Explain how the properties of phospholipids contribute to the formation of the first cell membranes during the origin of life.

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).

A central challenge in the origin of life is explaining how a stable boundary could form spontaneously to separate an internal chemical environment from the surrounding aqueous medium.

Model answer (4 marks)

Phospholipids are amphipathic, possessing a hydrophilic phosphate head and hydrophobic fatty‑acid tails.

In water the hydrophobic tails avoid contact with the solvent, so phospholipids spontaneously self‑assemble into bilayers or vesicles.

The bilayer forms a closed compartment that separates an internal aqueous space from the external medium, creating a proto‑cell boundary.

This compartmentalisation allows concentrations of organic molecules, early catalytic RNA and metabolic reactions to be maintained inside, increasing the likelihood of life emerging.

Examiner tips

Mention amphipathic nature first, then self‑assembly, then compartment, then functional advantage.
Use the word ‘self‑assemble’ and ‘bilayer/vesicle’ to match the mark scheme.
Show the logical sequence: structure → behaviour → boundary → benefit.

Common mistakes

Failing to state that phospholipids are amphipathic.
Describing only one property (e.g. only the hydrophilic head) and omitting the self‑assembly step.
Using vague terms like ‘membrane’ without explaining how it forms or why it matters.

Mark scheme (4 marks)

Phospholipids are amphipathic / have both a hydrophilic phosphate head and hydrophobic fatty acid tails
In an aqueous environment, phospholipids spontaneously self-assemble into bilayers or vesicles to minimise contact of hydrophobic tails with water
Vesicle/liposome formation creates a compartment that separates an internal aqueous environment from the external medium, providing a proto-cell boundary
This compartmentalisation would have allowed concentrations of organic molecules / early catalytic RNA / metabolic reactions to be maintained inside, increasing the probability of the emergence of life

Key terms in this question

phospholipid