Biological Classification

Introduction to Classification

Since the dawn of civilisation, attempts have been made to classify living organisms. Aristotle was the earliest to attempt a scientific basis for classification, using simple morphological characters to classify plants into trees, shrubs, and herbs, and animals into those with red blood and those without.

History of Systems

• Two Kingdom System: Proposed by Linnaeus (Kingdom Plantae and Kingdom Animalia). It did not distinguish between eukaryotes/prokaryotes, unicellular/multicellular, or photosynthetic/non-photosynthetic organisms.
• Five Kingdom Classification: Proposed by R.H. Whittaker (1969). The kingdoms are Monera, Protista, Fungi, Plantae, and Animalia.

2.1 Kingdom Monera

Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms and occur almost everywhere, including extreme habitats. Bacteria are grouped under four categories based on shape: Coccus (spherical), Bacillus (rod-shaped), Vibrium (comma-shaped), and Spirillum (spiral).

2.1.1 Archaebacteria

These live in harsh habitats. They differ from other bacteria in having a different cell wall structure, which aids survival in extreme conditions.

• Halophiles: Extreme salty areas.
• Thermoacidophiles: Hot springs.
• Methanogens: Marshy areas and gut of ruminant animals (responsible for biogas production).

2.1.2 Eubacteria ('True Bacteria')

Characterised by the presence of a rigid cell wall and, if motile, a flagellum.

• Cyanobacteria (Blue-green algae): Photosynthetic autotrophs having chlorophyll a. Can be unicellular, colonial, or filamentous. Some fix atmospheric nitrogen in specialised cells called heterocysts (e.g., Nostoc, Anabaena).
• Chemosynthetic autotrophs: Oxidise inorganic substances (nitrates, ammonia) to produce energy (ATP). Important for recycling nutrients.
• Heterotrophic bacteria: Most abundant decomposers. Helpful in making curd, antibiotics, and fixing nitrogen. Some are pathogens causing diseases like Cholera, Typhoid, Tetanus, and Citrus canker.
• Mycoplasma: Organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen.

2.2 Kingdom Protista

All single-celled eukaryotes are placed under Protista. Being eukaryotes, the cell body contains a well-defined nucleus and membrane-bound organelles.

• Chrysophytes: Includes diatoms and golden algae (desmids). Most are photosynthetic. In diatoms, cell walls form two thin overlapping shells embedded with silica (indestructible), forming diatomaceous earth.
• Dinoflagellates: Mostly marine and photosynthetic. Cell walls have stiff cellulose plates. Most have two flagella. Red dinoflagellates (e.g., Gonyaulax) multiply rapidly causing red tides.
• Euglenoids: Freshwater organisms found in stagnant water. Instead of a cell wall, they have a protein-rich layer called pellicle which makes the body flexible. They are photosynthetic in sunlight but heterotrophic in its absence (e.g., Euglena).
• Slime Moulds: Saprophytic protists. Under suitable conditions, they form an aggregation called plasmodium. During unfavourable conditions, the plasmodium differentiates to form fruiting bodies bearing resistant spores.
• Protozoans: Heterotrophs living as predators or parasites.
  • Amoeboid: Move using pseudopodia (e.g., Amoeba, Entamoeba).
  • Flagellated: Possess flagella; some cause diseases like sleeping sickness (e.g., Trypanosoma).
  • Ciliated: Actively moving due to thousands of cilia (e.g., Paramoecium).
  • Sporozoans: Have an infectious spore-like stage (e.g., Plasmodium which causes malaria).

2.3 Kingdom Fungi

Unique kingdom of heterotrophic organisms. Except for yeasts (unicellular), fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium. Cell walls are composed of chitin and polysaccharides.

Reproduction: Vegetative (fragmentation, fission, budding), Asexual (spores like conidia, zoospores), and Sexual (oospores, ascospores, basidiospores). The sexual cycle involves Plasmogamy, Karyogamy, and Meiosis.

Classes of Fungi

• Phycomycetes: Found in aquatic habitats or on decaying wood. Mycelium is aseptate and coenocytic. Spores are produced endogenously in sporangium (e.g., Mucor, Rhizopus, Albugo).
• Ascomycetes (Sac-fungi): Mostly multicellular (e.g., Penicillium) or unicellular (e.g., Yeast). Mycelium is branched and septate. Asexual spores are conidia; sexual spores are ascospores produced in asci (e.g., Aspergillus, Claviceps, Neurospora).
• Basidiomycetes: Includes mushrooms, bracket fungi, puffballs. Mycelium is branched and septate. Asexual spores generally absent. Vegetative reproduction by fragmentation is common. Sexual spores (basidiospores) are produced exogenously on the basidium (e.g., Agaricus, Ustilago, Puccinia).
• Deuteromycetes (Imperfect Fungi): Only asexual or vegetative phases are known. Reproduce only by asexual spores known as conidia. Mycelium is septate and branched (e.g., Alternaria, Trichoderma).

2.4 Plantae & 2.5 Animalia

• Kingdom Plantae: Eukaryotic chlorophyll-containing organisms. Cell wall mainly made of cellulose. Includes Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms. Life cycle exhibits alternation of generations (diploid sporophytic and haploid gametophytic).
• Kingdom Animalia: Heterotrophic eukaryotic organisms that are multicellular and lack cell walls. Mode of nutrition is holozoic. Sexual reproduction is by copulation followed by embryological development.

2.6 Viruses, Viroids, Prions and Lichens

Whittaker's classification did not mention acellular organisms like viruses, viroids, and prions, or lichens.

• Viruses: Non-cellular organisms characterized by an inert crystalline structure outside the living cell. They are obligate parasites.
  – Discovery: D.J. Ivanowsky (1892) recognised microbes smaller than bacteria (TMV). M.W. Beijerinck (1898) called the fluid "Contagium vivum fluidum". W.M. Stanley (1935) showed viruses could be crystallised.
  – Structure: Contains genetic material (RNA or DNA, never both) and a protein coat called capsid made of capsomeres.
• Viroids: Discovered by T.O. Diener (1971). Smaller than viruses, free RNA, lacking a protein coat. Cause potato spindle tuber disease.
• Prions: Agents consisting of abnormally folded protein. Transmit infectious neurological diseases (e.g., Mad cow disease/BSE, Cr-Jacob disease).
• Lichens: Symbiotic association between algae (phycobiont - autotrophic) and fungi (mycobiont - heterotrophic). They are good pollution indicators (do not grow in polluted areas).