58 The Structure of Protoplasm 



every ninth will be of lysine; and so on for the various amino acid 

 residues. 



While this notion of residue distribution still needs more sup- 

 porting evidence, there seems to be at least a probability, from the 

 biological point of view, that a certain amount of orderliness must 

 prevail in the formation of proteins. We are accepting this as plaus- 

 ible and are making use of it in a general way since it serves as a 

 basis for the arrangement of the residue end groups into patterns 

 or mosaics both on the surface and inside of the particle. Thus, if 

 it is necessary that the glutamic acid and lysine residue endings 

 occur at a specific distance from each other in order that a fatty 

 molecule or a respiratory prosthetic group may be attached, their 

 distribution may allow this to occur only once on a surface or at 

 most only a few times. 



Of the molecules occurring in cytoplasm which are thought to 

 play an important part in the structural features of protein aggre- 

 gates the fatty materials stand out as the most prominent. They 

 occur in active protoplasm in quantities of at least ten to fifteen 

 molecules for every protein molecule of 36,000 molecular weight. 

 They form a miscellaneous class in which the common feature is 

 the excessively large amount of carbon-hydrogen groups and the 

 relatively infrequent occurrence of oxygen and nitrogen atoms. 

 The significance of these proportions of component atoms comes 

 out when it is recalled that the oxygen and nitrogen atoms have a 

 strong affinity for water due to their negative residuals, while the 

 carbons are lacking in this attraction for water. The resulting 

 insolubility in water has led us to think of these materials as fatty 

 substances. 



The long-chain fatty acids, the neutral fats which are glycerol 

 esters of the long-chain fatty acids, the phospholipids, and the sterols 

 are included in this group. Of these the two most constant and 

 ubiquitous in occurrence in cytoplasmic material are the phospho- 

 lipids and sterols (69, 70, 71) . At least three methods indicate that 

 these materials may be found associated with protein material 

 within the heavier cytoplasmic granules. The methods include 

 solubility or dispersal of the large particles in fat solvents (72, 73, 

 74, 75, 76), blackening with osmic acid indicating the presence of 

 unsaturated fatty acids (77) and detection of the aldehyde grouping 

 of the acetal-phospholipids (78) . 



The phospholipids include two general groups, the ester-phospho- 

 lipids represented by lecithin and cephalin and the recently discov- 



