Explain how the first cells could have arisen from non-living matter through the progressive increase in molecular complexity on early Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The origin of life is thought to have involved a series of stages in which simple inorganic molecules gave rise to increasingly complex organic systems capable of self-replication and compartmentalisation.
Model answer (4 marks)
1. Simple inorganic molecules such as H₂O, CO₂, NH₃ and H₂S reacted abiotically (e.g. lightning, UV radiation, heat) to give organic monomers such as amino acids and nucleotides.
2. These monomers polymerised abiotically, often on mineral or clay surfaces that concentrated them, to form macromolecules such as polypeptides and polynucleotides.
3. Amphipathic or phospholipid‑like molecules spontaneously assembled into bilayers, enclosing the macromolecules and creating membrane‑bound compartments – protocells or liposomes.
4. Protocells that could replicate their nucleic acids and transmit heritable information to offspring were subject to natural selection, leading to the emergence of the first true cells.
2. These monomers polymerised abiotically, often on mineral or clay surfaces that concentrated them, to form macromolecules such as polypeptides and polynucleotides.
3. Amphipathic or phospholipid‑like molecules spontaneously assembled into bilayers, enclosing the macromolecules and creating membrane‑bound compartments – protocells or liposomes.
4. Protocells that could replicate their nucleic acids and transmit heritable information to offspring were subject to natural selection, leading to the emergence of the first true cells.
Examiner tips
- Use the exact sequence of stages (monomers → polymers → compartments → replication).
- Show each stage with a brief example (e.g. lightning for monomer formation).
- Mention natural selection as the driving force for the final step.
Common mistakes
- Confusing monomers with macromolecules; not distinguishing between them.
- Omitting the role of mineral/clay surfaces in concentrating monomers.
Mark scheme (4 marks)
- Simple inorganic molecules (e.g. H₂O, CO₂, NH₃, H₂S) reacted abiotically to form organic monomers such as amino acids or nucleotides under early Earth conditions (e.g. lightning, UV radiation, heat).
- Monomers polymerised abiotically to form macromolecules such as polypeptides or polynucleotides, possibly on mineral or clay surfaces that concentrated the molecules.
- Amphipathic / phospholipid-like molecules spontaneously formed bilayers that enclosed these macromolecules, creating membrane-bound compartments (protocells / liposomes).
- Protocells that could replicate their nucleic acids and pass heritable information to offspring were subject to natural selection, driving the emergence of the first true cells.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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