PENTOSE SUGARS IN PLANT METABOLISM 



371 



utilized in the metabolic processes of the leaf. Careful analyses 

 of starches from various sources and formed under known con- 

 ditions would be desirable information. Similar experiments 

 have been made in order to determine whether the pentoses form 

 glycogen in the liver. The results of various investigators, how- 

 ever, are somewhat contradictory; some having observed an in- 

 crease of glycogen or pseudoglykogen^ others reporting entirely 

 negative results. On critical examination of all the experimental 

 data it becomes evident, however, that in several of the cases 

 where negative results were reported the means employed to re- 

 duce the glycogen in the liver were such as to affect a more dras- 

 tic metabolic disturbance than would be compatible with nor- 

 mal conditions. 



It is evident then, that the five 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 ap- 

 propriation of carbon dioxide by the chlorophyllous leaf. The 

 question resolves itself into whether the pentoses are direct prod- 

 ucts 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 pro- 

 gressive addition of six molecules of formaldehyde the presence 

 of pentoses is to be expected. 



2 CH^O^C^H^O^; CH^O^ + CH^O -.C3H6O3 ; C3H6O3 + CHaO-^ 

 C4H8O4; C4H8O4 + CH^O^CsHioOa; C5H10O5 + CH^O-^ 

 C6H12O6 



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

 of glycerine aldehyde, hexoses or their condensation products 

 would be the only substances formed : 



2 CH2OH.CHOH.CH: 0->CH20H(CPIOH)4.CH: O 



^ Cremer, Ergebnisse der Physiol. 11: 898, 1902. Jaaresber. f. Fortschritte 

 der Tierchem. 38: 446, 1908. 



v^f^ 



