CHAPTER 7 

 CARBON SOURCES AND CARBON UTILIZATION 



Carbon occupies a unique position among the essential elements 

 required by living organisms. Almost half of the dry weight of fungus 

 cells consists of carbon. Protoplasm, enzymes, the cell wall, and reserve 

 nutrients stored within the cells are compounds of carbon. Carbon com- 

 pounds are equally important in fugus nutrition. Fungi secure energy 

 by oxidizing organic compounds. In addition to being the main struc- 

 tural elements, carbon compounds play an equally important functional 

 role. The number of carbon compounds known far exceeds the total 

 of known compounds of all the other elements, because of the property of 

 carbon of forming compounds in which carbon is linked to carbon in the 

 form of chains and rings. Various other elements such as nitrogen, 

 oxygen, and sulfur may serve as linking elements. While many carbon 

 compounds are stable at ordinary temperatures, others are extraordinarily 

 sensitive to a wide range of chemical reagents and to slight changes in the 

 physical environment. 



Organic compounds differ in composition, structure, and configuration. 

 These are key factors which must be considered in relation to utilization 

 of organic compounds by fungi. Since more is known about carbohy- 

 drates and related compounds as carbon sources, and about the manner 

 in which they are dissimilated and assimilated, than about any other 

 class of organic compounds, most of the discussion in this chapter will 

 be devoted to these topics. In the main, only naturally occurring organic 

 compounds will be considered. 



MONOSACCHARIDES AND RELATED COMPOUNDS 



The simple sugars, or monosaccharides, have the general formula 

 C„(H20)n. The carbon chain is unbranched except in a few, very rare 

 sugars. The functional groups present are primary ( — CH2OH) and 

 secondary ( — CHOH — ) alcohol groups, and an aldehyde ( — CHO) or 

 ketone ( — CO — ) group, actual or potential, is always present. The 

 primary alcohol and aldehyde groups are restricted to the end positions 

 of the carbon chain, while the ketone group is usually on the second 



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