6.4 Cell Size, Growth and Division Signals

A cell's volume approximates the amount of metabolically active material requiring substrates and producing waste. Its surface area limits exchange across the plasma membrane. If every linear dimension increases by factor k, area increases by k² but volume by k³; the area-to-volume ratio therefore falls as size rises. Diffusion paths to the interior also lengthen.

For a cube of side L, area is 6L², volume is L³ and SA:V is 6/L. A 1-unit cube has ratio 6:1, a 2-unit cube 3:1 and a 3-unit cube 2:1. The cube is a model, but the scaling rule applies to similar shapes. It explains why one giant cell cannot simply replace many smaller cells with the same total volume.