# The cell membrane is described as a selectively permeable barrier. Explain how the cell membrane controls the movement of substances into and out of the cell, including reference to diffusion, osmosis, and active transport.

> Eduqas A-Level Biology — 1.3 Cell membranes and transport · Explain · 5 marks

> A student places a plant cell into a solution that has a lower water concentration than the cell's cytoplasm. The cell membrane plays a key role in determining what enters and leaves the cell.

## Mark scheme (5 marks)

1. The cell membrane is selectively permeable, meaning it only allows certain substances to pass through
2. Diffusion moves substances from an area of high concentration to an area of low concentration across the membrane (down a concentration gradient)
3. Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a selectively permeable membrane
4. In the scenario described, water would leave the cell by osmosis because the external solution has a lower water concentration than the cytoplasm
5. Active transport moves substances against a concentration gradient (from low to high concentration) and requires energy

## Key terms

- [cell membrane](https://www.gradenine.co.uk/glossary/cell-membrane)
- [selectively permeable](https://www.gradenine.co.uk/glossary/selectively-permeable)
- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)
- [osmosis](https://www.gradenine.co.uk/glossary/osmosis)
- [active transport](https://www.gradenine.co.uk/glossary/active-transport)

## Related

- [Revision notes for Eduqas A-Level Biology](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/the-cell-membrane-is-described-as-4610d5b7) · Published by Druglandscape Ltd.