Proteins and Protoplasmic Structure 27 



modia of Myxomycetes could grow through pores in fine filter paper 

 averaging 1 u. If. however, a plasmodium was forcibly pressed 

 through bolting silk, the resulting pieces had to be larger than 200 u 

 to remain alive. Retardation of growth was noticed when the 

 diameter of the pores was less than 750 u. He suggests that pre- 

 formed rods, longer than 200 ^i and composed of submicroscopic 

 elements, are present in the protoplasm. Moore concludes that if 

 these are broken up or removed, life ceases. Centrifuging at 75,000 

 gravities for 5 minutes likewise retarded proliferation. This he 

 interprets as due to separation of these larger elements. On resting 

 for a sufficient time, it appeared that structure was re-established, 

 for proliferation began again. Here is an indication that not only 

 are long rod-shaped particles present, but also that they play a role 

 in the economy of the cell. 



At once the question arises, what is the chemical nature of these 

 particles? Of all substances present in protoplasm capable of aggre- 

 gating to form such rods, the most prominent place is held by the 

 proteins. From the time of Reinke, all of the gross analyses of 

 protoplasm have emphasized the dominant position of the proteins. 

 With proteins making up so large a part of its substance, it would 

 be anticipated that they would exert a great influence upon its 

 characteristics. Let us compare what is known of their properties 

 with the properties of protoplasm. 



GENERAL STRUCTURAL FEATURES OF PROTEINS 



Recent work permits us to classify the proteins in two chief 

 groups: (1) the fibrous or extended proteins, such as wool, and (2) 

 the corpuscular or highly folded proteins, such as egg albumin. Mem- 

 bers of both groups are constructed from the same kinds of amino 

 acids. These appear to be joined into chains by peptide linkages to 

 form what Astbury (3) has designated the "backbone" structure 

 (Fig. 1) . While the backbone of silk is apparently fully extended, in 

 other protein fibers, such as hair and muscle, it is folded somewhat, 

 as shown by their X-ray spacings. The individual properties of 

 proteins are dependent on their side chains. In a fiber, adjacent 

 backbones appear to be held together by bonds of various strengths. 

 These bonds are due to interactions of the side chains (R, R', R", . . . , 

 in Fig. 1) branching off from the backbone. These are of diversified 

 chemical nature (Fig. 2). Some are lipophilic (leucine) while others 

 are hydrophilic (aspartic acid, tyrosine, and lysine) . Of the latter 

 group, aspartic acid is an example of an acidic amino acid while 



