# Explain how the structure of the plasma membrane enables it to act as a selectively permeable barrier.

> IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. The phospholipid bilayer consists of hydrophilic heads facing outward and hydrophobic tails forming the interior, creating a barrier that restricts the passage of polar/charged molecules and ions.
2. Channel proteins and/or carrier proteins span the membrane and allow specific polar molecules, ions, or other hydrophilic substances to cross, contributing to selective permeability.
3. Small, non-polar/uncharged molecules (e.g. O₂, CO₂, steroid hormones) can dissolve in and pass freely through the hydrophobic core by simple diffusion.
4. The fluid nature of the membrane (fluid mosaic model) allows lateral movement of proteins, enabling dynamic regulation of permeability, and the mosaic of different proteins provides specificity for different substances.

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-f624b870) · Published by Druglandscape Ltd.