Molecular Structure in Protoplasm 69 



seems probable, however, that in the Uving organism several 

 organic molecules and ions become attached to a single protein 

 molecule to form a complex aggregate (115). The complexity to 

 which this is likely to lead may be brought out by referring to the 

 composition of active protoplasm as given in Table 2 where an 

 average situation is shown based on one molecule of protein of 

 molecular weight 36,000. A content of 2 per cent for the fatty 

 materials (69) corresponds to about 10 molecules, such as those of 

 the phospholipids, and the percentages of remaining materials indi- 

 cate about 20 small organic molecules and perhaps 50 to 100 inorganic 

 ions, also present on the average for each protein molecule of this 

 size. The small organic molecules include principally such substrate 

 molecules as those of hexose, triose, etc., since analyses indicate 

 very small amounts of the pyridine nucleotides (116) of cytochrome 

 (117) , thiamine-pyrophosphate (97) and riboflavinphosphate (118) . 

 In fact, computations show the number of these to be only 1 pyridine 

 nucleotide to about 20 molecules of protein of the size mentioned; 

 1 cytochrome to about 500 to 600 protein; 1 thiamine pyrophosphate 

 to about 1,700, and 1 riboflavinphosphate to about 1,300 protein 

 molecules of 50 A size. 



If we now think of the protein as a cube of 50 A with six faces and 

 about ten polar amino acid residues on at least four faces, we will 

 have perhaps fifty potential places of attachment for the various 

 smaller molecules and ions, and in all there must be at least 150 

 molecules and ions available in the cytoplasm for each protein par- 

 ticle of 50 A in size, thus making it seem probable that some of the 

 small organic molecules are not attached to the protein but instead 

 are contained in the channels and submicroscopic vacuoles. 



We now have some comprehension of an elementary protein par- 

 ticle to which several phospholipids as well as several inorganic ions 

 and perhaps a respiratory prosthetic group are attached. In the 

 living protoplasm, however, it seems reasonably certain that several 

 or many complexes of this sort are attached to form larger and 

 structurally more complex aggregates which serve particular pur- 

 poses. Down in the lower submicroscopic region, however, relatively 

 little is known concerning the aggregation of these conjugated pro- 

 teins with one another; further, there is very little evidence concern- 

 ing the quantitative distribution and localization of these materials 

 throughout the cell. 



Aggregation of a number of small 50 A protein particles into a 

 larger complex involves the nature of the outer faces of the particles, 



