# Explain how the polarity of water molecules enables water to act as a solvent for non-polar molecules in the presence of amphipathic molecules, using phospholipid bilayer formation as an example.

> IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks

> Biological membranes form spontaneously when phospholipids are mixed with water. This self-assembly depends critically on the properties of water as a solvent.

## Mark scheme (4 marks)

1. Water molecules are polar/have an uneven distribution of charge, with a partial negative charge on oxygen and partial positive charges on hydrogen atoms, allowing them to form hydrogen bonds with other polar or charged molecules.
2. Non-polar (hydrophobic) molecules cannot form hydrogen bonds or interact favourably with water, so water molecules maintain hydrogen bonds with each other and exclude non-polar regions.
3. Phospholipids are amphipathic, having a polar/hydrophilic phosphate head that interacts with water and a non-polar/hydrophobic fatty acid tail that is excluded from water.
4. The hydrophobic tails cluster together away from water (minimising disruption to hydrogen bonding) while the hydrophilic heads face the aqueous environment, driving spontaneous bilayer formation.

## Key terms

- [polarity](https://www.gradenine.co.uk/glossary/polarity)
- [amphipathic](https://www.gradenine.co.uk/glossary/amphipathic)
- [phospholipid bilayer](https://www.gradenine.co.uk/glossary/phospholipid-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-polarity-of-water-6468fd7d) · Published by Druglandscape Ltd.