PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 281 



The Sieve Hypothesis. — The foregoing considerations of the 

 chemical constitution of the protoplasmic membrane give some 

 indication of how it may function as a selectively permeable 

 system. We may now consider this latter property of the 

 membrane in greater detail. 



There are three main hypotheses of the mechanism responsible 

 for the selective properties of the protoplasmic membrane. One 

 views the problem mechanically; the other, chemically; and the 

 third, electrically. A membrane which functions in a purely 

 mechanical way may be likened to a sieve with pores that allow 

 fine particles but not larger ones to pass through. The chemical 

 view of membrane mechanics postulates solubility as the deter- 

 mining factor. Substances enter the cell by dissolving their way 

 through. From an electrical point of view, the membrane is 

 regarded as charged and therefore permits only particles of one 

 sign to pass. 



The German physiologist Traube (1867) is primarily respon- 

 sible for the suggestion that the plasma membrane functions as a 

 molecular sieve. The membrane is presumed to have pores that 

 permit small molecules but not large ones to pass. A membrane 

 satisfying these conditions bears Traube's name. A Traube 

 precipitation membrane is formed when copper sulphate comes 

 into contact with potassium ferrocyanide (Fig. 101). A crystal 

 of the former is put into a solution of the latter, and a membrane 

 of copper ferrocyanide is precipitated around the sulphate crystal. 

 This membrane permits the passage of water, but salt molecules 

 are held back. 



In so far as it exhibits a certain amount of selective perme- 

 ability, the copper ferrocyanide membrane emulates the living 

 plasma membrane, but let us see if the analogy is very close. 

 This question has been answered by CoUander. He showed that 

 the behavior of the two is quite different, as the table ^ on page 

 282 indicates: 



It is evident that there is no parallelism at all between the 

 relative permeability of the living cell {Rheo discolor) and that of 

 the artificial membrane. But the molecular volumes of the sub- 

 stances and their relative solubility in ether indicate two impor- 

 tant facts — that the rate of entrance of the substances into the 

 artificial membrane is a function of their size and that the rate of 



1 From R. Hober. 



