110 MEMBRANES (PLASMAHAUT) 



Structure. 



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

 explanation of the structure of a cell membrane 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. p. 405). 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, but 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 

 is a function of the size (or speed) of a particle in solution. By 

 a careful selection of membranes a mixed solution may be separated 

 into its constituent solutes. In general, a membrane acts like 

 a filter-paper made infinitely fine so that ultramicroscopic 

 particles may be retained on the filter. Indeed, the process of 

 separating substances in solution from one another has been 

 termed " ultra-filtration." 



A living membrane, however, alters in its permeability. It may 

 at one time allow a solute to pass through and at another prevent 

 its passage : or at times allow a comparatively large particle to 



