Biological Classification

Biological Classification

1. History of Classification

1. Early Man:
      Early humans classified organisms based on their uses in daily life. This rudimentary approach prioritized practical applications over biological relationships.

2. Aristotle's Classification:
      Aristotle introduced a more systematic approach by classifying organisms using easily observable morphological characteristics.
      - Plants:
        - Herbs
        - Shrubs
        - Trees
      - Animals:
        - Enalma: Animals with red blood.
        - Anaima: Animals without red blood.

3. Two-Kingdom Classification (by Carolus Linnaeus):
      Linnaeus classified organisms into two kingdoms based on the presence or absence of a cell wall:
      - Plantae:
        - Included plants, fungi, bacteria, dinoflagellates, and chrysophytes.
      - Animalia:
        - Included protozoans and animals.
      - Limitations:
        - Lack of clear differentiation between unicellular and multicellular organisms.
        - Confusion between prokaryotic and eukaryotic cells.
        - Distinctions between autotrophic and heterotrophic organisms were not well defined.
        - Differences in cell walls of bacteria (peptidoglycan), fungi (chitin), and plants (cellulose).

4. Three-Kingdom Classification (by Ernst Haeckel):
      Haeckel added a third kingdom, Protista, encompassing all eukaryotic organisms alongside Plantae and Animalia.

5. Four-Kingdom Classification (by Copeland):
      The fourth kingdom, Monera, was introduced, which included all prokaryotic organisms. The kingdoms now were:
      - Monera
      - Protista
      - Plantae
      - Animalia

6. Five-Kingdom Classification (by R.H. Whittaker, 1969):
      Whittaker proposed a widely accepted classification scheme that included a fifth kingdom called Fungi, leading to:
      - Monera
      - Protista
      - Plantae
      - Animalia
      - Fungi
      - Criteria for Five-Kingdom Classification:
        - Cell structure
        - Cellular organization
        - Mode of nutrition
        - Mode of reproduction
        - Phylogenetic relationships

2. Characteristics of the Five Kingdoms

3. Six-Kingdom Classification / Three Domain Classification

1. Six Kingdoms:
      Proposed by Carl Woese, the six kingdoms are:
      - Archaebacteria
      - Eubacteria
      - Protista
      - Fungi
      - Plantae
      - Animalia
      - These kingdoms are categorized into three domains:
        - Domain Archaea
        - Domain Bacteria
        - Domain Eukarya

4. Kingdom Monera

• Characteristics:
     - Prokaryotic with a less developed nucleus.
     - Ubiquitous; these organisms can survive in varied environments, including extreme conditions like hot springs, deep oceans, deserts, snow, and acidic environments.

5. Types of Bacteria

5.1 Divisions of Kingdom Monera:

   - Archaebacteria
     - Can survive in extreme environments.
     - Types:
       - Halophiles:
         - Live in extremely salty environments.
         - Examples: Halobacterium, Halococcus.
       - Thermoacidophiles:
         - Thrive in hot and acidic environments.
         - Examples: Sulfolobus, Desulfurolobus, Thermus aquaticus.
       - Methanogens:
         - Found in marshy areas and in the guts of ruminants.
         - Responsible for producing 65% of atmospheric methane.
         - Examples: Methanobacterium, Methanococcus, Methanogenium, Methanospirillum.
   - Eubacteria (True Bacteria):
     - Typical bacteria found in various environments.
     - Divided based on shape:
       - Coccus (spherical): Examples—Streptococcus, Staphylococcus.
       - Bacillus (rod-shaped): Example—Lactobacillus.
       - Spirillum (spiral-shaped): Example—Rhodospirillum, Vibrio.

5.2 Properties of Bacteria

• Staining Properties:
     - Studied by Christian Gram using crystal violet stain.
     - Gram-positive bacteria:
       - Stain retained, indicating a thick peptidoglycan cell wall.
     - Gram-negative bacteria:
       - Stain not retained due to a thinner peptidoglycan layer layered with lipopolysaccharides.

6. Nutrition in Bacteria

• Types based on Nutrition:
     - Photoautotrophs:
       - Carry out photosynthesis, using sunlight.
     - Chemoautotrophs:
       - Synthesize food from chemical reactions, converting chemical energy to ATP.
     - Saprotrophs:
       - Decompose dead organic matter.
     - Parasites:
       - Depend on living hosts; disease-causing pathogens.
     - Symbionts:
       - Form mutually beneficial relationships with other organisms, e.g., Rhizobium with legumes.

7. Kingdom Protista

• Characteristics:
     - Eukaryotic organisms, largely unicellular, with some multicellular forms.
     - Nutrition can be autotrophic or heterotrophic.
     - Comprised of various divisions:
       1. Chrysophytes (Golden algae): Autotrophic and eukaryotic, comprising producers in marine ecosystems.
       2. Dinoflagellates: Unicellular, characterized by two flagella and can cause red tides through rapid reproduction.
       3. Euglenoids: Mixotrophic, with pigments for photosynthesis; they can alter their nutritional method based on light availability.
       4. Slime Molds: Eukaryotic, saprophytic organisms that thrive in damp environments, capable of forming fruiting bodies.
       5. Protozoans: Animal-like, heterotrophic, varied locomotion mechanisms.
       

8. Structure of Viruses

• Composition:
     - Viruses are composed of genetic material (either DNA or RNA) encased in a protein shell called a capsid.
     - Types of viruses include:
       - Animal viruses,
       - Plant viruses, and
       - Bacteriophages which infect bacteria.

• Sub-viral Particles:
     - Viroids: Smaller than viruses, consist only of RNA; cause diseases in plants.
     - Prions: Proteinaceous infectious agents that lead to diseases in animals and are associated with neurodegenerative disorders.