Molecular Structure i7i Protoplasm 49 



carbon and oxygen, are joined by a single bond as in Figure 2, the 

 distance between their atomic centers is the sum of their single 

 bond radii, i. e., 0.77 + 0.66 = 1.43 A. 



It thus appears that protoplasmic molecular structures have a 

 certain basic similarity regardless of the kind of atoms involved. 



N O H 



XX a 



RADIUS 0.77A 0.7 1 A 0.66 A 0.29 A 



BONDS 4 3 2 1 



C-C C-N C-0 C-H 



DISTANCE 1.54A 1.48A 1.43A I.06A 



Fig. 2. Diagrams showing spatial properties of carbon, oxygen, nitrogen, 

 and hydrogen atoms. Lower portion shows interatomic distance between two 

 atoms joined by a single bond. 



since the values of the angles and of the radii are practically the 

 same in all. Structural and cohesional differences arise, however, 

 due to the residual negative charge on the oxygen and nitrogen 

 atoms and to the absence of these residuals on the carbon atom. 

 These residual forces, it will be seen later, are responsible for the 

 formation of the hydrogen bond (19, 20) , a mechanism of consider- 

 able importance in the cohesion of molecules. In this mechanism 

 the proton, or hydrogen nucleus, of an — OH or — NHo group is 

 pulled toward the negative residual of another oxygen or nitrogen 

 atom. This attraction results in a closer approach (2.8 A) of the 

 two groups than would otherwise occur (3.5-4.0 A) . The hydrogen 

 proton, as shown in Figure 3, is always involved, and for this 

 reason the bond is spoken of as the hydrogen bridge. 



The component of protoplasm which is universally recognized 

 (21, 22, 23, 24, 25) as being of most importance structurally is 

 protein. Various investigators using microchemical methods have 

 demonstrated its existence in cytoplasmic particles (26, 27, 28, 29, 

 30) ; and many other workers (31 and 64) have demonstrated the 

 importance of the proteins in respiratory mechanisms. At this point 



