KJG MEMBRANES {PLASM AH AUT) 



the amount of water held by the cell. The ratio of water to sohds 

 in a cell depends directly on the cholesterol and indirectly on the 

 lecithin content. Now, Leathes has shown that if a monomole- 

 cular layer of lecithin is prepared on the surface of water, the area 

 so covered is markedly reduced by the addition of cholesterol. In 

 some way this alcohol causes the long-chained phospholipine to 

 pack its molecules more closely together. Protein, cell-albumin 

 and a globulin and a nucleoprotein (so-called /3 globulin) also 

 enter into the composition of the cell membrane, and contribute 

 to its apparently capricious behaviour by virtue of their ampho- 

 teric nature. 



Structure. 



In spite of many attempts to overthrow it, the most satisfactory 

 explanation of the structure of a cell memljrane is the pore theory. 

 The question as to whether the pores are like those of a sponge 

 or like those of a honeycomb is not of importance, for the membrane 

 is of extreme thinness. It has been proved that the rate of passage 

 of a fluid through an artificial membrane is the same as the rate 

 of flow through capillary tubes. For our purpose, then, we may 

 consider that cell membranes are composed of some of the cell 

 material concentrated at the surface and admitting water, etc., 

 through the spaces between the molecules or other complexes 

 which compose this layer. 



Permeability. 



Artificial membranes may be prepared of any desired permea- 

 bility (Part II.). A membrane which allows water to pass 

 through and no solute is said to be semipermeable. A perfect 

 semipermeable membrane has never been prepared, though 

 Traube's copper ferrocyanide membranes are very nearly so. 



If an animal membrane, such as a pig's bladder, be stretched 

 across the end of a cylindrical tube so as to form a drum-head, 

 one has a simple dialysing membrane such as was employed by 

 Graham in his classical researches. When this membrane-covered 

 end is immersed in water, the liquid cannot rush into the dialysing 

 vessel all at once, Init slowly oozes through. A solution of sodium 

 chloride passes in almost as rapidly as water alone. Sugar passes 

 through the membrane slowly, while a starch solution fails to 

 penetrate at all. 



A list of hydrated ions could be drawn up in the order of their 

 magnitude or, which comes to the same thing, in the order of their 

 speed of migration. With certain apparent exceptions, which will 

 be mentioned immediately, the ability to pass through a membrane 



