Classification System (DP IB Biology)

Domains

• The science of taxonomy has frequently changed to match the latest discoveries about the features of organisms

  • Historically, the largest taxonomic groups were the plant and animal kingdoms, then fungi were discovered and added (incorrectly) to the plant kingdom

  • Microscopes led to the discovery of prokaryotes and eukaryotes, and the taxa were later divided into five kingdoms; plants, animals, fungi, protoctists, and prokaryotes

    • The protoctists are eukaryotic, primarily single-celled, organisms

  • rRNA analysis has recently shown that there are two distinct groups of prokaryotes, leading to a shift in taxonomic thinking and the beginnings of the three domain system

• The largest taxonomic group is now known as a domain

• There are three domains, which are:

  • Archaea (prokaryotes)

  • Eubacteria (prokaryotes)

  • Eukaryotes (eukaryotes)

The three domains diagram

The three domains are the eubacteria, archaea, and eukarya. The archaea are thought the be more closely related to the eukaryotes than to the prokaryotes.

Classifying the Archaea

• The Archaea are single-celled organisms that live in a range of habitats

  • Some live in extreme environments, e.g.

    • Thermophiles live in hot springs

    • Methanophiles metabolise methane

• The Archaea were originally classified with the rest of the bacteria in one taxon due to sharing several features with them

  • Prokaryotic cell structure

  • Circular chromosome

  • Presence of a cell wall

  • 70S ribosomes

• Closer analysis of the Archaea, much of which was carried out by Carl Woese in 1977, showed that some of their features were distinct from the rest of the prokaryotes

  • Their cell walls are made of a different material from the cell walls of the rest of the bacteria

  • Their cell membranes are made of a distinct type of lipid

  • The small subunit of their ribosomes is more similar to eukaryotic ribosomes than to the ribosomes of the rest of the prokaryotes

  • Multiple DNA sequences present in bacteria are absent in methanogenic archaeans

• These discoveries led to a change in the classification of the archaea, which became their own domain

The three domains features table