# Explain why all existing cells are thought to have originated from pre-existing cells rather than arising spontaneously from non-living matter today.

> IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks

## Mark scheme (4 marks)

1. 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.
2. 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).
3. 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.
4. Any organic molecules or proto-cells that might form today would be rapidly broken down / consumed by existing organisms, preventing new abiogenesis.

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- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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