54 



MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



narily be permeable to the larger particles of colloids, emulsions, and 

 suspensions. Such membranes which will allow these smaller particles 

 to pass through, but which hold back the larger particles, are called 

 differentially permeable membranes. The plasma membrane which 

 surrounds cells is such a differentially permeable membrane. To aid 

 in your understanding of this, let us turn to another rather far-fetched 

 analogy among the things with which you are familiar. Suppose we 

 had a room divided down the center from floor to ceiling with a net 

 of the general nature of a tennis net. On one side of the net suppose 

 we had a general assortment of fruit — grapes, limes, grapefruit, and 



Rubber membrane 



Animal membrane 



t 



• • . \ • •.* . . 

 •••♦... .. 

 *.*•* • ••• • 



_ • * • 



•*• • • " „ 



.. • . • • • 



No diffusion 



Diffusion 



Fig. 4.2. Diffusion through a membrane. Whenever a membrane is impermeable to 



dissolved particles there is no diffusion. When the membrane is permeable, however, 



the particles pass through and there is diffusion through the membrane. 



cantaloupes. We put several boys on each side of the net and tell them 

 to start throwing this fruit at the net. When the grapes and limes are 

 thrown, some of these will go through the openings in the net, but the 

 grapefruit and cantaloupes will be stopped by the net and will remain 

 on the same side. As the grapes and limes go through, the boys on the 

 other side will begin throwing them back. Allow this to go on for 

 some time, assuming that the fruit is not crushed in the process, and 

 we will find that the limes and grapes are evenly distributed on the two 

 sides of the net, but the grapefruit and cantaloupes remain on the side 

 where they were at the beginning. Now think of the grapes and limes 

 as small molecules of a dissolved substance and the grapefruit and can- 

 taloupes as colloidal particles and perhaps you can have a better under- 



