Cell Size and Volume
Plants and animals are multicellular; they contain many
small cells. Molecules often enter a cell and move about the
cytoplasm by diffusion, a physical process dependent on
concentration gradients. Diffusion is not a highly rapid or
efficient means of distributing materials over long cellular
distances. The dependence of cells on this process places a
constraint on their design and size. No portion of even the
largest of cells is more than 1 mm from the cell
membrane. Cells can be long and skinny like a nerve
cell, but they can't be too short and fat.
Most cells are much smaller than even 1 mm across. A
cuboidal epithelial cell is just about l5µm along one
side, for example. An explanation for why cells stay so
small and indeed why the body of a plant or animal is
multicellular can be found by considering the relationship
between surface area and volume relationships. Suppose a
cell was 1 mm in each dimension (height, width, depth);
its surface area would be:
6 x (1 mm x 1 mm) = 6 mm 2
The volume of the cell would be: 1 mm x 1 mm x 1 mm = 1 mm3
If the linear dimensions of the cell doubled, its surface
area would be 24 mm 2 and its volume would be 8 mm 3.
Notice that as a result of the cell doubling in size, the volume
actually octupled but the surface area only quadrupled.
Nutrients enter a cell and wastes exit a cell at the cell
membrane. A large cell requires more nutrients and produces more wastes than does a small cell. Therefore a large
cell that is actively metabolizing cannot make do with proportionately less surface area than a small cell. Yet as discussed above and illustrated in table 1, as a cell increases
in size, surface area decreases rapidly in proportion to
volume. This means that as cells increase in size, the amount
of surface area becomes inadequate to allow exchanges
within a reasonable length of time and the cell is unable to
exist.
For example, consider that egg cells are among the
largest known cells. A chicken's egg is several cm in di ameter, but the egg is not actively metabolizing and contains a large amount of storage material called yolk.
Once the egg is fertilized and metabolic activity begins, the
egg divides repeatedly. Cell division provides the surface
area needed to allow adequate exchange. Similarly, in the
body when cells are growing and actively metabolizing, cells
must divide in order to maintain enough surface area per
unit volume.
Cells that specialize in absorption have modifications
to greatly increase the surface area per volume of the
cell. The columnar cells along the surface of the intestinal
villi have microvilli to increase their surface area and so
do the cells that line the kidney tubules. Modifications are
necessary to increase surface area per volume and never
vice versa because of the mathematical relationships
noted in table 1.
Table 1
Linear dimensions
of cell
Surface (mm2)
1 mm
6
1
6:1
2 mm
24
8
3:1
4 mm
96
64
1.5:1
Volume (mm3)
Ratio