1.7.4Inquiry Process
Classification System — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 27 of 160
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 6Three Domains
Each domain shows a distinct set of molecular and cellular features.
Bacteria
- Bacteria Prokaryotes with peptidoglycan cell walls, no histones on their DNA and ester-linked membrane lipids.
Archaea
- Archaea Prokaryotes lacking peptidoglycan, with histones bound to DNA and ether-linked lipids in their membranes.
Eukarya
- Eukarya Organisms with a nucleus and membrane-bound organelles, histones, introns and 80S ribosomes.
Basis
- Basis The system was proposed using differences in ribosomal RNA, which showed three deep lineages.
Part 2 of 6Comparing the Domains
Comparing the Domains
Cell wall
- Cell wall Bacteria have peptidoglycan, while archaeal walls lack it and are made of other proteins or polysaccharides.
DNA packaging
- DNA packaging Histones are present in archaea and eukaryotes, whereas bacterial DNA is naked.
Introns
- Introns Introns are found in eukaryotic genes and some archaeal genes but are rare in bacteria.
Ancestry
- Ancestry Archaea are more closely related to eukaryotes than to bacteria despite looking like bacteria.
| Feature | Bacteria | Archaea | Eukarya |
|---|---|---|---|
| Nucleus | No | No | Yes |
| Peptidoglycan wall | Yes | No | No |
| Histones | No | Yes | Yes |
| Ether-linked lipids | No | Yes | No |
Part 3 of 6Using the System
1
Check whether the cell has a nucleus; if so it is Eukarya.
2
If there is no nucleus, test for peptidoglycan in the cell wall.
3
Peptidoglycan present in the cell wall points to the domain Bacteria.
4
Absence together with histones or ether-linked lipids points to Archaea.
5
Many archaea live in extreme habitats, such as hot springs or salt lakes, but others live in ordinary soil and oceans.
Part 4 of 6Why Archaea Matter
Archaea were once grouped with bacteria because both are prokaryotic and similar in size.
Why Archaea Matter
Extremophiles
- Extremophiles Some archaea thrive in boiling springs, salt lakes or acid, with enzymes that remain stable at high temperature.
Methanogens
- Methanogens Certain archaea in marshes and animal guts make methane as a waste product of their metabolism.
Evolutionary link
- Evolutionary link Gene expression machinery in archaea resembles that of eukaryotes, suggesting a shared ancestry.
Everyday habitats
- Everyday habitats Many archaea live in soil, oceans and the human gut, not only in extreme environments.
Part 5 of 6Worked Example
Question
- An unknown single-celled organism has no nucleus, has histones bound to its DNA, has ether-linked membrane lipids and lacks peptidoglycan in its wall.
- Assign it to a domain.
- No nucleus means it is prokaryotic, so Eukarya is ruled out.
- Lack of peptidoglycan argues against Bacteria.
- Histones bound to DNA and ether-linked membrane lipids are characteristic features of Archaea.
Answer The organism belongs to the domain Archaea.
Part 6 of 6Exam Checklist & Tips
1.7.4 – Key Points for the Exam
- Domain The highest taxonomic rank.
- Archaea Prokaryotes lacking peptidoglycan, with histones.
- Histone A protein around which DNA is wound.
- Intron A non-coding region within a gene.
Common Mistakes
- Thinking archaea are a type of bacteria.
- Stating that all archaea live in extreme environments.
- Forgetting that archaeal DNA is associated with histones.
- Using only appearance to assign a domain.
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