30 The Structure of Protoplasm 



of divinyl-benzene was added to the styrene before polymerization, 

 cross-linkages were produced. The resultant polymer, although 

 indistinguishable by chemical tests from pure polystyrene, when 

 placed in the same solvents swelled enormously but did not dissolve. 

 In the case of a protein, like gelatin, gel formation depends on cross- 

 linkages between the main chains. These bonds may vary in 

 strength, from primary valence bonds, such as salt and disulfide 

 linkages, to van der Waals' forces (13, 35, 44) . 



When collagen, the fibrous protein that constitutes connective 

 tissue, is heated in water, it disperses to form gelatin. Gelatin forms 

 statistically isotropic gels, yet, upon stretching these, the micells 

 become extended and parallel. A gelatin gel that has been dried in 

 the stretched state gives the X-ray pattern of a collagen fiber (14) . 

 Under these conditions, it is optically anisotropic and has a maximal 

 breaking strength in the direction parallel to the axis along which 

 it had been originally stretched. In other words, a fiber has been 

 produced. The modern conception of the structure of an unstretched 

 gelatin gel (37) would picture it as a tangle of chainlike molecules 

 held together at certain points where chains cross (Fig. 3a) . If por- 

 tions of these molecules become oriented parallel to each other, 

 a local anisotropy is set up (Fig. 3b) . Stretching tends to produce 

 a more orderly arrangement of longer portions of the chains until 

 the statistically isotropic areas of the gel nearly disappear (Fig. 3c) . 



Seifriz (44) and, later, Frey-Wyssling (13) have proposed that 

 a similar structure characterizes cytoplasm, except that the cross- 

 linkages between the micells are constantly changing with changes 

 in its local metabolic states. Perhaps it is well to emphasize here 

 the importance of local regions in protoplasm. These become espe- 

 cially important when the question of shifts in bonds is considered. 

 It is possible, for instance, to arrive at values for the mean oxidation- 

 reduction potential or the pH of cytoplasm by the injection of suitable 

 indicators. These overall values may be of considerable use when 

 one cell is to be compared with another. But in the submicroscopic 

 realm in protoplasm, such experiments lose significance. In the 

 measurement of pH or oxidation-reduction potentials in nonliving 

 systems, a state of equilibrium throughout the system is essential. 

 It is obvious that living protoplasm is not in a state of equilibrium. 

 It undoubtedly has some regions of quite different pH from other 

 sections. For example, we have no right to predict from its over- 

 all pH value that the proteins in cytoplasm are above or below their 

 isoelectric points, for it is possible that local changes in hydrogen- 



