24 77//; CHEMISTRY AND PHYSICS OF THE CELL 



the proteins,) and when so bound it cannot be seen, nor can it be 

 stained by the dyes ordinarily used for that purpose ; onl}- when de- 

 generative changes of certain khids have liberated it from combina- 

 tion does it become visible and stainable by ordinary methods (Rosen- 

 feld). By the special fixation method of Ciaccio the fatty compounds 

 of even normal cells may be made stainable (Bell),^ showing that 

 the so-called masked fat is really in a not altogether invisible form. 

 Whether the intracellular fat has any function other than that of 

 serving as a food-stuff is not known, but there can be no question of 

 the importance of the phosphorized fats, or phospholipins. 



Lecithin is a primarj^ cell-constituent, and is probably important 

 both in metabolism and physically. liammarsten regards it as con- 

 cerned in the building up of the nucleus. As will be shown later, 

 many of the most essential physical properties of the living cell de- 

 pend upon the presence in it of lipoids, of which lecithin is appar- 

 ently the chief. Of the ether-soluble substances in the heart, for ex- 

 ample, 60 to 70 per cent, is lecithin, which constitutes about 8 per 

 cent, of the dry weight of the myocardium, 



There are several varieties of lecithin, depending upon the fatty 

 acid radical they contain. The structural formula of one lecithin, 

 stearyloleyl lecithin, is as follows: 



CH,— 0— C— H3,0 



CH— 0— Cis— H33O 



I 

 CH„— 0— PO— OH 



I 



0— CH„— CH2— N =E (CHala. 



OH 



It differs from ordinary fats, therefore, in having two special groups, 

 one the phosphoric acid, the other the choline radical, which last may 

 be of some importance in pathological processes. In its ]ihysical 

 properties it is quite similar to tlie ordinary fats, although it forms 

 even finer emulsions in water, which are .practically colloidal solutions 

 (W. Koch). 



Cephalin, a closely related body differing in having but one methyl 

 grouj), is also pi-obably as M^dely spread in the tissues as lecithin, 

 according to Koch and Woods. ^° 



Cholesterol, which is another lipoiil, is nearly as universally present 

 as lecithin. ^^ It exists both free and in combination with fatty 

 acids, for chf)lesterol is an alcohol and not at all similar to the fats 

 chemically, aitliough very similar physically. The empirical formula 

 is CoJT.,,011 or C,-H.t,blT, and it is ivJal.'d I0 llic terpenes. It 



n.Tour. ^^\od. Eos., mil (10), 539. 



if>.Tour. Biol. C'hoin., 100.5 (1), 20.1. 



11 Recent literature piven liv Clikin, llioclicm. Cciilr.. lOOS (7). 2Sn. 



