PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 271 



tissue exhibit true cellular permeability. This is true of the 

 kidney, the tubules of which function in their permeability 

 like that of a typical cell. There is also the possibility that 

 tissue such as that of the lining of the intestines may owe its 

 permeability to purely mechanical processes. For example, 

 the intestines and other living membranes permit water to pass 

 through more rapidly in one direction than in another. This 

 may not be a true permeability process at all but due to certain 

 cellular activities, such as rhythmic contraction of the intestinal 

 villi. 



Another so-called permeability phenomenon which has 

 aroused considerable interest and discussion, and which may 

 not be a permeability process at all but one of metabolism, is 

 that of the relative rate of entrance of different sugars into the 

 cell; thus, it is generally known that glucose enters cells more 

 readily than does sucrose. Further, glucose is fully absorbed 

 in the body, while fructose and manose are not. It is further 

 said that the dextrorotary form of certain substances (alanin) 

 is taken up by the animal body, while the levorotary form is not 

 absorbed and appears in the urine. 



Attempts have been made to explain these phenomena in 

 terms of strict permeability processes — e.g., on the basis of size 

 of molecule. The whole problem of sugar absorption appears 

 to be primarily, if not solely, one of metabolism. Glucose is 

 used in respiration, while sucrose is not. Consequently, as 

 glucose is destroyed within the cell, there is room for more, 

 strictly on the basis of a concentration equilibrium. As the 

 sucrose is not used, no more enters. The protoplasmic mem- 

 brane may thus play no part whatever in determining the rate 

 of entrance of sugars into the cell, metabolic processes alone 

 being responsible. This may also be true for the little under- 

 stood passage of sugar from one cell to another, in which sugar 

 enters a cell at one end and leaves it at another. 



In contrast to the foregoing phenomena, others equally diffi- 

 cult of interpretation rest apparently upon selective perme- 

 ability pure and simple; such is the more rapid entrance of 

 sodium chloride than that of sodium sulphate into cells. 



Selectiveness is the outstanding characteristic of cell perme- 

 ability; next in significance is change. The permeability of the 

 plasma membrane is not a fixed property. Change in perme- 



