254 The Structure of Protoplasm 



Globular molecules are said to occur in globulins, such as edestin, 

 and in normal albumen (egg white) , but this is true only in dilute 

 solutions and only when the molecules are not at surfaces. Further- 

 more, all yield X-ray photographs when denatured. According to 

 Astbury^"', the supposed globular protein molecule is a folded 

 polypeptide chain in which the linear fiber has been thrown into a 

 series of rings. 



In some proteins, the natural molecular configuration is an 

 extended chain, which may be fully extended as in silk, (3 keratin, 

 and [3 myosin, or regularly folded in one dimension, as in the a- and 

 super-contracted forms of keratin and myosin (Fig. 4, p. 32) . When 

 the chains are fully folded, so-called "globular" proteins result; 

 among them are egg albumen, haemoglobin, pepsin, and insulin. 

 These "globular" proteins unfold on denaturation, and the liberated 

 polypeptide chains may then sometimes be spun into artificial protein 

 fibers. In the living cell there is probably an unlimited number of 

 reversible interchanges between various states of folding. From this, 

 it follows that a spherical protein may be transformed into a fibrous 

 one; possibly the reverse transformation may also occur. The 

 former change has been proven experimentally (see Moyer, p. 28, 

 this Monograph) . 



Evidence is predominantly in favor of but one stereochemistry of 

 the proteins based on the convolutions of the polypeptide chain. It 

 may be, however, that the proteins fall into two groups, morpho- 

 logically considered, the soluble proteins of globular molecular 

 form, such as egg albumen, insulin, and edestin, and the insoluble 

 proteins with chain molecules, such as silk, hair, and wool. Proto- 

 plasm will contain both forms; the globular proteins will serve pri- 

 marily as nutrition and the linear ones for construction. It is to 

 the chain molecule that the elastic and general structural properties 

 of protoplasm are due. 



CONTRACTILITY 



Elasticity and contractility are very similar phenonena, for 

 elastic substances contract. Consequently, much that is said of the 

 one is applicable to the other. In the conclusion to this chapter, I 

 shall emphasize the wide-spread occurrence of rhythmic contrac- 

 tility in tissues, but it is a special case of which I wish first to speak, 

 that of the continuous rhythmic pulsation of the protoplasm of 

 slime molds. 



' Nature, 237, 803. 1936. 



