THE PERMEABILITY OF MEMBRANES 137 



on the colloids of the cell membrane. Another important fact in this connection is that blood 

 corpuscles are much more sensitive to saponin when suspended in isotonic sodium chloride 

 than in isotonic cane-sugar, as found by Handovsky (1912, p. 413). For example, 0'002 

 per cent, saponin produced 98 per cent, haemolysis in the former cafe, but only 20 per cent., 

 under similar conditions, in the latter. The way in which this effect is produced is not quite 

 clear. Although saponin may not be in colloidal solution in water, the experiments of 

 Dumanski on molybdenum oxide, referred to on page 95 above, suggest that the presence of 

 electrolytes may cause it to assume the necessary aggregated condition, and thus the electrolyte, 

 also changing the sign of the charge on the corpuscles, may facilitate adsorption by electrical 

 means. 



r In the second place, there are reasons, as we have seen, for rejecting the 



"Hypothesis of a membrane consisting of a simple kind of substances, lipoid or 



protein, alone, and for regarding it as a .complex colloidal system of all cell 



constituents, together with those of the outer liquid, which diminish the surface 



energy at the interface. 



Lepeschkin (1911), in fact, comes to the conclusion, as the result of an elaborate series of 

 experiments, that a simple mosaic structure of lipoid and protein is not a satisfactory hypothesis, 

 but that a colloidal complex is necessary. The effect of the addition of varying proportions of 

 glycerol or castor oil to the collodion of which an artificial membrane is made will occur to the 

 reader (page 95 above). The function of lipoids is suggested by Lillie (1912, 2, p. 17) to be 

 that of increasing the stability of the other colloids, in fact as a protection from excessive 

 aggregation, as described in the preceding Chapter (page 97). 



Although lipoids must enter into the composition of the membrane, it seems 

 evident that their relationship to substances which are supposed to be "lipoid- 

 soluble is not that of solvents, in which case the laws of partition would be 

 obeyed, but rather that of surface adsorption, owing to their state of colloidal 

 dispersion. A colloidal solution of lecithin in benzene behaves quite differently 

 from one of benzene in lecithin, that is, according to which is the external or 

 continuous phase and which the internal or dispersed phase. . 



Ruhland (1913) gives strong evidence that, at all events as regards dyes 

 and enzymes, permeability is not a question of solubility in the membrane, 

 but of the dimensions of particles or molecules; that is, the membrane may 

 be looked upon as a sieve. In this paper a full account of the literature on the 

 subject is given. 



An important point to remember is that the membrane must not be looked 

 upon as an invariable permanent structure. Its permeability can be changed 

 by reagents applied to the outside, as in the experiments of Osterhout, where 

 sodium salts make it permeable to the Na ion, while the addition of calcium 

 re-establishes the normal state of semi-permeability ; other cases have been given 

 above. Functional changes of the cell itself are also associated with changes in 

 permeability, as will be shown in the next section. If, however, we look upon the 

 cell membrane as an integral part of the protoplasmic system, as locally concen- 

 trated constituents of the cell, this behaviour will not seem so difficult to 

 understand. 



PHENOMENA IN WHICH CHANGES OF PERMEABILITY OCCUR 



Supposing that the cell membrane becomes impermeable to substances to 

 which it was previously permeable, what effects may be expected to follow ? We 

 know that, in a reversible reaction, the position of equilibrium depends on the 

 relative concentration of the constituents of the system. Such a reaction will 

 therefore continue to take place in one direction if the products are allowed to 

 escape from the cell, but, if the membrane becomes impermeable to them, the 

 reaction will come to an equilibrium and cease. 



Take the case of starch or glycogen stored in a cell, which cell also contains an enzyme 

 capable of causing their hydrolysis to sugar ; if the membrane is impermeable to this sugar, 

 the reaction soon comes to an end, partly on account of the back reaction, -partly because the 

 action of the enzyme is more or less paralysed by the accumulation of the products of i 

 activity, as we shall see in Chapter X. But, as soon as the products are allowed to escape 

 again, the reaction starts afresh. This consideration applies to any reversible reaction taking 

 place in the cell. 



From the powerful action of electrolytes on colloidal systems, such as that of 



