(JHAPTER XIV 

 CARBON METABOLISM 



AS we have seen, many organisms are cajDable of 

 building up all their cell constituents and can 

 maintain and reproduce themselves, using only a 

 single organic substance and ammonium salts as the 

 sources of raw materials and energy. Substances which 

 can serve in this way to support growth may belong to 

 almost any type of compound, saturated and unsaturated 

 fatty acids, hydroxy-acids, keto-acids, di- and tri-basic 

 acids, alcohols, carbohydrates, amines, amino -acids, 

 amides, and aromatic compounds among others. Gener- 

 ally speaking, only a comparatively small proportion of 

 the compound destroyed ultimately finds its way into the 

 composition of the cell ; the bulk of the compound is 

 more or less profoundly altered during the processes by 

 which energy is obtained. This altered part of the sub- 

 strate accumulates in the medium as the products of 

 fermentation which are characteristic of the various 

 organisms. Some organisms, the yeasts, for instance, 

 break up sugars with formation of alcohol and carbon 

 dioxide, others break up sugars with production of such 

 substances as lactic acid, acetic acid, butyl alcohol, or 

 acetone. All these fermentation products are, from the 

 point of view of the organism, waste products, although 

 they may be very valuable to mankind. 



The part of the compound which is converted into 

 cell material and taken into the composition of the 

 newly produced cells is said to be assimilated. The 

 portion which is broken down to provide the energy 



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