# Explain why biologists consider it unlikely that the first cells on Earth were surrounded by a phospholipid bilayer membrane of the type found in modern cells.

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

> The earliest evidence of life on Earth dates to approximately 3.5 billion years ago. Understanding how the first cells arose requires consideration of the chemical conditions present on the early Earth and the properties of biological molecules.

## Mark scheme (4 marks)

1. Phospholipids are complex molecules that require enzymatic/biological synthesis, which would not have been available before cells existed.
2. Simpler amphipathic molecules such as fatty acids could self-assemble into vesicles/protocells under early Earth conditions.
3. The first membranes are thought to have been composed of abiotically synthesised molecules, consistent with the conditions of early Earth (e.g. Miller–Urey type chemistry / hydrothermal vents).
4. A membrane boundary was still necessary for the first cells to concentrate molecules / maintain chemical conditions different from the surroundings / allow a proto-metabolism.

## Key terms

- [phospholipid bilayer](https://www.gradenine.co.uk/glossary/phospholipid-bilayer)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-biologists-consider-it-unlikely-e6407d4d) · Published by Druglandscape Ltd.