2.3 Cell Size

Surface area vs. volume

As a cell grows, its volume increases faster than its surface area (volume scales with the cube of length, surface area only with the square). So bigger cells have a lower surface-area-to-volume (SA:V) ratio.

Why cells stay small

A cell exchanges materials across its surface but uses them throughout its volume. A high SA:V ratio means efficient exchange, so cells stay small — or become long, thin, or folded. Structures like microvilli add surface area to speed absorption.

Key terms

Surface-area-to-volume ratio

Surface area divided by volume; higher ratios allow more efficient exchange.

Why cells stay small

To keep a high SA:V ratio so materials can be exchanged fast enough.

Volume vs. surface area scaling

Volume grows with length cubed; surface area only with length squared.

Microvilli

Tiny membrane folds that increase surface area for absorption.

Diffusion distance

Smaller cells keep the distance materials must travel short and efficient.

Try an AP-style question

What are the surface-area-to-volume ratios of Cube A and Cube B?

1. 1 : 6 and 3 : 1
2. 6 : 1 and 6 : 1
3. 6 : 1 and 2 : 1
4. 6 : 1 and 18 : 1