2.3 Cell Size

01Why cells stay small

Everything a cell needs (oxygen, nutrients) and everything it gets rid of (wastes, heat) crosses the plasma membrane. So the membrane's surface area is the cell's supply line, while its volume is the demand.

As a cell grows, volume grows faster than surface area (volume scales with r³, area with r²). The surface-area-to-volume ratio (SA:V) drops, and eventually the membrane can't keep up.

02How organisms cheat the limit

Cells and tissues increase surface area without growing in volume: microvilli on intestinal cells, root hairs in plants, alveoli in lungs, flattened shapes like red blood cells.

Larger organisms also lose heat more slowly per unit mass because of their low SA:V — an elephant holds heat; a mouse must eat constantly to stay warm.

03Notebook box

Cube: SA = 6s², V = s³. Sphere: SA = 4πr², V = (4/3)πr³.

Worked example: Compare a 1 cm cube to a 3 cm cube. 1 cm: SA = 6, V = 1, ratio 6:1. 3 cm: SA = 54, V = 27, ratio 2:1. The smaller cube exchanges materials three times more efficiently per unit volume.

Key takeaways

✦Bigger cell → smaller SA:V → slower exchange per volume.
✦Folds, villi and flattening raise SA without raising V much.
✦Show your SA, V and ratio when doing calculations.

Watch outA HIGHER SA:V ratio is better for exchange. Students flip this when a graph shows ratio falling with size.

Quick check

Did it stick?

1.A cube with side 2 cm has a SA:V ratio of…

2.As a spherical cell doubles its radius, its SA:V ratio…

3.Microvilli on intestinal cells mainly…