PROPERTIES OF PROTOPLASMIC MEMBRANES 133 



layer of protoplasm is also intelKgible on this point of 

 view. 



When we say that a cell or tissue is permeable to a 

 given substance, we state merely the ability of the 

 substance to penetrate; the method of penetration is 

 not stated. Thus a dissolved substance may penetrate 

 a partition by diffusing through the interspaces, or by 

 dissolving in or combining chemically with the substance 

 of the partition. In the case of ions there may be an 

 apparent penetration resulting from a change in the 

 electrical conditions at the surface of the partition. For 

 example, when a current is passed by zinc electrodes 

 through a bath of a zinc salt solution divided into two 

 compartments by a zinc partition, it is not strictly 

 correct to say that the zinc ions penetrate the partition. 

 The effect is the same as if they did, but in reahty zinc 

 ions are deionized and deposited as zinc on one face, 

 while from the other face the metal passes into solution 

 as zinc ions. There is apparently a penetration in such 

 a case, but not in reality; and it is possible that under 

 certain conditions the passage of a current through a 

 plasma membrane may similarly depend on the chemical 

 combination of ions on the one face simultaneously 

 with their release from the other. The physical and 

 chemical conditions of these various types of permeability 

 require careful examination. 



The factors determining the penetration of dissolved 

 substances through living plasma membranes are various, 

 and for the purpose of the present discussion they may 

 be grouped under the following heads: (i) structural 

 conditions, including thickness, density (water-content), 

 state of dispersion of structural colloids, size of interstices 



