Explain why all existing cells are thought to have originated from pre-existing cells rather than arising spontaneously from non-living matter today.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Pasteur’s controlled experiments showed that sterilised broth remained free of microbes unless exposed to non‑sterile air, disproving spontaneous generation.
Binary fission is the only known mechanism for producing new cells; therefore each cell must arise from a pre‑existing cell (Omnis cellula e cellula).
The early Earth had conditions (e.g. anoxic atmosphere, high energy input, abundant organic monomers) that no longer exist, so abiogenesis does not occur today.
Any organic molecules or proto‑cells that might form now would be rapidly degraded or consumed by existing organisms, preventing new abiogenesis.
Binary fission is the only known mechanism for producing new cells; therefore each cell must arise from a pre‑existing cell (Omnis cellula e cellula).
The early Earth had conditions (e.g. anoxic atmosphere, high energy input, abundant organic monomers) that no longer exist, so abiogenesis does not occur today.
Any organic molecules or proto‑cells that might form now would be rapidly degraded or consumed by existing organisms, preventing new abiogenesis.
Examiner tips
- Use Pasteur’s experiment as the key evidence for disproving spontaneous generation.
- Mention binary fission and the principle Omnis cellula e cellula to justify cell origin.
- Explain why early Earth conditions are unique and why they are absent today.
- Show that existing organisms would destroy any new organic molecules, preventing abiogenesis.
Common mistakes
- Confusing spontaneous generation with abiogenesis; failing to cite Pasteur’s experiment.
- Lacking the binary fission point or mis‑stating it as mitosis.
- Overlooking the role of early Earth conditions or the absence of free oxygen.
Mark scheme (4 marks)
- Pasteur's experiments (or equivalent controlled experiments) demonstrated that sterilised broth did not produce microbial growth unless exposed to non-sterile air, disproving spontaneous generation.
- Cell division / binary fission is the only known mechanism by which new cells are produced, so each cell must come from a pre-existing cell (Omnis cellula e cellula).
- The early Earth had conditions (e.g. lack of free oxygen / high energy input / abundance of organic monomers) that no longer exist, so abiogenesis is not occurring today.
- Any organic molecules or proto-cells that might form today would be rapidly broken down / consumed by existing organisms, preventing new abiogenesis.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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