1 50 



CARBON METABOLISM III 



Figure 3. Glucose concen- 

 tration and fat synthesis by 

 Aspergillus fischeri. Plotted 

 from data of Prill, Wenck, 

 and Peterson (428). 



Glucose utilized, gm 

 (logarithmic scale) 



(318), and effects of metals, which are definite but not susceptible of 

 generalization (426, 466). Fat formation in most yeasts and fungi is 

 optimal at neutral or slightly alkaline reaction (238, 428, 485, 560). 



A supply of oxygen is invariably essential for fat synthesis (50). 

 However, cultivation with shaking reduces the fat content, but not 

 always the efficiency of conversion of glucose to fat (561). Here the 

 effect is probably that of encouraging respiration and growth and 

 thereby reducing the amount of carbon available for lipid synthesis. 



Maximum yields of fat have been of interest because of possible 

 industrial applications (270). Rippel (446) calculated that, consider- 

 ing the usual values for protein content and for the economic co- 

 efficient, the maximum synthesis by microorganisms, the "fat co- 

 efficient," is 15 gm. of fat per 100 gm. of sugar utilized. Values usually 

 found in the yeasts and fungi are in the range 5-15 (158, 318), but 

 cultivation of Rhodotorula gracilis under conditions which restrict 

 protein formation to a minimum allows a value of 18 to be attained 

 in aerated culture (179). Since lipids are more reduced than carbo- 

 hydrate, much of the glucose carbon must be oxidized to carbon 

 dioxide in order that the necessary hydrogen be supplied. 



The composition of the fat of a given species is relatively constant, 

 but external conditions appear to affect it in two ways. First, the ratio 

 of unsaturated to saturated fatty acids is claimed to be greater at low 

 than at high growth temperatures (413, 508). No such effect, however, 

 was found in careful studies on Aspergillus fischeri (428). The ratio 

 of saturated to unsaturated fats is also affected by glucose concentra- 



