210 RESPIRATION 



of these and other minerals are sufficient. Aerobic oxidation with 

 exogenous substrate is generally markedly increased if a source of 

 nitrogen is provided (199, 265, 275); this, of course, converts the 

 preparation at least potentially to a growing culture carrying on amino 

 acid or even protein synthesis with an attendant demand upon energy 

 and upon metabolic intermediates of the citric acid cycle. The effect 

 of such organic materials as yeast extract may be the same, but there 

 is the added complication that respirable materials are also being 

 supplied. 



3. ENDOGENOUS RESPIRATION 



Endogenous respiration, or "autorespiration," defines the metabo- 

 lism of the cell in the absence of an external substrate insofar as it is 

 expressed by gas exchange. In yeast there is normally no anaerobic 

 endogenous carbon dioxide liberation (270), although 2,4-dinitro- 

 phenol initiates fermentation of carbohydrate reserves (251). Fusa- 

 rium lini, on the other hand, has a vigorous endogenous fermentation 

 (98). 



The substrate of endogenous respiration in fungi is not known; 

 respiratory quotients are consistent with lipid oxidation (186, 230), but 

 suitable analytical data to discriminate between this and other possi- 

 bilities are not available. Cell-free extracts of Neurospora crassa are 

 stimulated in endogenous respiration by triphosphopyridine nucleo- 

 tide (301). 



Scopulariopsis brevicaulis cells are unable to assimilate nitrogen in 

 the absence of exogenous substrate, i.e., the endogenous respiration 

 cannot be coupled to amino acid or protein synthesis (186). This is 

 in contrast to both Chlorella vulgaris (284) and Torulopsis utilis (332). 



The high endogenous rate characteristic of many fungi and actino- 

 mycetes complicates experimental study; obviously, if some limiting 

 terminal system is fully saturated by the endogenous system, then an 

 addition of substrate will not cause any further increase in oxygen up- 

 take. However, it cannot be assumed that under these circumstances 

 the exogenous substrate is inert — conceivably some of it will be oxi- 

 dized in competition with the oxidation of endogenous materials. 

 Partial responses are also possible: if, for example, the terminal system 

 were 50 per cent saturated by endogenous metabolism, addition of 

 substrate could at most double the oxygen uptake rate. But — and this 

 may be a real problem — one can imagine that in fact the substrate is 

 competing on, say, equal terms with the endogenous metabolism for 



