# Explain how the phospholipid bilayer and membrane proteins together enable a cell to control the movement of ions across its plasma membrane.

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

## Mark scheme (4 marks)

1. The phospholipid bilayer is hydrophobic in its core / non-polar interior, which prevents free diffusion of ions (charged/polar particles) through the bilayer.
2. Channel proteins / ion channels provide hydrophilic pores that allow specific ions to move across the membrane by diffusion (down their concentration gradient / electrochemical gradient).
3. Carrier proteins / pumps can actively transport ions against their concentration gradient using ATP, allowing the cell to move ions to where they are in higher concentration.
4. The number, type, and gating (opening/closing) of membrane proteins can be regulated, giving the cell control over which ions move and when / in what quantity.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard 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-phospholipid-bilayer-and-c509e48e) · Published by Druglandscape Ltd.