138 MEMBRANES {PLASMAHAVT) 



hours they may be found ahiiost entirely in this phase. Iodine and 

 eosin and similar substances pass in the same direction, but more 

 slowly and less completely, while ferric chloride, cupric acetate, etc., 

 take a very long time to leave the aqueous phase. On the other 

 hand, the interface seems quite impermeable to the chlorides of 

 cobalt, chromium and nickel. All the salts to which the living cell 

 is impermeable are also excluded completely, or almost so, from 

 passage across this laboratory-made interface. 



Of course a cell is in close juxtaposition to several other cells, 

 and therefore the composition, structure, and permeability of any 

 one cell membrane may vary from place to place according to the 

 nature of the interface. One interface may be such as to allow 

 free passage of solutes to which other interfaces may be semi- 

 permeable. 



Alterations in permeability may be due to (1) alterations in 

 the membrane or (2) alterations in the material presented to it 

 from either side. 



(1) The nature of the membrane is of great importance in 

 studies of permeability. The cell membrane is unique, and one 

 cannot guard too carefully against the adoption of generalisations 

 drawn from experiments in which collodion, parchment or other 

 artificial membranes have been used. Even the behaviour of dead 

 animal membranes or of that of the erythrocyte is quite different 

 from the true plasmahaut. Further, the membrane itself may 

 undergo change in composition and permeability as the cell 

 contents or the environment change in {a) composition, or in 

 (6) physical state. The composition of the surface layer depends 

 on the substances present in solution in the interior and on the 

 nature of the interface. Any alteration in the chemical state of 

 either of these phases will produce such an alteration at the surface 

 as will alter permeability. The electrical double layer on the 

 surface plays a considerable part in deciding the composition of the 

 membrane. If a solute of opposite electrical sign to the membrane 

 come within the electrical sphere of attraction it will be adsorbed 

 and will either thicken the membrane or may occlude, wholly or 

 partially, some of the interstices. In any case, adsorption will 

 alter the permeability of the adsorbing surface. It may have a 

 further effect. The adsorbed material may enter into combina- 

 tion, chemical or physical, with the membrane, producing a second 

 alteration in permeability. It may even cause a third alteration, 

 by ultimately passing through the membrane and going into 

 solution on the other side. 



If the adsorbed material be an amphoteric colloid, then the 

 electrical charge on the membrane may be modified and so produce 



