WHY ARE MOST CELLS SMALL?
Most cells
are small and can be seen only with the aid of a microscope. (Exceptions include the eggs of
most vertebrates [fishes, amphibians, reptiles, and birds] and
some long nerve cells.) One reason for the smallness of cells is that the ratio
of the volume of the cell’s nucleus to the volume of its cytoplasm must not be
so small that the nucleus, the cell’s major control center, cannot control the
cytoplasm. Another aspect of cell volume works to limit cell size. As the
radius of a cell lengthens, cell volume increases more rapidly than cell
surface area (figure 2.3). The need for nutrients and the rate of waste
production are proportional to cell volume. The cell takes up nutrients and eliminates
wastes through its surface plasma membrane. If cell volume becomes too large,
the surface area-to-volume ratio is too small for an adequate exchange of
nutrients and wastes.
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