# 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

## Mark scheme (4 marks)

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

## 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 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-706ee482) · Published by Druglandscape Ltd.