# Explain how the fluid mosaic model accounts for the selective permeability of the plasma membrane.

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

## Mark scheme (4 marks)

1. The phospholipid bilayer forms a hydrophobic core that restricts the passage of polar/charged molecules (e.g. ions, glucose)
2. Small, nonpolar/lipid-soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely through the phospholipid bilayer
3. Intrinsic/integral proteins (channel or carrier proteins) span the membrane and allow specific polar molecules or ions to cross
4. The 'fluid' nature of the membrane (lateral movement of phospholipids and proteins) allows the membrane to adjust its protein composition, maintaining selectivity under varying conditions / proteins can be inserted or removed to regulate transport

## Key terms

- [fluid mosaic model](https://www.gradenine.co.uk/glossary/fluid-mosaic-model)
- [selective permeability](https://www.gradenine.co.uk/glossary/selective-permeability)

## 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-fluid-mosaic-model-63be009f) · Published by Druglandscape Ltd.