IB Biology · Theme A: Unity and diversity · HL only
A2.1 Origins of cells
A one-page summary of A2.1 Origins of cells, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What plausible hypothesis could account for the origin of life?
What intermediate stages could there have been between non-living matter and the first living cells?
What A2.1 covers
From chemistry to compartments
- A2.1.1Conditions on early Earth
- A2.1.2What makes something alive?
- A2.1.3Why the first cell is hard to explain
- A2.1.4The Miller–Urey experiment
- A2.1.4More evidence: meteorites
- A2.1.5Fatty acids self-assemble
- A2.1.5From sheet to vesicle
- A2.1.6RNA: messenger and machine
Tracing life's origin through evidence
- A2.1.7What is LUCA?
- A2.1.7The evidence: a shared toolkit
- A2.1.8 · Dating method 1Fossil evidence
- A2.1.8 · Dating method 2Carbon isotopes
- A2.1.8 · Dating method 3Genomic analysis: the molecular clock
- A2.1.9LUCA near hydrothermal vents
A2.1 Origins of cells: summary
Early Earth
- No O₂/ozone → UV; more CO₂/CH₄ → warmer
- Miller-Urey: amino acids form abiotically
What is a cell?
- Smallest self-sustaining unit of life
- Viruses can't metabolise/reproduce alone
The chicken-and-egg problem
- DNA needs protein; protein needs DNA
- Vesicles, RNA, vents: three partial answers
Compartments & RNA
- Fatty acids self-assemble into vesicles
- RNA world: ribozymes carry code + catalysis
LUCA
- Last shared ancestor — not the first cell
- Evidence: shared DNA/code/ribosomes/ATP
Dating & vents
- Fossils, isotopes, molecular clock
- LUCA's chemistry matches hydrothermal vents
Key terms
- Prebiotic
- before life existed; chemistry occurring without any living organism.
- Amphipathic
- having both a hydrophilic head and a hydrophobic tail.
- Vesicle
- a hollow sphere enclosed by a self-assembled membrane.
- Ribozyme
- an RNA molecule that can catalyse a chemical reaction.
- LUCA
- the last population all living things are descended from — not the first cell.
- Hydrothermal vent
- an ocean-floor fissure releasing mineral-rich, heated water.
Sample exam questions
Three of the 29 multiple-choice questions for A2.1. Try each one before opening the answer.
Question 1. Which statement best describes the atmosphere of prebiotic (early) Earth?
- It contained more free oxygen than today
- It contained little or no free oxygen
- It was almost pure nitrogen with no other gases
- It had the same composition as the atmosphere today
Show the answer
Answer: B. Free oxygen only accumulated later, after photosynthetic organisms evolved; the early atmosphere was rich in gases such as carbon dioxide, methane and water vapour but essentially lacked O₂.
Question 2. Higher concentrations of carbon dioxide and methane on early Earth are thought to have caused:
- a thick protective ozone layer to form
- a lower average surface temperature
- an abundance of free oxygen gas
- a higher surface temperature
Show the answer
Answer: D. Carbon dioxide and methane are greenhouse gases; high concentrations trapped heat and kept the surface warmer than it would otherwise have been.
Question 3. Why is LUCA not considered to be the first cell that ever lived?
- Earlier cells existed but left no descendants
- LUCA had no genetic material of its own
- The first cell was in fact a eukaryote
- LUCA lived long after multicellular life evolved
Show the answer
Answer: A. LUCA already had complex features such as a genetic code and metabolism, so simpler cells must have preceded it; those earlier lineages simply died out, leaving LUCA as the last shared ancestor.
Linking questions
Questions that connect A2.1 to other parts of the course, the kind that come up in Paper 2.
- How does the structure of RNA allow it to act as both an information molecule and a catalyst in the RNA world hypothesis? (see A1.2, D1.2)
- How do the amphipathic properties of phospholipids explain both the self-assembly of early vesicles and the structure of modern cell membranes? (see A1.1, B2.1)
- How does the presence of a universal genetic code in all organisms provide evidence for both LUCA and common ancestry in evolution? (see A4.1, D1.2)
- How do the differences between cells and viruses inform the debate about what counts as living when explaining the origin of the first cells? (see A2.2, A2.3)
Practise A2.1
Study notes, every question and full markschemes for A2.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.