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

> IB DP Biology Higher 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 prevents the passage of polar/charged molecules (e.g. ions, glucose)
2. Small non-polar / lipid-soluble molecules (e.g. O₂, CO₂, steroid hormones) can pass freely through the phospholipid bilayer by simple diffusion
3. Integral/transmembrane channel or carrier proteins span the bilayer and allow specific polar/charged substances to cross
4. The mosaic/fluid arrangement allows proteins to be distributed according to the cell's transport needs, so different membrane regions can have different permeabilities / the composition of the membrane can vary

## 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 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-fluid-mosaic-model-434db2a4) · Published by Druglandscape Ltd.