Cells are small because larger cells could not function as efficiently. Commands from the nucleus of the cell must travel to other parts of the cell, and the less distance they have to travel, the quicker communication can occur. The second reason for small cells is surface-to-volume ratio. As cell size measured by, for example, diameter, increases surface area grows much more slowly than volume. For a round cell surface area increases as the square of the diameter, but volume increases as the cube of the diameter.
Example:
Cell diameter (cm) 0.5cm 1cm 1.5cm
Surface area (cm2) 0.79 3.14 7.07
Volume (cm3) 0.06 0.52 1.77
Surface to volume ratio 13.2: 1 6.0:1 4.0:1
Therefore, for an increase in cell diameter of 10 fold, surface area increases by approximately 100 and volume by approximately 1000.
Radius 1 cm 10 cm
Surface area 4 pi r2 12.57 cm2 1257 cm2
Volume = 4/3 pi r2 4.189cm2 4189 cm2
A cell’s surface provides the only outlet for interaction with the outside and because large cells have proportionally much less surface area they cannot exchange materials as efficiently as smaller cells.
One way to increase surface area for a cell without increasing volume is to have outfoldings and infoldings of the cell to increase the surface area. Many cells have such structures.