Explain how the abiotic synthesis of organic monomers could have occurred on the early Earth, and outline the significance of polymerisation in the transition from chemistry to biology.

IB DP Biology Higher Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The early Earth, approximately 3.8–4 billion years ago, had an atmosphere very different from today's, lacking free oxygen and rich in reducing gases such as methane, ammonia, hydrogen and water vapour.

Model answer (4 marks)

Energy sources such as lightning, UV radiation or volcanic heat supplied the energy to drive the synthesis of organic monomers from inorganic gases in the early atmosphere. In experiments like Miller–Urey, these conditions produced amino acids, simple sugars and other monomers. Polymerisation then joined these monomers into larger molecules such as polypeptides or polynucleotides. These polymers could store genetic information or catalyse reactions, giving the first self‑replicating or catalytic systems that marked the shift from simple chemistry to living biology.

Examiner tips

  • Use the word ‘synthesised’ for monomers, ‘polymerisation’ for joining them, and link to information storage or catalysis.
  • Show the energy source first, then the Miller–Urey example, then the role of polymers in heredity and evolution.
  • Keep each point brief – one sentence per mark.
  • Use UK spelling (synthesised, catalyse).

Common mistakes

  • Confusing ‘polymerisation’ with ‘replication’; they are distinct processes.
  • Omitting the energy source or the Miller–Urey experiment; both are required for full marks.
  • Using vague terms like ‘life’ instead of ‘living systems’ or ‘self‑replicating polymers’.

Mark scheme (4 marks)

  1. Energy sources (e.g. lightning, UV radiation, or heat from volcanic activity) provided the energy needed to drive synthesis of organic molecules from inorganic precursors.
  2. Organic monomers such as amino acids, nucleotides, or simple sugars were synthesised abiotically from inorganic molecules under these conditions, as demonstrated by the Miller–Urey experiment.
  3. Polymerisation joins monomers into polymers (e.g. polypeptides or polynucleotides), which can store information or carry out catalysis — functions essential for living systems.
  4. Polymerisation represents the transition from simple chemistry to biology because self-replicating or catalytic polymers could undergo natural selection, allowing heredity and evolution to begin.

Key terms in this question

abiotic synthesis · organic monomers · polymerisation

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