Explain how the structure of a phospholipid allows it to form a bilayer in aqueous environments.

IB DP Biology Standard Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A phospholipid is amphipathic, having a hydrophilic phosphate head and two hydrophobic fatty‑acid tails.
In water the polar heads are attracted to the solvent and orient outward, while the non‑polar tails avoid water and point inward.
Thus the molecules arrange themselves so that the heads face the aqueous environment and the tails form a non‑polar core, creating a bilayer.
This arrangement minimises free energy and therefore self‑assembles spontaneously in aqueous media.

Examiner tips

Use the word ‘amphipathic’ to describe the phospholipid structure.
Explain the orientation of heads and tails in water.
Mention that the bilayer is thermodynamically stable and self‑assembles.

Common mistakes

Confusing the orientation of heads and tails; writing heads inward or tails outward.
Omitting the term ‘amphipathic’ or failing to link structure to function.
Not stating that the bilayer formation is driven by minimising free energy.

Mark scheme (4 marks)

A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails (amphipathic nature).
In an aqueous environment the hydrophilic heads face outward towards the water / are attracted to water molecules.
The hydrophobic tails face inward / away from water, forming a non-polar interior to the bilayer.
This arrangement is thermodynamically stable / minimises free energy, allowing the bilayer to self-assemble spontaneously.

Key terms in this question

phospholipid · bilayer