# Explain how the structure of a phospholipid makes it suitable for forming a bilayer in cell membranes.

> IB DP Biology Higher Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks

## Mark scheme (4 marks)

1. A phospholipid has a hydrophilic phosphate head group and two hydrophobic fatty acid tails, giving it an amphipathic nature.
2. In an aqueous environment, the hydrophobic tails orientate away from water and the hydrophilic heads orientate towards water, driving self-assembly of the bilayer.
3. The hydrophobic interior of the bilayer acts as a barrier to the passage of polar/charged molecules and ions, maintaining selective permeability.
4. The fluid nature of the bilayer arises because phospholipids are not covalently bonded to each other, allowing lateral movement, which maintains membrane flexibility and integrity.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher 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-how-the-structure-of-a-68930495) · Published by Druglandscape Ltd.