Biological Classification
A Deep Dive into the Kingdoms of Life
1. History of Classification
The earliest attempts at classifying organisms were based on simple, observable characteristics. Aristotle made the first scientific attempt, classifying plants into trees, shrubs, and herbs, and animals based on the presence or absence of red blood.
Two Kingdom Classification
Proposed by Carolus Linnaeus, this system divided all life into two kingdoms: Plantae and Animalia. However, it had major drawbacks as it failed to distinguish between prokaryotes and eukaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms (like fungi).
2. The Five Kingdom Classification
Proposed by R.H. Whittaker in 1969, this is the most widely accepted system. It classifies organisms into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. The main criteria for this classification were:
- Cell Structure (Prokaryotic/Eukaryotic)
- Body Organisation (Unicellular/Multicellular)
- Mode of Nutrition (Autotrophic/Heterotrophic)
- Reproduction
- Phylogenetic Relationships (Evolutionary history)
| Character | Kingdom Monera | Kingdom Protista | Kingdom Fungi | Kingdom Plantae | Kingdom Animalia |
|---|---|---|---|---|---|
| Cell Type | Prokaryotic | Eukaryotic | Eukaryotic | Eukaryotic | Eukaryotic |
| Cell Wall | Non-cellulosic | Present in some | Present (Chitin) | Present (Cellulose) | Absent |
| Body Organisation | Unicellular | Unicellular | Multicellular/loose tissue | Tissue/Organ | Tissue/Organ/System |
| Mode of Nutrition | Autotrophic & Heterotrophic | Autotrophic & Heterotrophic | Heterotrophic | Autotrophic | Heterotrophic |
3. Kingdom Monera
This kingdom includes all prokaryotic organisms, with bacteria as its sole members. They are the most abundant microorganisms and are cosmopolitan in habitat.
Shapes of Bacteria
Bacteria are grouped based on shape: Coccus (spherical), Bacillus (rod), Vibrium (comma), and Spirillum (spiral).
a) Archaebacteria
These "ancient bacteria" live in harsh habitats due to their unique cell wall structure. They include halophiles (salty areas), thermoacidophiles (hot springs), and methanogens (marshy areas, gut of ruminants).
b) Eubacteria ("True Bacteria")
These are characterized by a rigid peptidoglycan cell wall.
- Cyanobacteria (Blue-Green Algae): Photosynthetic autotrophs with chlorophyll 'a'. Some can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena).
- Chemosynthetic Autotrophs: Oxidize inorganic substances for energy. They play a key role in nutrient cycling.
- Heterotrophic Bacteria: The majority are decomposers. Some are pathogens (Vibrio cholerae), while others are beneficial (making curd, antibiotics).
c) Mycoplasma
These are the smallest living cells, which completely lack a cell wall. They can survive without oxygen and are often pathogenic.
4. Kingdom Protista
This kingdom includes all unicellular eukaryotic organisms and forms a link between the other kingdoms. They are primarily aquatic.
Major Groups of Protists
- Chrysophytes: Includes diatoms and golden algae. Diatoms have indestructible silica cell walls that form diatomaceous earth. They are the "chief producers" in oceans.
- Dinoflagellates: Marine, photosynthetic protists. Some, like Gonyaulax, cause red tides.
- Euglenoids: Mixotrophic organisms like Euglena, which are photosynthetic in light but heterotrophic in the dark.
- Slime Moulds: Saprophytic protists that form a large aggregation called a Plasmodium.
- Protozoans: Heterotrophic protists divided into four groups: Amoeboid, Flagellated, Ciliated, and Sporozoans.
5. Kingdom Fungi
A unique kingdom of heterotrophic, eukaryotic organisms. Their bodies are made of thread-like hyphae (collectively called mycelium), and their cell walls are composed of chitin.
| Class | Common Name | Key Features | Examples |
|---|---|---|---|
| Phycomycetes | Algal Fungi | Aseptate, coenocytic hyphae. | Mucor, Rhizopus (bread mould) |
| Ascomycetes | Sac Fungi | Septate hyphae. Sexual spores (ascospores) produced in a sac (ascus). | Aspergillus, Yeast, Neurospora |
| Basidiomycetes | Club Fungi | Septate hyphae. Sexual spores (basidiospores) on a club-shaped basidium. | Agaricus (mushroom), Puccinia (rust) |
| Deuteromycetes | Imperfect Fungi | Sexual reproduction is absent or not known. | Alternaria, Trichoderma |
6. Viruses, Viroids, and Lichens
These are not included in the five-kingdom system because they are acellular or symbiotic entities, not truly "living" in the conventional sense.
a) Viruses
Non-cellular, inert crystalline structures that are obligate intracellular parasites. They consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid). A virus that infects bacteria is a bacteriophage.
b) Viroids
Discovered by T.O. Diener, they are smaller than viruses and consist only of a free RNA molecule, lacking a protein coat. They cause diseases like potato spindle tuber disease.
c) Lichens
Symbiotic associations between an alga (phycobiont) and a fungus (mycobiont). The alga provides food, and the fungus provides shelter and absorbs nutrients. Lichens are excellent pollution indicators.
