130 PRINCIPLES OF GENERAL PHYSIOLOGY 



in liquids which dissolve lipoids, such as urea, fatty acids, some ammonium salts, 

 etc., are found to penetrate the cell membrane. Those to which the membrane 

 is impermeable are not dissolved by lipoid solvents; such are sugar, amiim acids, 

 inorganic salts, mineral acids, etc. We note, for example, that sodium hydroxide, 

 insoluble in benzene, does not penetrate, while ammonium hydroxide, which is 

 soluble in benzene, readily does so. But it will doubtless occur to the reader that 

 these two bases differ in many other ways besides that of solubility in benzene. 



Again, Loeb (1909) showed that the lower fatty acids are more effective in 

 modifying certain cell processes, such as- those involved in the fertilisation of 

 ova, than the mineral acids are. The fact can be explained on the ground of 

 the " lipoid solubility " of the former. 



The aniline dyes were made extensive use of by Overtoil (1899, ii.) to test the 

 hypothesis, and it was found that those soluble in lipoids, that is, the salts of colour 

 bases, passed into the cell, while those not soluble, salts of colour acids, did not. The 

 meaning to be attached to the phrase " lipoid -solubility " in this connection will appear 

 hereafter. We may note also that the "basic" dyes which enter, are uniformly electro- 

 positive as regards the coloured substance to which we direct our attention, while the 

 "acid" dyes are electro-negative, so that lipoid solubility cannot be adduced as the 

 only difference between the two classes. Further, just as remarked above with 

 reference to the hydroxides of sodium and ammonium, it cannot be held that 

 " lipoid-solubility " is the only difference between acetic and hydrochloric acids. 



In fact, a layer of benzene shows the same selective permeability in respect of organic or 

 weak bases and acids, on the one hand, and strong inorganic bases and acids on the other 

 hand, as the cell membrane does, but no one* supposes that this membrane is composed of 

 benzene. Benzene, however, does not dissolve even the "basic" dyes, although solutions of 

 certain lipoids in chloroform, etc., appear to do so. It will be seen presently, however, that 

 there is strong evidence that this js really an adsorption on the surface of the lipoid, which is 

 only in colloidal solution. 



Notwithstanding what has just been said, it seems from the work of Overton 

 that we must admit that " lipoids " play an important part in the properties of 

 the cell membrane, although we shall see later that it is impossible to assign the 

 total composition of the cell membrane to them. Moreover, we shall find that 

 there are difficulties in looking upon them as solvents in the ordinary sense. 



At this point, then, we may profitably consider some of the chemical and 

 physical properties of the cell constituents to which the name " lipoids " has been 

 somewhat loosely applied. 



We find sometimes that all those substances extracted by alcohol are called 

 lipoids. This is clearly calculated to cause confusion. Glucose, urea, free bases, 

 such as choline, may be mentioned as being soluble in alcohol, but not of a lipoid 

 nature. Overton himself includes cholesterol, although, strictly speaking, the 

 name should be restricted to substances chemically related to the fats proper. 

 For the present purpose, perhaps, it may be allowed to remain in the class of 

 lipoids, owing to the similarity of its physical properties. 



The simple ordinary fats, glycerol esters of both saturated and unsaturated 

 higher fatty acids, are common constituents of the cell, but the most interesting 

 are those complex fats, to which the name " lipines," with its derivatives, has 

 been given by Leathes (1910). Lipines themselves are compounds of fatty 

 acids with a nitrogen-containing group, but contain no phosphorus nor carbo- 

 hydrate. Phospholipines contain phosphorus in addition, and are sometimes 

 called " phosphatides," while " galactolipines " contain no phosphorus, but a 

 carbohydrate group, galacto.se, and correspond to the cerebrins or cerebrosides of 

 some authors. The most familiar of these lipoids is the phospholipine, lecithin, 

 of which the formula is usually given thus : 



CH 2 .O.OC.C, 7 H.j 

 CH.O.OC.C 15 H ai 

 CH.,.O-P = O 



OH O.C 2 H 4 .CH, 



>N.CH 3 

 HO .CH, 



