Biological Classification Lecture Notes

Historical Overview of Biological Classification

• Early Attempts at Classification

  • Since the dawn of civilization, classification was done instinctively based on human needs for food, shelter, and clothing.
  • Aristotle: The first to attempt a more scientific basis for classification.
    • Plants: Classified based on simple morphological characters into three groups: Trees, Shrubs, and Herbs.
    • Animals: Divided into two groups based on the presence or absence of red blood.
  • Linnaeus' Two Kingdom System:
    • Established the kingdoms Plantae and Animalia.
    • Inadequacies:
      • Did not distinguish between eukaryotes and prokaryotes.
      • Did not distinguish between unicellular and multicellular organisms.
      • Did not distinguish between photosynthetic (e.g., green algae) and non-photosynthetic (e.g., fungi) organisms.
      • Many organisms did not fit into either category, making the system insufficient.

• Evolution of Classification Criteria

  • Modern systems moved beyond gross morphology to include:
    • Cell structure.
    • Nature of the cell wall.
    • Mode of nutrition.
    • Habitat.
    • Methods of reproduction.
    • Evolutionary (phylogenetic) relationships.

The Five Kingdom Classification

• R.H. Whittaker ($1969$): Proposed the Five Kingdom Classification.
  • The Five Kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.
  • Main Criteria for Classification:
    • Cell structure (Prokaryotic vs. Eukaryotic).
    • Thallus organisation.
    • Mode of nutrition.
    • Reproduction.
    • Phylogenetic relationships (Evolutionary history).

Comparative Characteristics of the Five Kingdoms

• Kingdom Monera

  • Cell Type: Prokaryotic.
  • Cell Wall: Noncellulosic (Polysaccharide + amino acid).
  • Nuclear Membrane: Absent.
  • Body Organisation: Cellular.
  • Mode of Nutrition: Autotrophic (chemosynthetic and photosynthetic) and Heterotrophic (saprophytic/parasitic).

• Kingdom Protista

  • Cell Type: Eukaryotic.
  • Cell Wall: Present in some.
  • Nuclear Membrane: Present.
  • Body Organisation: Cellular.
  • Mode of Nutrition: Autotrophic (photosynthetic) and Heterotrophic.

• Kingdom Fungi

  • Cell Type: Eukaryotic.
  • Cell Wall: Present (without cellulose; made of chitin).
  • Nuclear Membrane: Present.
  • Body Organisation: Multicellular / loose tissue.
  • Mode of Nutrition: Heterotrophic (saprophytic / parasitic).

• Kingdom Plantae

  • Cell Type: Eukaryotic.
  • Cell Wall: Present (cellulose).
  • Nuclear Membrane: Present.
  • Body Organisation: Tissue / organ.
  • Mode of Nutrition: Autotrophic (photosynthetic).

• Kingdom Animalia

  • Cell Type: Eukaryotic.
  • Cell Wall: Absent.
  • Nuclear Membrane: Present.
  • Body Organisation: Tissue / organ / organ system.
  • Mode of Nutrition: Heterotrophic (holozoic / saprophytic, etc.).

Impact of Classification Changes

• Reclassification Highlights:
  • Bacteria and Blue-Green Algae: Formerly under 'Plants' because they have cell walls, but now moved to Monera because they are prokaryotic.
  • Fungi: Moved to a separate kingdom because their walls contain chitin, unlike the cellulosic walls of plants.
  • Kingdom Protista: Created to house all unicellular eukaryotic organisms. It brought together:
    • Chlamydomonas and Chlorella (formerly plants/algae with cell walls).
    • Paramoecium and Amoeba (formerly animal kingdom without cell walls).

Kingdom Monera

• General Characteristics:

  • Bacteria: Sole members of this kingdom; most abundant micro-organisms.
  • Habitats: Occur almost everywhere (soil, hot springs, deserts, snow, deep oceans). Many are parasites.
  • Morphological Categories (Shapes):
    • Coccus (pl.: cocci): Spherical.
    • Bacillus (pl.: bacilli): Rod-shaped.
    • Vibrium (pl.: vibrio): Comma-shaped.
    • Spirillum (pl.: spirilla): Spiral.
  • Metabolic Diversity: Exhibit the most extensive metabolic diversity as a group.
    • Autotrophs: Synthesize food from inorganic substrates (photosynthetic or chemosynthetic).
    • Heterotrophs: Majority of bacteria; depend on other organisms or dead organic matter.

