# Explain why the phospholipid bilayer acts as a selective barrier, allowing some molecules to cross freely whilst preventing others from crossing without the assistance of membrane proteins.

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

> Cell membranes are described as selectively permeable structures. The phospholipid bilayer forms the fundamental barrier of all cell membranes, yet cells must be able to exchange a wide variety of substances with their surroundings to maintain homeostasis.

## Mark scheme (4 marks)

1. The hydrophobic core of the bilayer is formed by the non-polar fatty acid tails, which repel polar and charged molecules.
2. Small non-polar / lipid-soluble molecules (e.g. oxygen, carbon dioxide, steroid hormones) can dissolve in and diffuse freely across the hydrophobic core.
3. Polar molecules and ions cannot cross the hydrophobic core unaided because they are hydrophilic / repelled by the non-polar interior.
4. Membrane proteins (channel or carrier proteins) provide a hydrophilic pathway, allowing polar molecules and ions to cross the bilayer.

## Key terms

- [phospholipid bilayer](https://www.gradenine.co.uk/glossary/phospholipid-bilayer)
- [selective barrier](https://www.gradenine.co.uk/glossary/selective-barrier)
- [membrane proteins](https://www.gradenine.co.uk/glossary/membrane-proteins)

## 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-acts-4adccc14) · Published by Druglandscape Ltd.