CHAPTER XXVI 



REGULATORY SUBSTANCES 



The activities of cells are in great measure determined by 

 specialized substances which appear to be of a complex nature. 

 Perhaps they appear so because of limitations in our knowledge 

 which preclude an as yet definite classification of them with the 

 salts, carbohydrates, fats, or proteins. These specialized sub- 

 stances are the enzymes, hormones, vitamins, and pigments. 

 Certain of them, or the responsible constituents within them, 

 have been found to be very simple compounds — even single 

 elements — consequently, the presumably complex substance 

 (e.g., a hormone) may be simply an element (e.g., boron) or a 

 radical (e.g., the sulphhydryl radical SH). 



Enzymes, hormones, vitamins, and pigments serve as deter- 

 miners of the activities of single cells and of intercellular activities 

 of the organism as a whole. Regulatory substances thus control 

 and harmonize body functions. While protoplasmic in origin, 

 they usually enter the body fluids and there serve their purpose 

 as organizers. Through them the cells of the body become more 

 intimately associated. Without them, there would be no 

 organized whole. While we shall consider them primarily as 

 regulatory substances of the entire living body, it should be 

 remembered that they are concerned just as much in determining 

 and controlling the activities of a single cell. 



ENZYMES 



Enzymes are important nutritional growth-producing and 

 activating substances occurring in cells and organisms in great 

 numbers. They are regarded as catalysts accelerating metabolic 

 reactions, such as the conversion of proteins, fats, and carbo- 

 hydrates into their simpler component parts. Enzymes are often 

 resorted to in explanation of reactions even though the evidence 

 of their presence may simply be the knowledge that something 

 must be responsible for a certain activity. We believe that 

 chlorophyll contains something that is responsible for the syn- 



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