The size of a cell is limited by the relationship of the cell's outer
surface area to its volume, or its surface area-to-volume ratio.
As a cell grows, its volume increases much faster than its surface area
does, as shown in Figure 4-5. This trend is important because the
materials needed by a cell (such as nutrients and oxygen) and the
wastes produced by a cell (such as carbon dioxide) must pass into
and out of the cell through its surface. If a cell were to become very
large, the volume would increase much more than the surface area.
Therefore, the surface area would not allow materials to enter or
leave the cell quickly enough to meet the cell's needs. As a result,
most cells are microscopic in size.