CARBON SOURCES 145 



plexity. Not all fungi utilize all natural organic compounds, nor do all 

 species utilize a given compound with the same facility. The composi- 

 tion, structure, and configuration of organic compounds affect utilization, 

 but the effect of these factors may be different for different fungi. 



The carbohydrates are the most common and important sources of 

 carbon for the fungi. Sugars (and other compounds) having the same 

 structure, but with mirror-image configuration, differ physiologically. 

 Usually only one enantiomorph is utilized, or one is utilized much more 

 rapidly than the other. Glucose is utilized by more fungi than any other 

 sugar. Few fungi are unable to utilize glucose. A few species are appar- 

 ently unable to utilize any sugar; e.g., Leptomitus lacteus. The species 

 that utilize the pentoses, sugar alcohols, acids, and other simple organic 

 compounds are fewer in number than those which utilize glucose. 



The oligo- and polysaccharides are utilized by fewer species than is 

 glucose. The nature of the glycoside linkage as well as the sugar residues 

 is important in determining whether these compounds are utilized by a 

 given fungus. It is probable that most fungi hydrolyze oligosaccharides 

 before utilization occurs. This does not exclude direct utilization in some 

 instances. An oligosaccharide and its hydrolytic products are not always 

 physiologically equivalent. The general order of availability of the three 

 common disaccharides appears to be maltose, sucrose, and lactose. 

 Among the polysaccharides, cellulose and starch are the most abundant. 

 These compounds are insoluble and must be hydrolyzed or otherwise 

 degraded to low-molecular-weight compounds before utilization. Only 

 those fungi which form cellulase and amylase are able to utilize these 

 compounds. This "digestion" is accomplished by enzymes. Ability to 

 utilize other polysaccharides is also dependent upon possession of the 

 necessary hydrolytic enzymes. 



Some fungi utilize carbon dioxide, but not as a sole source of carbon. 

 It is postulated that carbon dioxide combines with pyruvic acid and other 

 keto acids to form key intermediate products which are necessary for the 

 formation of amino acids. 



The fate of the carbon supplied to a fungus is best determined by 

 carbon-balance studies; i.e., by complete chemical analyses of the myce- 

 livim and other metabolic products, including the carbon dioxide produced. 



The first step in utilization of sugars and other carbon sources is the 

 formation of certain key intermediate metabolic compounds. These key 

 compounds are in part utilized for synthesis and in part oxidized to pro- 

 vide energy. The metabolic pathways leading to the formation of key 

 intermediates differ, depending upon the environmental conditions and 

 the fungus involved. Among the key intermediates pyruvic acid is 

 especially noteworthy. Reduction of this compound yields lactic acid, 

 while amination and reduction leads to alanine. Decarboxylation pro- 



