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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Hierarchy and Binomials
Taxonomy arranges species into groups called taxa, with no overlap between groups, based on evolutionary relationships.
Ranks
- Ranks From the top: domain, kingdom, phylum, class, order, family, genus, species, with each taxon nested inside a larger one.
Species
- Species The lowest rank;
- similar species are grouped in a genus, and members of a species can breed to produce fertile offspring.
Binomial
- Binomial The scientific name made of genus then species in Latin, with the genus capitalised, the species in lower case, and both italicised.
Examples
- Examples Humans are Homo sapiens and dogs are Canis familiaris, and the name is the same all over the world.
Evidence
- 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
- Prokaryotae Prokaryotic cells with no nucleus, typically less than 5 μm in size, such as bacteria.
Protoctista
- Protoctista Eukaryotes that do not fit the other kingdoms;
- protozoa have no cell wall while algae have cellulose walls and chloroplasts.
Fungi
- 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
- 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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