THE PERMEABILITY OF MEMBRANES 127 



There is clearly no question of parallelism, as would be the case if the 

 corpuscles were always permeable to glucose; neither does the content of the 

 corpuscles remain constant, as would be the case if they were always impermeable. 

 As a rule, rise in the content of the plasma is associated with a rise in that 

 of the corpuscles, but not in invariable proportion. The facts suggest the 

 possibility that the normal semi-permeability of the membrane to glucose is 

 connected with a particular difference of concentration on the two sides, but 

 that the actual value of this difference may be changed by other influences. The 

 membranes may be, as it were, tuned to different concentrations of glucose by 

 the action of other substances. Similar conditions may perhaps apply to cells in 

 general, but the data as yet available are not sufficiently decisive. 



The action of electrolytes on the permeability of the membrane suggests that electrical forces 

 play a part in the phenomena. The relation to precipitation of colloids will occur to the 

 reader. It seems also possible that the presence or absence of an electrical charge on the 

 membrane itself may be of importance in determining the permeability to ions. Suppose that 

 a membrane has a negative charge, it would, to a certain extent, oppose the passage of 

 electro-negative ions. Certain experiments by Girard (1910, p. 479) seem to support this 

 view. A membrane of gelatine allowed magnesium chloride to pass more freely when given 

 a positive charge by the presence of a trace of acid. The change produced in the structure 

 of the membrane, however, must be taken into consideration. In any case, it is difficult to 

 see how the presence of an electrical charge could exercise a permanent influence on the 

 distribution of an electrolyte between the two sides of a membrane, although the time taken 

 to attain equilibrium might be affected, a factor of importance in rapid changes of state. The 

 experiments of Mines (1912) on the production of potential difference will be referred to in 

 Chapter XXII. 



The work of Overton (1899, i. and ii.) has shown a striking correspondence 

 between the nature of a dye, as the salt of a colour-acid or a colour-base, and its 

 passage into cells. While the cell membrane is impermeable to the former, it is 

 readily permeable to the latter. The fact is brought by this investigator into 

 relation with lipoid solubility and the lipoid nature of the membrane, a question 

 to be discussed presently. Here, we may direct attention to the fact that these 

 two classes of dyes, or the coloured ions into which they dissociate, have opposite 

 electrical charges. The so-called " acid " dyes, that is, those in which the coloured 

 part of the salt is the acid radical, are electro-negative, while the " basic " dyes 

 are electro-positive, a fact which would undoubtedly have much influence on 

 their adsorption by constituents of the membrane and of the cell itself. In fact, 

 Endler (1912) has shown that the rate at which the diffusible dyes enter the 

 cell is greatly affected by the presence of various electrolytes and brings the fact 

 into relation with changes in electric charge, although it does not seem quite clear 

 whether the effects described by him are not rather, of a "lyotropic " origin. 



Hardy and Harvey (1911, p. 220) find that unicellular plants and animals 

 possess, as a rule, a surface charge, which varies with functional activity. This 

 latter fact is shown by the circumstance that different individuals of the same 

 species in a mixed culture were found to migrate in an electric field at different 

 rates. Red blood corpuscles, on the other hand, have a markedly uniform rate 

 of migration and may be regarded as having very slight chemical activity, 

 although living. The activity they possess is also very, uniformly distributed 

 between individuals. 



THE CONSTITUTION OF THE CELL-MEMBRANE 



To begin with, we must remember that the film covering the outer surface of 

 protoplasm, or, in fact, any surface where it is in contact with another phase, is 

 not of such a nature that it can be separated off, even optically, from the rest 

 of the cell. After death, under the action of toxic substances, it seems that a 

 distinct membrane may be visible. There are, of course, membranes covering 

 whole organs, which can be separated from the cells beneath them, such as the 

 interesting one on the barley corn, whose properties have been investigated by 

 Adrian Brown (1909). Such membranes play an important part in the physiology 

 of organisms, but are to be distinguished from those with which we are immediately 

 concerned. 



