Chapter IVC 



METABOLISM OF THE B VITAMINS 



It is proposed to outline in this chapter the essential facts concerning 

 the processes which the B vitamins undergo from the time they are 

 synthesized or ingested by the organism until they are excreted. As in 

 other chapters in this section, the information in most cases is empirical ; 

 a true understanding of the subject does not exist. In both plants and 

 animals, however, there are certain broad categories which may be referred 

 to as "stages" in the metabolism. For convenience, these may be referred 

 to as digestion, absorption, distribution, anabolism and catabolism, and 

 excretion, and the following discussion proceeds in this order. Since plant 

 metabolism involves many differences in detail, and its consideration is 

 limited by the meager amount of data bearing upon the subject, it is con- 

 sidered independently. 



Metabolism in Plants 



Origin in the Plant. The precise pathways involved in B vitamin 

 biosynthesis have been considered briefly in an earlier section. The cyto- 

 logical location of this synthesis in the single-celled plants is not known, 

 but it seems relatively certain that the synthesis occurs in the leaves of 

 the higher green plants, which are generally considered to be the major 

 focus of synthetic activity in the plant. Since many alkaloids are also 

 synthesized in this highly active metabolic area, 224 it is no wonder that 

 some B vitamin moieties may be recognized in certain alkaloids (arecoline, 

 guvacine, nicotine, ergot alkaloids) . B Vitamins are also supplied to the 

 higher plants from the soil and from symbiotic microorganisms, although 

 the extent to which this source is important to the higher plants is 

 unknown. The B vitamins reach the heterotrophic portions of the plant 

 largely through the translocation stream by which other leaf synthetic 

 products are transported, but also by diffusion from neighboring cells and 

 by absorption from the exterior environment. 



Digestion and Cellular Absorption in Plants. Since the B vitamins are 

 frequently found in plants in bound forms, it is apparent that the plants 

 must first release the stored vitamins in order to utilize them for their 

 own purposes, and annual cyclic variations in the relative amounts of 

 free and bound vitamins are known to occur in some cases. Since plants 



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