibiotic production (Penicillium).
- **Forgetting that fungi have chitin, not cellulose:** Plant cell walls contain cellulose; fungal cell walls contain chitin. This is a frequent MCQ trap.
- **Mixing up disease-causing organisms:** Malaria is caused by Plasmodium (a protozoan), not a bacterium or virus. Tuberculosis is bacterial, not viral. Match each disease to the correct type of pathogen.
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Quick Reference
- **Five kingdoms:** Monera → Protista → Fungi → Plantae → Animalia
- **Bacteria = prokaryotic; Fungi = eukaryotic with chitin walls; Virus = acellular**
- **Rhizobium fixes nitrogen; Lactobacillus makes curd; Yeast ferments sugar**
- **Antibiotics work on bacteria only, not on viruses**
- **Pasteurisation kills pathogens in milk without boiling**
- **Viruses reproduce only inside living host cells—they are obligate parasites**