92 PHYSICAL CHEMISTRY 



isotonic sugar solution, to be equal to that of a 0.15 per cent NaCl 

 solution, whereas the Kohlrausch conductivity = 0.03 per cent 

 NaCl. It seems probable, therefore, that the chief impermeable 

 layer of all cells is the plasma membrane, although other mem- 

 branes, nuclear membranes and vacuole membranes, may also 

 be impermeable to many substances. 



These experiments of Hober are selected because the cells are 

 not injured in the least. If space permitted, a large number of 

 experiments which go to show that the plasma membrane is the 

 semipermeable part of the cell could be described. The word 

 semipermeable was first used to describe permeability to water 

 and to nothing else. This is never true of the plasma mem- 

 brane, since it is always permeable to oxygen, for instance. The 

 word is now used to denote limited permeability. It will be 

 shown later that the plasma membranes of some cells are im- 

 permeable to water. 



Before considering the permeability of cells to various sub- 

 stances it is well to discuss the diffusion of substances in general, 

 from one phase to another of a diphasic system in particular. 



According to Euler the diffusion of undissociated molecules 

 through water is inversely proportional to the square root of 

 the molecular weight, large molecules therefore diffusing more 

 slowly than small ones. The speed of diffusion of ions is given 

 on page 24. 



Diffusion is retarded by increase in the viscosity of a solution, 

 hence the emulsoids, which enormously increase the viscosity, 

 decrease diffusion. From this it also follows that those sub- 

 stances that increase the viscosity of water diffuse slowly. 



The molecules of a substance in solution move in all directions, 

 the substance diffusing from regions of higher to regions of 

 lower concentration. The rate of diffusion is proportional to 

 the concentration gradient. 



In the diffusion of a substance from one phase into another, 

 three processes are involved: first, the diffusion to the phase 

 boundary; second, solution in the second phase; third, diffusion 

 away from the phase boundary. 



Substances which lower the surface tension collect at the 

 phase boundary, this step in the process being rapid in this case. 

 This concentration in the surface film is due to two factors, 



