52 



MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



surrounding- areas. Hence, there will be frequent collisions among the 

 dye molecules in this area ; but as they move out to the areas where they 

 are less concentrated, such collisions are less frequent. The molecules 

 of the dye will thus gradually spread out from the area around the crys- 

 tal and eventually become evenly distributed. Once this has been ac- 

 complished the movements will be equal in all directions, and the solu- 

 tion will remain of equal density in all of its parts. To sum up, we say 

 that diffusion is the movement of molecules from a region where they 

 are more concentrated to a region where they are less concentrated. 



Fig. 4.1. Demonstration of the principle of diffusion. A crystal of dye dropped into 

 a beaker of water begins to dissolve and the particles move away from the crystal 

 toward the areas of lesser concentration. The second beaker shows the results of this 

 reaction; the crystal has completely dissolved and the particles of dye are uniformly 



distributed throughout the water. 



Let us consider an analogy which might help us to remember the 

 principle of diffusion. Suppose we had a hundred blindfolded people 

 tightly bunched in one corner of a large gymnasium and held in this 

 corner by a surrounding rope. These people could move to a certain 

 extent, but would be so crowded that they could not move about among 

 one another very well. These we might compare to the crystal of dye 

 with its molecules in the solid state. Now suppose that the rope was 

 cut and the people were told to walk and keep walking. Those at the 

 outer edge of the bunch could not move inward, but they could begin 

 walking outward. As they moved out those farther in could be free to 

 begin walking. There would be frequent collisions at first among the 

 congestion of the newly loosed people, but each time they collided they 

 would turn and move in a different direction. Those that moved out- 



