62 MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



Surface-Volume Ratio of Cells 



In Chapter 2 it was pointed out that cell duplication was necessary 

 to keep cells from growing so large that they could not have sufficient 

 exchange of materials through their plasma membranes to support 

 life. The efficiency of cells is dependent upon their small size. A 

 small object will have a greater surface area in proportion to its vol- 

 ume than a larger object of the same shape. Since the rate of diffusion 

 is dependent upon the exposed surface, it becomes evident why the 

 small size is important. 



To explain this principle more fully let us think of a cube measuring 

 four inches in each direction. This cube would have ninety-six square 



ches 



2 inches 



1 inch 



1.5 : 1 



3: 1 



6:1 



Surface - volume ratio 



Fig. 4.6. Effect of size on the surface-volume ratio. As the size of a cube becomes 

 smaller, it has a relatively greater exposed surface. 



inches on its surface and sixty-four cubic inches of volume with a 

 surface-volume ratio of 1.5 to 1. If we now take a cube which measures 

 only two inches in each direction, we find that it has twenty-four square 

 inches of surface, but is only eight cubic inches in volume. The surface- 

 volume ratio here is 3 to 1. A one-inch cube would have six square 

 inches of surface and one cubic inch of volume, a ratio of 6 to 1. 

 Thus, we see how the surface-volume ratio increases as objects get 

 smaller. 



We see this principle illustrated over and over again in the physi- 

 ology of living things. Human red blood cells are very small and are, 

 therefore, much more efficient than they would be if they were larger. 



