MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 53 



ward from the greater concentration of people would travel farther 

 without collisions. When they hit the walls they would turn and go 

 in a different direction. In the course of time there would be a grad- 

 ual dispersion of people in the room until they would be about equally 

 spaced in the room and would remain that way even though they con- 

 tinued walking and bumping. If we think of the particles of a dis- 

 solved substance in place of the people and the energy of heat as the 

 force which keeps them moving, we have a rough impression of what 

 happens in diffusion. 



Diffusion will occur in gases as well as in liquids. If we open a bot- 

 tle of strong perfume in one corner of a room, it will not be long before 

 we can detect the odor in all parts of the room. The molecules of per- 

 fume diffuse from the area where they are more concentrated to areas 

 of lesser concentration. It will even occur to a certain extent in solids, 

 but since the molecules of a solid are not free to move about one an- 

 other readily, such diffusion is extremely slow. 



Before closing this discussion we should keep in mind the fact that 

 the molecules of the solvent (water, etc.) are in motion as well as the 

 molecules of the solute (dye, perfume, etc.). These moving molecules 

 behave according to the same principles of diffusion as described for 

 the solute and aid in the general mixing of particles in solutions of dif- 

 ferent density. 



Diffusion Through a Membrane 



A membrane does not stop the process of diffusion if the membrane 

 is permeable to the particles of the dissolved substance. A sheet of 

 rubber stretched between two solutions of different concentration would 

 prevent diffusion because the rubber molecules which form the rubber 

 sheet fit together so tightly that they will not permit any dissolved 

 particles to pass through. There are many membranes, however, which 

 contain openings between the molecules of which they are formed that 

 are large enough to permit dissolved particles to pass through. Dif- 

 fusion can take place through such permeable membranes. Minerals 

 in solution in the soil diffuse through the outer membrane of the cells 

 of the root hairs, and food in solution in your intestine diffuses into the 

 cells lining the intestine because the membranes around these cells are 

 permeable to dissolved particles. 



Since the size of dispersed particles in a mixture varies greatly, it 

 stands to reason that all such particles cannot pass through the open- 

 ings between the molecules of membranes. A membrane which is per- 

 meable to the small dissolved particles of a true solution will not ordi- 



