1 51 



LIPIDS l J ' 



tion and by pH (318), and in an as yet undetermined manner by cell 

 pigments (403). Second, the content of free fatty acids is greater at 

 high carbohydrate levels. The acid value is also stated to be affected 

 by pH (425), but the observed effects may be attributable to other 



causes. 



The relation of fat formation to growth is exemplified in the data 

 of Figure 4. As discovered early by Lindner (345), two phases of meta- 

 bolic activity are distinguishable in many organisms: a phase of growth 

 and a phase of fat accumulation. This is exactly comparable to the 

 sequence of events in citric acid accumulation (Figure 2). Separation 

 of growth and fat synthesis is not, however, always so clear (429); 

 Figure 5 shows parallel courses of dry weight and fat production. 



In animal and bacterial systems it seems clear that fatty acids are 

 synthesized ultimately from 2-carbon fragments, specifically acetyl- 

 coenzyme A (114). In the fungi, precursor studies, reviewed by Foster 

 (200), early suggested the involvement of 2-carbon compounds in fat 

 synthesis. More conclusive evidence has been obtained with several 

 isotope techniques. Thus, in a mutant of Neurospora crassa which is 

 unable to form acetate from carbohydrate, the carbon of fatty acids 

 is derived almost entirely from exogenous acetate (407). The im- 

 portance of acetate in fatty acid synthesis appears also from studies 

 on isotope distribution in the fatty acids of Phycomyces blakesleeanus 

 (43, 45), Fusarium lini (146), and Ustilago zeae (190). 



Whether the unsaturated acids are derived directly from the 

 saturated, by dehydrogenation, is an open question. Two types of 

 evidence— nutritional responses of a fatty acid requiring mutant 



Figure 4. Growth, sugar uti- 

 lization, and fat formation in 

 cultures of Rhodotorula gra- 

 cilis. Redrawn from Enebo, 

 Anderson, and Lundin (179), 

 by permission of Academic 

 Press, Inc. 



Residual sugar 



40 60 



Time, hours 



80 



