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4.4.1Unit 4: Energy, Environment, Microbiology and Immunity

The Three Domains of Life — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 21 of 28 Unit quiz

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Part 1 of 4Hierarchy and Binomials

4.4.1 The Three Domains of Life

Taxonomy arranges species into groups called taxa, with no overlap between groups, based on evolutionary relationships.

Ranks
  • From the topdomain, kingdom, phylum, class, order, family, genus, species, with each taxon nested inside a larger one.
Species
  • The lowest rank;
    • similar species are grouped in a genus, and members of a species can breed to produce fertile offspring.
Binomial
  • The scientific name made of genus then species in Latin, with the genus capitalised, the species in lower case, and both italicised.
Examples
  • Humans are Homo sapiens and dogs are Canis familiaris, and the name is the same all over the world.
Evidence
  • Appearance alone can mislead, as whales and sharks look alike but are unrelated, so DNA sequencing is now used.

Part 2 of 4The Five Kingdoms

Organisms were first placed in five kingdoms according to shared general features.

The Five Kingdoms
Prokaryotae
  • Prokaryotic cells with no nucleus, typically less than 5 μm in size, such as bacteria.
Protoctista
  • Eukaryotes that do not fit the other kingdoms;
    • protozoa have no cell wall while algae have cellulose walls and chloroplasts.
Fungi
  • Eukaryotes with walls often made of chitin, which feed by extracellular digestion of dead matter or as parasites and reproduce by spores.
Plantae and Animalia
  • Multicellular eukaryotes: plants have cellulose walls, chloroplasts and are autotrophs,
    • while animals have no walls and are heterotrophs.

Part 3 of 4Three-Domain System

Molecular phylogeny changed how the highest ranks are drawn.

1
Molecular phylogeny compares DNA, RNA or protein sequences, and the more similar they are the more recent the common ancestor.
2
Analysis of ribosomal RNA genes showed that the prokaryotes are not one group but two very different ones.
3
Prokaryotae were therefore split into two domains, Archaea and Bacteria, which are less closely related than first thought.
4
The four eukaryotic kingdoms were placed together in the third domain, Eukarya, giving Archaea, Bacteria and Eukarya.
5
Scientific classification can change whenever new evidence appears, which shows that scientific knowledge is provisional.

Part 4 of 4Exam Checklist & Tips

4.4.1 – Key Points for the Exam

  • Taxon A group of organisms in a classification, such as a genus or family.
  • Binomial A two-part scientific name made of genus and species.
  • Domain The highest taxonomic rank: Archaea, Bacteria or Eukarya.
  • Molecular phylogeny Working out evolutionary relationships by comparing molecules such as DNA or RNA.

Common Mistakes

  • Writing the binomial with a capital letter for the species or without italics or underlining.
  • Listing the ranks in the wrong order; remember Kingdom Phylum Class Order Family Genus Species below domain.
  • Saying that Archaea and Bacteria are the same because both are prokaryotes, when molecular evidence shows they are distinct domains.
  • Assuming organisms with similar appearance must be closely related, when features may be similar by convergence.

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