THE CHEMISTRY AND PHYSICS OF THE CELL 27 



Nucleo=albumins (or phospho-proteids), by an unfortunate 

 similarity of name, are often confused with nucleo-proteids by 

 non-chemical writers, a difficulty increased by an actual resem- 

 blance to the extent that they also yield phosphoric acid, and 

 are somewhat similar in solubility and digestibility. They are 

 essentially different, however, in that they do not yield nucleic 

 acid or purin bases on cleavage. Probably members of this 

 group are also constant components of cells. 



Glycoproteids (or gluco-proteids) and phospho-glycoproteids 

 are also believed to occur frequently or constantly in proto- 

 plasm. They are compounds of proteids with a sugar or sugar- 

 like group, which probably usually contains nitrogen, thus 

 differing from the ordinary hexoses and pentoses. 



Insoluble proteids, or bodies resembling the coagulated pro- 

 teids in their lack of solubility in various fluids, are left behind 

 after the other proteids have been extracted from the cells. 

 Their significance is not known : whether to a large extent arti- 

 ficially produced or whether a normal structural element of the 

 cell. 



FATS AND LIPOIDS 



Ordinary fats occur in nearly all cells, and probably in all, 

 but their demonstration is not readily possible. The micro- 

 scopic appearance of a cell, even when special stains for fat are 

 used, gives no correct idea of the amount of fat actually present. 

 Thus normal kidneys contain 15 to 18 per cent, of fat in their 

 dry substance, but none of this can be detected with the micro- 

 scope. A kidney which seems microscopically the site of marked 

 fatty degeneration may show no more fat when examined chem- 

 ically than a normal kidney, which in section appears to be 

 quite free from fat. This is because some of the intracellular 

 fat is bound chemically with the proteids, and when so bound 

 it cannot be seen, nor can it be stained by the dyes used for 

 that purpose ; only when degenerative changes of certain kinds 

 have liberated it from combination does it become visible and 

 stainable (Rosenfeld). 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 phosphor- 

 ized fat, lecithin. 



Lecithin is a primary cell-constituent, and is probably im- 

 portant both in metabolism and physically. Hammarsten 

 regards it as concerned in the building up of the nucleus. As 

 will be shown later, many of the most essential physical proper- 

 ties of the living cell depend upon the presence in it of lipoids, 

 of which lecithin is apparently the chief. Of the ether-soluble 



