Proteins and Protoplasmic Structure 31 



ion activity can produce a local situation completely different from 

 that occurring in a near-by region of the same cytoplasm. Yet it is 

 the local conditions prevailing in this unstirred system that can 

 bring about the formation or breakage of a bond between two adja- 

 cent protein molecules. 



As local regions change under the influence of enzymatic 

 action, radicals break apart and join to other groups or remain in the 

 sol state for a time. It appears, however, that the framework can be 



(a) (b) (0 



Fig. 3. Gel structure, (a) Statistical isotropy in unstretched gel. (b) Effects 

 of moderate stretching, (c) Crystallite formation produced by extreme 

 stretching. 



readily reformed by gelation. Just as with nonliving gels, conditions 

 inducing micellar orientation result in visible birefringence of the 

 cytoplasm. 



Reactions to Shearing Stresses. Gels, in general, exhibit struc- 

 tural viscosity at low rates of shear. That is, their rate of flow is 

 not proportional to the shearing stress. When the shearing stress 

 becomes great enough, the structure of the gel may be broken down 

 so that it becomes a truly viscous sol. This has been observed with 

 dilute gelatin gels by Freundlich and Abramson (11) . On standing, 

 these sols reverted isothermally into gels; in other words, they were 

 also thixotropic. Not all gels exhibit this latter phenomenon, only 

 gels capable of forming very loose interconnections. 



Pfeiffer (36) has reported that protoplasm shows structural 

 viscosity which depends on the rate of shear and that it, too, can 

 be thixotropic. His experiments were performed by sucking proto- 

 plasm, into a micropipette at various pressures. It has been observed 

 by Chambers (7) , among others, that agitation of the cytoplasm of a 



