describe the classification of organisms into three domains: Archaea, Bacteria and Eukarya

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Classification

Classification of Organisms

Learning Objective

By the end of this lesson you should be able to describe the classification of organisms into the three domains: Archaea, Bacteria and Eukarya, and explain the major characteristics that distinguish each domain.

Why a Three‑Domain System?

The traditional five‑kingdom system was based largely on morphological traits. Molecular studies, especially comparisons of ribosomal RNA (rRNA) sequences, revealed that life is best divided into three fundamentally different lineages. These lineages are recognised as the domains Archaea, Bacteria and Eukarya.

Key Features of the Three Domains

FeatureArchaeaBacteriaEukarya
Cell typeProkaryotic (no nucleus)Prokaryotic (no nucleus)Eukaryotic (membrane‑bound nucleus)
Cell wall compositionPseudo‑peptidoglycan or S‑layer proteinsPeptidoglycan (muramic acid present)Cellulose (plants), chitin (fungi), or none (animals)
Membrane lipidsEther‑linked lipids, branched isoprenoid chainsEster‑linked fatty acidsEster‑linked phospholipids (cholesterol in many)
rRNA (16S/18S) sequenceDistinctive archaeal signaturesDistinctive bacterial signatures18S rRNA (eukaryotic)
Typical habitatsExtreme environments (hot springs, high salinity, anaerobic)Wide range – soil, water, human gut, etc.All non‑extreme environments; multicellular organisms
Representative groupsThermoplasmata, Halobacteria, MethanogensProteobacteria, Cyanobacteria, FirmicutesPlants, Animals, Fungi, Protists

Domain Archaea

Archaea are prokaryotes that share some biochemical features with eukaryotes but retain a simple cell organization. They are often called “extremophiles” because many thrive in conditions that would be lethal to most other life forms.

  • Metabolism: Includes methanogenesis, sulfur reduction, and aerobic respiration.

  • Genetic traits: Histone‑like proteins help package DNA, similar to eukaryotes.

  • Ecological role: Important in carbon cycling (methane production) and in extreme ecosystems.

Domain Bacteria

Bacteria constitute the most familiar prokaryotes. They are ubiquitous, ranging from harmless commensals to serious pathogens.

  • Cell wall: Contains peptidoglycan, a polymer of N‑acetylglucosamine and N‑acetylmuramic acid.

  • Metabolic diversity: Photoautotrophs (cyanobacteria), chemoautotrophs, heterotrophs, nitrogen fixers.

  • Medical relevance: Many antibiotics target bacterial cell‑wall synthesis.

Domain Eukarya

Eukaryotes possess a true nucleus and membrane‑bound organelles. This domain includes both unicellular and multicellular organisms.

  • Organelles: Mitochondria, chloroplasts (in plants and algae), endoplasmic reticulum, Golgi apparatus.

  • Genetic organization: Linear chromosomes with histone proteins.

  • Reproduction: Sexual (meiosis) and asexual (mitosis) cycles.

Comparative Summary

The table above highlights the principal differences. The most fundamental distinction is the nature of the cell membrane lipids and the composition of the cell wall, which reflect evolutionary divergence revealed by molecular phylogenetics.

Evolutionary Relationships

Phylogenetic analyses based on 16S rRNA (for prokaryotes) and 18S rRNA (for eukaryotes) support the following simplified tree of life:

Suggested diagram: A rooted tree showing the split between Archaea and Bacteria, with Eukarya branching from within Archaea (the “eocyte” hypothesis) or as a sister group, depending on the model.

Key Points to Remember

  1. All three domains contain prokaryotes except Eukarya, which has a nucleus.

  2. Archaea and Bacteria differ markedly in membrane chemistry and cell‑wall structure.

  3. rRNA sequencing is the primary tool for determining deep evolutionary relationships.

  4. Understanding domain characteristics is essential for topics such as microbiology, ecology, and biotechnology.

Sample Exam Question

Explain why the presence of ether‑linked lipids in the cell membrane is a distinguishing feature of Archaea and discuss how this adaptation benefits organisms living in extreme environments.