METABOLIC FUNCTIONS OF B VITAMINS 217 



been specifically demonstrated. The production of partial genetic blocks 

 which result, for example, in impaired synthetic abilities on the part of 

 the cells may cause cells to require for rapid metabolism a substance 

 which otherwise would never be a limiting factor. In such cases, how- 

 ever, the cells have not lost their synthetic ability completely, and the 

 difference between these cells and the unaltered ones is quantitative, 

 not qualitative. The capacity of cells to adapt themselves to the utiliza- 

 tion of completely new substances indicates that they possess latent 

 potentialities with respect to enzymatic reactions which are ordinarily 

 not observed. 



Variations in the fundamental processes in different cells, in different 

 species, and in different individuals of the same species are of extreme 

 importance and interest, but their discussion does not belong in a sum- 

 mary of the chemical processes which appear to be common to cells in 

 general. 



There would be certain advantages in preparing an elaborate diagram- 

 matic scheme showing all the known and postulated relationships between 

 the fundamental biochemical compounds. Such a chart could be used to 

 indicate in a concise manner where the individual reactions discussed in 

 the previous chapter fit into the general processes of cell metabolism. It 

 is technically impossible, however, to prepare a diagram that would not 

 be more confusing than enlightening, since some of the intermediate 

 compounds, like pyruvic acid, are involved in a multitude of reactions. 

 The metabolic processes, therefore, will be treated in turn according to 

 the classical divisions of biochemical substances: carbohydrates, lipides, 

 and amino acids and proteins. Following this will be a discussion of the 

 role of the B vitamins in the fundamental physiological processes involv- 

 ing energy transformations. 



The Utilization of Carbohydrates 



Most of the chemical steps involved in the utilization of carbohydrates 

 have been well established, and a number of excellent reviews on this 

 aspect of metabolism have appeared during the last ten years. 1-7 The 

 general metabolic pathways by which the carbohydrates are stored, de- 

 graded, or converted to intermediates that can be used for the synthesis 

 of compounds of other types involve a large number of reactions. These 

 can be most conveniently considered as components of four different 

 phases: (1) the synthesis and cleavage of the polysaccharides; (2) the 

 glycolytic process — glycogen (or starch) ±? pyruvate; (3) the anaerobic 

 utilization of pyruvate; and (4) the aerobic utilization of pyruvate. 



The Synthesis and Cleavage of Polysaccharides. The initial phase of 

 carbohydrate utilization includes the reactions by which complex sugars 



