338 THE BIOCHEMISTRY OF B VITAMINS 



pantothenic acid is not synthesized in the leaves of green plants until the 

 photosynthetic process commences, but whether this is a cause or an 

 effect is uncertain. Beyond this, little is known of annual cyclic variations 

 in those plants that maintain their photosynthetic ability throughout 

 the year. 



In the Thallophyta, and within single cells in general, very little is as 

 yet known with regard to the possible localization of B vitamins. It is not 

 possible to report any reliable information as to distribution between the 

 nucleus, cytoplasm, and cell wall, or as to distribution in different parts 

 of the cytoplasm, or as to distinct changes during mitosis. There will un- 

 doubtedly be much intensive study of this subject during the next few 

 years as a result of the intensive research efforts now in effect in the field 

 of cytochemistry, and of the rapidly increasing number of techniques now 

 being reported which should facilitate such study. 220 



Practically nothing is known with regard to the breakdown products 

 of B vitamins in green plants. A fertile and relatively simple field for 

 exploration awaits the investigator of this point, with the information 

 found in animal catabolism well developed to serve as a guide. In the 

 case of the Thallophyta, furthermore, the breakdown products of B vita- 

 mins have been but little more studied, despite the vast amount of infor- 

 mation available on the bacterial catabolism of other metabolites. 



Plants excrete B vitamins into the surrounding medium, and vitamin 

 production by certain bacteria has already been noted (p. 299). Indeed 

 in certain molds {Eremothecium ashbyae, Ashbya gossypii) riboflavin 

 excretion is so pronounced as to form riboflavin crystals about the myce- 

 lium. 5 Higher plants secrete vitamins into the soil from their roots, and in 

 several instances (i.e., thiamine, inositol) extended studies have been made 

 of the subject; these have indicated that the high localized vitamin con- 

 tent of the soil is a factor in increasing the bacterial population in these 

 areas. 6 The exact process involved in excretion, as in absorption, however, 

 is unknown, and such factors as threshold values are as yet unavailable. 



Digestion in the Animal Organism 



The many complex activities that proceed in the animal gastrointestinal 

 tract may produce three major changes in the nutritional vitamin forms: 

 liberation, activation, and destruction. Most frequently, the bound unab- 

 sorbable vitamin complexes may be broken down and the vitamin liber- 

 ated and made available for absorption. Associated with this process, the 

 cellular structure of the food may (or may not) be destroyed. Less often, 

 the vitamin may be so modified in the intestinal tract as to provide a 

 more active fprm than that ingested, for example, a form more closely 

 related to the functional form. Finally, destruction or inactivation of 



