# Explain why the phospholipid bilayer is described as selectively permeable, referring to the different properties of the molecules that cross it.

> OCR A-Level Biology A (H420) — 2.5 Biological membranes · Explain · 4 marks

> Cell membranes control the movement of substances into and out of cells. Experimental evidence shows that small non-polar molecules cross the membrane rapidly, while large polar molecules and ions cross very slowly or require assistance.

## Mark scheme (4 marks)

1. The hydrophobic core of the bilayer prevents passage of charged or polar molecules/ions
2. Small non-polar / lipid-soluble molecules can diffuse freely through the hydrophobic core
3. Large or polar molecules / ions require protein channels or carrier proteins to cross the membrane
4. This selective control means only certain molecules can cross, making the membrane selectively permeable rather than freely permeable

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-why-the-phospholipid-bilayer-is-c01868fe) · Published by Druglandscape Ltd.