Why Cells Are Small

Cells are the basic structural and functional units of life.   As life on


earth has evolved into organisms of varying complexities, two basic laws


of nature have dictated why cells have remained so small.


Shorter is


faster.   This is true both in terms of diffusion and in terms of chemical


and electrical movement.   By minimizing the the distance between a cellÕs


nucleus and and the numerous proteins and organelles that it must


constantly regulate , a cell is maximizing the speed in which


intercellular communications can take place while providing the ideal


conditions for diffusion: a vital function in the life of a cell.  


Like


wise, the surface area and volume of a cell are directly influential in


the efficiency of the cellÕs nutrient absorption and waste expulsion


processes.   Since the cell membrane of a eukaryotic cell is its only


source of nutrition, itÕs surface area must be large enough to allow the


cellÕs organelles to receive the materia ls it needs.   This is done by


maximizing the surface area to volume ratio.   By using the surface area


and volume equations for a sphere(4¹r2 and 4/3¹r3) you can estimate the


surface area of a small cell(5 µm) to be nearly 1,200,000 : 1.   By


modeling the   growth rate of the surface area and volume of a sphere on a


linear graph itÕs easily discernible that as the size of the sphere


increases the ratio of surface area to volume dramatically decreases


until finally the volume of the sphere surpasses the surf ace area.





Simply, by minimizing its size, a cell is maximizing the speed at which


it can communicate, the rate at which diffusion can occur, and the amount


of surface area at itÕs disposal.