THE ORIGIN AND E&LE OF PENTOSE SUGARS. 75 



X. THE ORIGIN AND ROLE OF PENTOSE SUGARS. 



The pentose sugars in plants have been most familiar as pentosans, as 

 components of the cell walls and vessels of the plants, and as found in 

 various gums in the form of xylan and araban. While widespread in the 

 vegetable kingdom, the pentose sugars have but recently been regarded in 

 their important bearing, and, in fact, were for a long time not recognized 

 as belonging to a separate group. In fact, the presence of monosaccharide 

 pentoses has been but very recently established. That the pentoses are of 

 great physiological importance to the plant becomes evident in the light 

 of recent investigations on the chemistry of the cell-nucleus. Among the 

 chief components hereof are the so-called nucleic acids. These are highly 

 complex substances consisting roughly of a combination of phosphoric acid, 

 purines, pyrimidines, and several carbohydrate groups. The plant nucleic 

 acids so far studied have been found to contain the pentose group. 1 The 

 function and fundamental importance of the nucleus in the metabolism of 

 the plant need no further discussion here ; however, a more intimate knowl- 

 edge of the chemical composition and action will undoubtedly lead to a 

 clearer understanding of the intricate reactions of this most important 

 organ. 



It is evident, then, that the 5 carbon atom group of sugars is a common 

 component of plants and is of great importance in some of the most vital 

 metabolic activities of the organism. Nevertheless, the origin and mode of 

 formation of the pentose sugars is still quite obscure. This problem is of 

 special interest because any light thereon would be of great value in gaining 

 a clearer understanding of the process of the photosynthetic appropriation 

 of carbon dioxid by the chlorophyllous leaf. The question resolves itself 

 into whether the pentoses are direct products of photosynthesis or are derived 

 from other sugars through subsequent metabolic activity. If, for instance, 

 the formation of sugar in the green leaf actually takes place by means of a 

 progressive addition of six molecules of formaldehyde, the presence of 

 pentoses is to be expected. 



2CH 2 O->C 2 H 4 O 2 ; C 2 H 4 O 2 + CH 2 O->C 3 H 8 O 8 ; 



C,H,0. + CH 2 0->aH 8 O 4 ; C 4 H 2 O 4 + CH*0->C 6 H lt 5 ; 



C 6 H 10 O 6 + CH 2 0->C 6 H 12 6 . 



If, again, the sugars are formed by the union of two molecules of gly- 

 cerine aldehyde, hexoses or their condensation products would be the only 

 substances formed: 



2CH 2 OH CHOH CH : O, CH 2 OH( CHOH) 4 CH : O. 



This, however, is purely chemical speculation and so far has aided little 

 in the solution of the problem of photosynthesis, the real course of which is 

 probably far more complicated than has been generally assumed. Not 



1 LEVENE, J., and W. JACOBS. Ueber die Pankreas-Pentose. Ber. d. deut. chem. Ges., 



43, 3147-3150, 1910. 

 . Ueber die Triticon Nucleinsaeure. Ibid., 43, 3164-3167, 1810. 



