# Explain how the process of simple diffusion differs from facilitated diffusion in terms of the molecules involved, the membrane structures used, and the direction of net movement.

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

## Mark scheme (4 marks)

1. Simple diffusion involves small, non-polar/lipid-soluble molecules (e.g. O₂, CO₂, steroids) moving directly through the phospholipid bilayer, whereas facilitated diffusion involves polar, charged, or large molecules (e.g. glucose, ions) that cannot cross the bilayer unaided.
2. Facilitated diffusion requires intrinsic/integral membrane proteins — either channel proteins (forming hydrophilic pores) or carrier proteins (that undergo conformational change) — whereas simple diffusion requires no protein.
3. Both simple and facilitated diffusion result in net movement of molecules down their concentration gradient (from high to low concentration), and neither process requires ATP/metabolic energy.
4. The rate of simple diffusion increases linearly with concentration gradient, whereas the rate of facilitated diffusion plateaus/reaches a maximum (Vmax) when all carrier/channel proteins are saturated.

## Key terms

- [simple diffusion](https://www.gradenine.co.uk/glossary/simple-diffusion)
- [facilitated diffusion](https://www.gradenine.co.uk/glossary/facilitated-diffusion)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-process-of-simple-243a3ac9) · Published by Druglandscape Ltd.