FACTORS IN RESPIRATION 



207 



Figure 1. The effect of culture age 

 on the endogenous respiration of 

 Fusarium solani mycelium. Curve 

 1, dry weight of culture; curve 2, 

 respiratory rate, in ^1 oxygen per 

 milligram dry cells per hour. 



2 4 



Age, days 



(93) or that provision of an ester of the acid increases its biological 

 effect (13). Or, intact cells of a fungus may fail to oxidize a substrate 

 while cell-free extracts do so rapidly (13, 65, 89, 258). All of these 

 types of evidence suggest that the intact cell is impermeable to the 

 substrate. In general, ionizable substances, e.g., phosphorylated com- 

 pounds and organic acids, are the most likely to be excluded by a 

 permeability barrier. 



However, failure of exogenous substrate to be utilized cannot al- 

 ways be ascribed to membrane impermeability. Thus, in yeast, frozen 

 cells are permeable to citrate but metabolize it only after further 

 treatment with chloroform (90). It is possible for enzymes in the cell 

 to be inaccessible by virtue of subcellular organization; in most cases 

 we cannot distinguish between this situation and true membrane 

 effects. 



Induction of Respiratory Enzymes. The enzymes of whatever funda- 

 mental respiratory pathways an organism has are probably always 

 present whatever the substrate. But it is often observed that some 

 compounds are oxidized only after a long lag or only if the cells have 

 previously been grown in contact with them. This suggests that cer- 

 tain enzymes, probably those concerned in the preliminary steps of 

 respiratory breakdown, are not constitutive but are induced by sub- 

 strate. The oxidation of phenylacetic acid is an example (Chapter 6), 

 and attack on other aromatic compounds by Hormodendrum sp. 



