Explain why Carl Woese developed the three-domain system to replace the five kingdom classification system.

Edexcel GCSE Biology (1BI0) — 4.2 Classification · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Scientists have developed different systems for classifying living organisms over time. Carl Woese proposed the three-domain system, which groups all living organisms into three domains: Archaea, Bacteria and Eukarya.

Model answer (4 marks)

The five‑kingdom system grouped all prokaryotes together as Prokaryotae, but Woese recognised that prokaryotes are not a single homogeneous group. By analysing ribosomal RNA sequences he showed that prokaryotes split into two distinct lineages – Archaea and Bacteria – which differ in membrane lipids, cell‑wall composition and genetic machinery. The third domain, Eukarya, contains all organisms with a nucleus and membrane‑bound organelles, reflecting their shared cellular complexity. Thus the three‑domain system, based on molecular genetic evidence, provides a more accurate, evolutionary classification than the five‑kingdom scheme.

Examiner tips

  • Use the word ‘because’ to link the reason to the outcome; mention ribosomal RNA as the key evidence; state that Archaea are distinct from Bacteria; note that Eukarya share a nucleus.
  • Keep the answer concise – 4 points, one sentence per point; avoid extra explanation or examples.

Common mistakes

  • Confusing the five‑kingdom system with the six‑kingdom system; mixing up ‘prokaryotes’ and ‘eukaryotes’ in the wrong domain; failing to mention ribosomal RNA as the evidence for the new system.

Mark scheme (4 marks)

  1. The five kingdom system placed all prokaryotes into one kingdom (Prokaryotae), but the three-domain system separates prokaryotes into two distinct domains (Archaea and Bacteria)
  2. Archaea are primitive bacteria that live in extreme environments and are sufficiently different from true bacteria to warrant their own domain
  3. The Eukarya domain groups together all eukaryotic organisms, reflecting their common possession of a nucleus / membrane-bound organelles
  4. The three-domain system is based on molecular / genetic evidence (e.g. differences in DNA or RNA sequences), making it a more accurate / scientific classification

Key terms in this question

three-domain system · five kingdom classification system

Related

More Classification questions

▶ Try answering this question with AI marking (free) →