62 CARBON NUTRITION 



organism to use fructose, mannose, or galactose depends upon its 

 ability to convert the sugar in question into a phosphor y la ted deriva- 

 tive of glucose able to enter the main respiratory pathways. The 

 metabolic relationships of these lour sugars in yeast and animal cells 

 are shown in the diagram (67): 



Polysaccharide Galactose 



Jt I 



Glucose-1 -phosphate ^ Galactose-1 -phosphate 



Glucose — -> Glucose-6-phosphate ;=± Fructose-6-phosphate 



! I if. 



i ! si 



Mannose — * Mannose-6-phosphate 



Fructose 



From this diagram it may be suggested that in adaptive growth on 

 fructose, mannose, or galactose, the enzyme actually lacking in the 

 cell and induced by the environment is the specific kinase responsible 

 for the phosphorylation of the free sugar. However, no data are 

 available and other possibilities and other pathways are of course 

 conceivable. The occurrence of mannose- 1 -phosphate in the mycelium 

 of Penicillium chrysogenum (105), for example, suggests a pathway of 

 mannose utilization similar to that of galactose in the scheme outlined 

 above. 



Studies on the economic coefficient (p. 89) and similar indices of 

 growth indicate that poorly utilizable monosaccarides are such because 

 they are not attacked or do not penetrate the cell, rather than because 

 once attacked they do not provide energy or usable metabolic products. 

 This is especially clear from experiments with Aspergillus niger, for 

 which the small amount of a poor carbon source which disappears from 

 the medium is as efficient in supplying carbon for growth as an equiva- 

 lent amount of glucose (208, 209). The sugars which do not support 

 growth adequately remain, that is, in the medium. Whether this is 

 the result of specific enzymatic deficiencies as suggested above or 

 whether it is to be attributed to slow penetration of the sugar remains 

 to be determined. The two possibilities are not necessarily mutually 

 exclusive if penetration of sugars depends, as it may, on a surface 

 enzymatic reaction. It is perhaps significant that the yeast cell is im- 

 permeable to sorbose, galactose, and arabiriose (171), all of which are 

 generally poor sources of carbon for microorganisms. 



L-Sorbose, a ketohexose differing from fructose in the configuration 

 of carbon atom 5, supports normal growth of a few fungi and actinomy- 

 cetes (14, 116, 146). Although unequivocal data are not available, it 

 appears not to be utilizable by most forms and it is definitely toxic to 



