68 



GROWTH 



The Proteins. Special importance has long been attached to 

 protein because of the predominating role it plays, not only in 

 the structure of the cell, and of the body, but also in the various 

 physiological processes. 



It has been recognized for years that different proteins have 

 unequal values as regards their more important physiological 

 role, but the explanation of this fact was deferred until some 

 knowledge was gained as to the constitution of the protein 



molecule. After it was once 

 established that this class of 

 compounds yields amino acids 

 on hydrolysis, the way became 

 clear to a better understanding 

 of the limiting factors as re- 

 gards the biological value of 

 proteins. 



The structure of the pro- 

 tein molecule may be illus- 

 trated in a diagrammatic, and 

 rather crude, manner that will 

 be sufficiently accurate for our 

 purpose. Suppose we take the 

 smallest possible sample of a 

 protein that it is theoretically 

 possible to take without chang- 

 ing its percentage composition. We will call this sample a pro- 

 tein molecule, and though it would be much too small to see, 

 even with the most powerful microscope, there is little doubt 

 that this molecule occupies a fairly constant volume and has a 

 definite geometrical pattern. Our information on these points is 

 very slight at best, so at the present time we are unable to de- 

 scribe them with any degree of accuracy. We feel very certain, 

 however, that if we could enlarge our molecule sufficiently, and 

 see its component parts, we would see that the protein molecule 

 is formed by the union of many smaller molecules, such as 



Figure 44. A diagram drawn to illustrate 

 the fact that on hydrolysis the protein mole- 

 cule yields amino acids. The large figure 

 represents the protein molecule, the smaller 

 ones the molecules of the amino acids that 

 compose it. 



