Classification of Living Organisms

Historical Overview

• Classification of organisms dates back centuries.
• Initial classifications focused on organisms useful for food, shelter, clothing.
• Aristotle: Developed first scientific classification method:
  • Plants: Classified into trees, shrubs, herbs.
  • Animals: Classified based on blood type: red blood vs. non-red blood.
• Carl Linnaeus: Proposed two kingdom system:
  • Kingdom Plantae: All plants.
  • Kingdom Animalia: All animals.
  • Limitation: Did not account for unicellular/multicellular, prokaryotes/eukaryotes.

Limitations of Early Classifications

• The two kingdom system did not capture:
  • Unicellular vs. multicellular organisms.
  • Prokaryotes vs. eukaryotes.
  • Photosynthetic vs. non-photosynthetic organisms.
• Many organisms did not fit into the existing categories.
• Significant variability in organisms within the same group in characteristics such as cell structure, nutrition, reproduction, and evolution.

Five Kingdom Classification by R.H. Whittaker (1969)

• Proposed a five kingdom system:
  1. Monera (bacteria)
  2. Protista (unicellular eukaryotes)
  3. Fungi (heterotrophs)
  4. Plantae (autotrophs)
  5. Animalia (multicellular heterotrophs)
• Criteria used for classification:
  • Cell structure, phallus organization, mode of nutrition, reproduction methods, phylogenetic relationships.

Kingdom Monera

• Bacteria: Found in various habitats, including extreme environments.
• Types of Bacteria:
  • Autotrophic Bacteria: Synthesize their own food.
  • Heterotrophic Bacteria: Depend on other organisms for food.
  • Based on shape:
    • Coccus: Spherical
    • Bacillus: Rod-shaped
    • Vibrio: Comma-shaped
    • Spirillum: Spiral-shaped
• Eubacteria: Characterized by rigid cell walls and flagella.
• Cyanobacteria (Blue-Green Algae): Autotrophic, capable of nitrogen fixation.
• Archaebacteria: Live in extreme conditions (e.g., Halophiles, Thermoacidophiles, Methanogens).
• Mycoplasmas: Lack cell walls, smallest living beings, pathogenic.

Kingdom Protista

• Characteristics: Unicellular eukaryotes, mostly aquatic.
• Categories of Protists:
  • Chrysophytes: Diatoms, golden algae.
  • Dinoflagellates: Colorful, can cause red tides.
  • Euglenoids: Flexible pellicle, photosynthetic.
  • Slime Molds: Saprophytic, form plasmodium.
  • Protozoans: Primitive relatives of animals, classified as:
    • Amoeboid
    • Flagellated
    • Ciliated
    • Sporozoans (e.g., Plasmodium causes malaria).

Kingdom Fungi

• Characteristics: Found in diverse habitats, mostly filamentous.
• Structure: Composed of hyphae, forming mycelium.
• Types of Fungi:
  • Phycomycetes: Aquatic, have aseptate mycelium.
  • Ascomycetes: Sac fungi, have septate mycelium.
  • Basidiomycetes: Include mushrooms, have septate mycelium.
  • Deuteromycetes: Imperfect fungi, only asexual reproductive phases known.
• Reproduction:
  • Vegetative: Fragmentation, fission, budding.
  • Asexual: Spores (conidia, zoospores).
  • Sexual: Plasmogamy, karyogamy, meiosis.

Kingdom Plantae

• Characteristics: Eukaryotic, autotrophic, cellulose cell walls.
• Types of Plants:
  • Algae, bryophytes, pteridophytes, gymnosperms, angiosperms.
• Life Cycle: Alternation of generations (diploid sporophytic and haploid gametophytic phases).

Kingdom Animalia

• Characteristics: Multicellular, heterotrophic eukaryotes, lack cell walls.
• Nutrition: Holozoic, extracorporeal digestion.
• Reproduction: Primarily sexual, with complex development.

Exclusions from the Five Kingdom System

• Viruses: Acellular, obligate parasites, not classified due to lack of cell structure.
• Viroids: Smaller than viruses, lack protein coats, cause diseases.
• Lichens: Symbiotic association of algae (phycobiont) and fungi (mycobiont).
• Examples of Diseases Caused by Viruses: Common cold, smallpox, AIDS, etc.