Some Physical Properties of Protoplasm 261 



retain their physiological individuality, the chief feature of which 

 is, for us at the moment, the characteristic rhythm each possesses, a 

 rhythm not necessarily in perfect synchronism with that of other 

 units. 



The foregoing essentially biological discussion of rhythmic con- 

 tractility in protoplasm leads naturally to a physical interpretation 

 of the phenomenon. This would be a highly speculative venture were 

 it not for the great advance made by physical and X-ray chemists in 

 an understanding of the molecular structure of contractile organic 

 substances such as silk, wool, and hair. Just what has been accom- 

 plished in the case of the elasticity of rubber I have already stated 

 (p. 253) , and I added that as elasticity and contractility are part of the 

 same general phenomenon, consideration of the physical basis of 

 these properties of protoplasm would be postponed. We may, then, 

 now consider the molecular structure most likely responsible for 

 elastic and contractile properties in certain organic systems, particu- 

 larly protoplasm. 



Protoplasmic contractility has been attributed to a variety of 

 forces. In the case of muscle, change in surface tension was a widely 

 accepted theory. But today it is generally conceded that proto- 

 plasmic contractility is due to the same molecular mechanism as are 

 the contractile properties of wool and hair, namely the folded poly- 

 peptide chain. 



The elasticity of jellies and the high water-holding capacity of 

 thixotropic gels led to the general acceptance of a linear structural 

 unit. Contractile properties have now led to another feature of the 

 structural unit of elastic organic systems, namely, molecular folding 

 (Fig. 4, p. 32) . With it our picture is relatively complete. 



All proteins are built of polypeptide chains with side chains 

 joining one molecule to another. Most proteins are crystalline. 

 Usually, two classes of proteins are recognized, the fibrous and the 

 non-fibrous or corpuscular ones, but there is so much in favor of a 

 close relationship between the two, and a small energy difference, 

 "that it is difficult to avoid the conclusion that fundamentally all 

 proteins are fibrous."^" 



The general molecular pattern of proteins is now well known 

 (Fig. 4, p. 51, etc.). 



Astbury'"^ divides proteins into four types according to their 



'• Trans. Faraday Soc, 36, 233. 878. 1940. 

 "Tabulae Biologicae, J7, 90. 1939. 



