60 



The Structure of Protoplasvi 



our present understanding of the large group of compounds known 

 as the sterols (85) . The common feature of the sterols is now known 

 (71) to be a complex ring system plus miscellaneous small side 

 chains forming, in general, a rather flattened molecule about 5 A 

 in thickness, 7-7.5 A in width, and approximately 20 A in length 

 (85) . Minor structural variations give rise to the various sterols 

 of which perhaps the best known are cholesterol (86) , shown in 



Figure 8, and ergosterol. They are found in both 

 plants and animals; those in the higher plants are 

 generally termed phytosterols. At the present 

 time very little is known of the specific role, either 

 structural or physiological, of the sterols in the 

 living cell. 

 ^ »s In addition to the proteins and fatty substances 



J#-r* which occur in relatively large amounts in cyto- 



V^ "A^^— plasm, a small quantity of organic molecules of 

 V^IH^H great diversity play an important part in the 

 activities of the cell, although in the structural 

 framework they are insignificant. A considerable 

 number of them are known to be involved in 

 respiration, and when attached to proteins as 

 prosthetic groups form the active part of cellular 

 mechanisms, or enzymes (87) . Most of these have 

 been shown (64, 88, and 89) to occur in conjunc- 

 tion with specific proteins which appear to be 

 albumins and globulins. 



The existence of cytoplasmic granules which 

 show intense respiration and hence contain 

 respiratory prosthetic groups has been amply demonstrated in 

 preparations from materials such as Arbacia eggs (126, 127) , liver 

 tissue (128) , pig's heart muscle (12) , and the breast muscle of the 

 pigeon (13). It has been reported (111) that similar material, con- 

 taining lipoids in addition to protein and respiratory prosthetic 

 groups, has been isolated from normal chick embryo. The associa- 

 tion of these three materials in cytoplasmic aggregates seems to be 

 more generally recognized as experimental work progresses (111) . 

 In order to show the diversity in structure of these small mole- 

 cules, a list of the better known is given, with here and there 

 photographic reproductions of molecular models and brief discus- 

 sions concerning the individual dimensions and properties. The 

 model photographs in Figures 9-12 inclusive are made to the same 



Fig. 8. Photo- 

 graph of model of 

 Cholesterol. Black 

 balls indicate car- 

 bon atoms; grey 

 ball at bottom, 

 oxygen atom; and 

 small white ones, 

 hydrogen. 



I 



