THE PERMEABILITY OF MEMBRANES 



133 



the cell, is rather of the nature of a hindrance, since it holds fast the substances 

 instead of passing them on. At the same time, the fact has to be explained why it 

 is just these particular things that enter the cell so easily, although some property 

 other than partition according to solubility will have to be brought into the 

 account. 



Before proceeding further, a few words may be said as to cholesterol. The 

 ubiquitous presence of this chemically-inert substance is a remarkable fact and 

 suggests that it must have some important part to play in the regulation of the 

 mechanisms of the cell. In strictness, it is not a lipoid, although for convenience 

 usually reckoned with them. In chemical constitution, it is the monatomic 

 alcohol of a substance related to the terpenes ; according to Windaus and Stein 

 (1904), the complex terpene in question is methyl-isopropyl-phenanthrene. The 

 most familiar terpenes are the essential oils of plants, cymene, for example, from 

 oil of caraway seed and from oil of eucalyptus is methyl-isopropyl benzene. It is 

 of some interest to find in the animal a representative of this class of substances 

 so widely spread in the vegetable kingdom. Cholesterol is soluble in ether, 

 benzene, chloroform and fats ; insoluble in water and in cold alcohol. In the 

 work of Loewe, above mentioned, it was found that it could take up lipoid-soluble 

 dyes to a very small degree only and apparently in accordance with the partition 

 law and not with that of adsorption. 



Although it is necessary to hold that lipoids form a part of the constituents of 

 the cell membrane, there is reason to doubt that they are the sole substances 

 taking part in it. For one thing, it is very difficult to understand how the 

 permeability is capable of regulation by processes occurring inside and outside of 

 the cell unless the membrane has a very complex composition. There is, also, 

 more direct evidence that a more complex structure than a mere lipoid one is 

 concerned, as we shall see presently. But, whatever explanation may be given 

 of the fact, it seems certain that cells are always permeable to substances 

 soluble in lipoid solvents, while being only at times permeable to those not so 

 soluble, such as sugars, amino-acids and most salts. There are, as it were, two 

 kinds of permeability, of which the latter one alone is subject to functional 

 change. 



The presence of more than one constituent in the case of the membrane of the 

 red blood corpuscles is shown by the experiments of Ryvosh (1907) on haemolysis 

 by saponin. This glucoside causes the corpuscles to break up by a kind of 

 solvent or dispersive action on the cell membrane. It has remarkable powers of 

 being adsorbed at interfaces between phases, driving out most other substances 

 from this situation. At the same time, it is difficult to demonstrate that it 

 lowers surface tension to any considerable degree. It is probable that this 

 difficulty arises from the fact, discovered by Ramsden (1904), of the formation of 

 a rigid filfti at the surface where its solution is in contact with another phase. 

 We have seen that haemolysis is also brought about by mixing the blood 

 corpuscles with a hypotonic solution. The phenomenon in this case is due to 

 swelling by osmosis. Now the corpuscles of different animals have a different 

 relative power of resistance towards these two methods of haemolysis, and in such 

 a way that the corpuscles of some animals require the difference between their 

 own osmotic pressure and that of the hypotonic solution, in order that haemolysis 

 may occur, to be greater than those of other animals. Also those of certain species 

 require a higher concentration of saponin than in the case of other species. The 

 important point is that the more resistant a particular kind of corpuscle is 

 towards saponin, the more sensitive it is to a hypotonic solution and vice versa. 

 The two series below illustrate this fact, the most resistant species in both 

 columns being at the top : 



Hypotonicity. 



Guinea Pig 

 White Rat 

 Dog 



Grey Rat 

 Pig 



Grey Mouse 



Cat 



Ox 



Goat 



Sheep 



Saponin. 



Sheep 



Goat 



Ox 



Cat 



Grey Mouse 



Pig 



Grey Rat 

 Dog 



White Rat 

 Guinea Pig 



