20.1. Kingdoms and Domains: An Overview

The Six Kingdoms of Life

  • All living organisms are divided into six kingdoms based on specific characteristics.
  • Key characteristics for grouping:
    • Cell Type: Prokaryote or Eukaryote
    • Cell Walls: Present or Absent
    • Body Type: Unicellular, Multicellular, or a Mixture
    • Nutrition: Heterotrophic (consumers), Autotrophic (producers), or a Mixture
  • The six kingdoms:
    • Eubacteria
    • Archaebacteria
    • Protista
    • Plantae
    • Fungi
    • Animalia

The Three Domains of Life

  • Initial classification: Prokaryotes and Eukaryotes.
  • Karl Woese's discovery (1977): Archaebacteria are fundamentally different from other prokaryotes.
  • Revised classification (1996): Three domains
    • Bacteria
    • Archaea
    • Eukarya
  • Domain Assignments:
    • Bacteria: Kingdom Eubacteria
    • Archaea: Kingdom Archaebacteria
    • Eukarya: Kingdoms Animalia, Fungi, Plantae, and Protista

Domain Bacteria

  • Contains one kingdom: Eubacteria (sometimes called Kingdom Bacteria).
  • Characteristics:
    • Prokaryotes
    • Lipids in cell membranes similar to eukaryotes.
    • Size: 0.1 to 15 micrometers.
    • Lack internal compartments.
    • Nutrient Acquisition: Various methods.
    • Cell Wall: Strong, made of peptidoglycan.
    • Gene Translation Apparatus: Amino acid sequences of ribosome proteins and RNA polymerases differ from Archaebacteria and eukaryotes.
  • Categorization of Bacteria:
    • Shape
    • Type of Metabolism
    • Nature of Cell Wall
  • Ecological roles and uses:
    • Some bacteria cause diseases.
    • Some bacteria are used in food processing.
    • Some bacteria control agricultural pests.
    • Some bacteria are photosynthetic.
    • Some bacteria are heterotrophs that require oxygen.
    • Heterotrophic bacteria recycle nitrogen, phosphorus, and carbon.
  • Abundance: Bacteria are the most abundant organisms on Earth.
    • Example: More bacteria live in your mouth than there are mammals on Earth.

Domain Archaea

  • Contains one kingdom: Archaebacteria.
  • Characteristics:
    • Prokaryotes, early descendants of bacteria, and more closely related to eukaryotes than to bacteria.
    • Cell Wall: Lacks peptidoglycan; contains unique lipids.
    • Gene Structure: Genes interrupted by introns (similar to eukaryotes).
    • Ribosomal Proteins: Similar to eukaryotes, different from bacteria.
  • Types of Archaebacteria:
    • Methanogens:
      • Found in swamps.
      • Energy acquisition: combines carbon dioxide (CO2) and hydrogen gas (H2) to form methane gas (CH_4).
      • Poisoned by oxygen.
    • Extremophiles:
      • Thrive in extreme environments.
      • Thermophiles: thrive in very hot conditions (up to 106 degrees Celsius or 222.8 degrees Fahrenheit).
      • Halophiles: live in very salty lakes.
      • Some found in acidic environments (pH below 1) or under enormous pressure (up to 800 atmospheres).
    • Non-extreme Archaebacteria: Found in the same environments as bacteria.

Domain Eukarya

  • Consists of four kingdoms: Protista, Fungi, Plantae, and Animalia.
  • Characteristics:
    • Eukaryotes: Contain eukaryotic cells with complex internal structures.
    • Highly Organized Cell Interior: Nucleus and internal compartments for specialized functions.
    • Multicellularity: Coordinated activities and cell contact (in multicellular organisms).
    • Sexual Reproduction: Integral part of the life cycle; meiotic cell division leads to formation of haploid gametes, which unite to form a diploid cell in fertilization.
    • Meiosis: Genetic recombination, raw material for evolution and produces offspring that can vary.
  • Types of Eukaryotes:
    • Protista: Mostly unicellular, but also multicellular organisms (many aquatic).
    • Fungi: Mostly multicellular, except for yeast (unicellular).
    • Plantae: Entirely multicellular; almost all are autotrophs with cells walls.
    • Animalia: Entirely multicellular; all are heterotrophs without cell walls.