CELL WALL DYNAMICS 43 



sources or in the presence of glucose. When glucose- 1-C 14 is sup- 

 plied exclusively, some label appears at the 3- and 4-positions. 

 Although most of the hexose supplied appears to undergo direct 

 conversion to cellulose, therefore, a significant proportion first 

 undergoes cleavage. Following evidence that cellulose formation 

 by Acetobacter is extracellular, it has recently been shown that 

 a cell-free enzyme system for the direct polymerization of glucose 

 does indeed exist in this organism. This system is characterized by 

 a pH optimum of 8.5-9 and an absolute requirement for adenosine 

 triphosphate. It is entirely possible that cellulose synthesis in 

 vascular plants involves different enzymes and mechanisms, and, 

 indeed, it seems that intact Acetobacter itself has a secondary 

 indirect mechanism. Nevertheless, the relative simplicity and 

 directions of this cell-free system are appealing, and its demon- 

 stration is of great significance. It has been established, moreover, 

 that glucose- 1-C 14 is a good precursor of cellulose in wheat, the 

 label appearing mainly at C { . 



When glucose-1-C 14 is fed to wheat and other plants, the Q 

 label appears not only in cellulose, but also in xylan, D-xylose, 

 L-arabinose, and galacturonic acid. Labeled xylose and ribose are 

 poor and indirect precursors of xylan, whereas glucuronolactone- 

 1-C 14 , a poor cellulose precursor, is readily converted into 

 xylan. Current evidence supports a pathway to hemicelluloses 

 involving oxidation at the C 6 position in glucose to form interme- 

 diate uronic acid with subsequent decarboxylation to pentose. 

 In contrast, xylose is first converted to glucose-6-phosphate via 

 fra/js-ketolase and /ra/?s-aldolase and then transformed into xylan 

 by (^-decarboxylation. 



The concept of (^-decarboxylation is relatively novel, and 

 counter to the somewhat earlier picture which emphasized reactions 

 at the glucose Q . Even earlier, it was believed that hemicellulose 

 arose from cellulose by oxidation to polyuronide and decarbo- 

 xylation to xylan. Although there were many appealing features 

 in this decarboxylation theory, it became untenable as more de- 

 tailed information about pentosan structure was obtained. The 

 modified decarboxylation theory which has been recounted, centers 

 about monosaccharides rather than polysaccharides. The most 



