# Explain how the structure of a phospholipid bilayer allows water molecules to move across the plasma membrane by osmosis.

> IB DP Biology Standard Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Phospholipids are arranged in two layers (bilayer) with hydrophobic fatty acid tails facing inward / towards each other
2. The hydrophobic core restricts / prevents free passage of most polar or charged solutes across the membrane
3. Water molecules, being small and polar, can pass slowly through the phospholipid bilayer / directly between phospholipid molecules
4. Water moves by osmosis from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) across the partially permeable membrane

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-b1233abd) · Published by Druglandscape Ltd.