CONCEPTS AND TERMS 



hold with increasing concentration until levels are reached so ex- 

 treme that one may suspect the nature of the membrane will have 

 been changed. The diffusion rate depends further on the material 

 traversed, represented in the diffusion equation by the diffusion con- 

 stant X D . This diffusion equation gives the velocity of migration for 

 unit membrane area as: 



V = Kjy(Si — $2) 



The temperature coefficient of free diffusion in aqueous solu- 

 tion is low, so that the Qi is undoubtedly not above 1.5. On the 

 other hand, Davson and Danielli (1943) have pointed out that diffu- 

 sion across a thin lipid layer from one aqueous phase to another 

 may be very slow and show a very high Q10, so that a Q10 of 2 or 

 3 or even higher cannot be taken to exclude diffusion as the rate- 

 limiting step. 



The diffusion constant K u measures the permeability of the 

 membrane to the solute, that is, the ease with which the solute finds 

 its way through the membrane. A membrane in the phvsicochemical 

 sense is any boundary we choose to designate as separating two 

 phases. In the biological sense, we usually mean to designate an 

 anatomic structure. Notice that permeability is an appropriate term 

 only within the subject of passive migration. To speak of the whole 

 subject of transport as the permeability problem is to miss the mark 

 badlv, and to reflect an optimism of two or three decades ago that 

 the whole matter of biological distribution could eventually be ex- 

 plained by passive permeation. 



E. N. Harvey wrote (1943) about the cell barrier: 



Just as chemistry could not have developed without test tubes 

 to hold reacting substances, so organisms could not have evolved 

 without relatively impermeable membranes to surround the cell 

 constituents. This barrier between the inside and the outside, 

 the inner and external world of each living unit, has been and 

 always must be considered one of the fundamental structures 

 of a cell. No one can fail to be impressed with the great differ- 

 ence in properties of living and dead cells. The dead are com- 

 pletely permeable to diffusible substances, while the living retain 

 one material and pass another. This difference, selective per- 

 meability, is so marked that it becomes the surest test to dis- 

 tinguish the living from the dead, holding where all other meth- 

 ods fail. It can truly be said of living cells, that by their mem- 

 branes ye shall know them. 



*3 