• Archaebacteria:

  • Live in harsh conditions due to a unique cell wall structure.
  • Halophiles: Extreme salty areas.
  • Thermoacidophiles: Hot springs.
  • Methanogens: Marshy areas and the gut of ruminant animals (cows, buffaloes); produce methane (biogas) from dung.

• Eubacteria ("True Bacteria"):

  • Possess a rigid cell wall and often a flagellum for motility.
  • Cyanobacteria (Blue-green algae): Unicellular, colonial, or filamentous. Photosynthetic (contain chlorophyll a). Some have specialized cells called heterocysts for nitrogen fixation (e.g., Nostoc, Anabaena).
  • Chemosynthetic Autotrophs: Oxidize inorganic substances ($nitrates$, $nitrites$, $ammonia$) to produce $ATP$. Recycle nutrients like nitrogen, phosphorous, iron, and sulphur.
  • Heterotrophic Bacteria: Most abundant. Act as decomposers. Helpful in making curd, antibiotics, and fixing nitrogen in legumes. Some are pathogens causing diseases like cholera, typhoid, tetanus, and citrus canker.
  • Reproduction: Primarily by fission. Under unfavorable conditions, they produce spores. Also capable of primitive DNA transfer (sexual reproduction).
  • Mycoplasma: Smallest known living cells; completely lack a cell wall; can survive without oxygen; often pathogenic.

Kingdom Protista

• General Characteristics:

  • Single-celled eukaryotes; primarily aquatic.
  • Link between plants, animals, and fungi.
  • Contain well-defined nuclei and membrane-bound organelles. Some have flagella or cilia.
  • Reproduction: Asexual and sexual (cell fusion and zygote formation).

• Major Groups of Protists:

  1. Chrysophytes: Diatoms and golden algae (desmids). Found in fresh/marine water. Cell walls form two overlapping shells (soap box fit) embedded with silica. Accumulations form diatomaceous earth, used in polishing and filtration. Diatoms are the chief 'producers' in oceans.
  2. Dinoflagellates: Mostly marine and photosynthetic. Colors vary (yellow, green, brown, blue, red) based on pigments. Stiff cellulose plates on cell wall. Usually have two flagella (longitudinal and transverse). Gonyaulax causes "red tides" and releases toxins that kill marine life.
  3. Euglenoids: Mostly stagnant fresh water. Lack a cell wall; have a protein-rich pellicle for flexibility. Two flagella (one short, one long). Mixotrophic: photosynthetic in sunlight, heterotrophic in darkness. Example: Euglena.
  4. Slime Moulds: Saprophytic. Vegetative body is an aggregation called plasmodium. Produce resistant spores with true walls that survive for many years and are dispersed by air.
  5. Protozoans: Heterotrophic predators or parasites. Four groups:
     • Amoeboid: Use pseudopodia (false feet) to move/capture prey (e.g., Amoeba). Some are parasites (e.g., Entamoeba).
     • Flagellated: Have flagella. Parasitic forms cause sleeping sickness (e.g., Trypanosoma).
     • Ciliated: Use thousands of cilia for movement and steering food into the gullet (e.g., Paramoecium).
     • Sporozoans: Infectious spore-like stage. Plasmodium (malarial parasite) causes malaria.

Kingdom Fungi

• General Characteristics:

  • Unique heterotrophic organisms; cosmopolitan distribution (air, water, soil, plants, animals).
  • Prefer warm and humid places.
  • Morphology: Mostly filamentous (except unicellular yeast). Body consists of hyphae (thread-like structures). A network of hyphae is a mycelium.
    • Coenocytic hyphae: Continuous tubes with multinucleated cytoplasm.
    • Septate hyphae: Have cross walls (septae).
  • Cell Wall: Composed of chitin and polysaccharides.

• Nutrition and Life Strategies:

  • Saprophytes: Absorb soluble organic matter from dead substrates.
  • Parasites: Depend on living plants/animals.
  • Symbionts: Lichens (with algae) or mycorrhiza (with roots of higher plants).

• Reproduction:

  • Vegetative: Fragmentation, fission, and budding.
  • Asexual: By spores (conidia, sporangiospores, or zoospores).

• Important Examples:

  • Yeast: Unicellular; used in bread and beer.
  • Puccinia: Causes wheat rust.
  • Penicillium: Source of antibiotics.
  • Common edible fungi: Mushrooms and toadstools.