ENERGY SUPPLY OF THE CELL 25 



In discussing the energy supply of cells, it must be 

 considered that most cells can utilize various kinds of 

 food, and may even be able to use the same kind of food 

 in two different ways. It is easily possible to cultivate 

 some typical sugar-fermenters in the absence of sugar. 

 Yeast will grow in sugar-free broth; lactic acid bacteria 

 can grow in peptone solutions; Hydrogenomonas as well 

 as urea bacteria are commonly cultivated on ordinary 

 nutrient gelatin. Most molds seem to be omnivorous, 

 while some of the prototrophic bacteria are limited to 

 only one source of energy. Yeast will ferment sugar 

 to alcohol, but can oxidize it completely if an abundance 

 of oxygen is at hand. Most organisms show certain 

 preferences or, more accurately speaking, certain com- 

 pounds are more readily fermentable by certain organ- 

 isms than others. It is a common experience that 

 protein decomposition (e.g. gelatin liquefaction) is 

 retarded by addition of sugar. Apparently, the energy 

 needs of a cell can be more readily supplied from sugar 

 than from protein (see p. 205). 



(6) AMOUNTS OF ENERGY 



The amounts of energy liberated by the various fer- 

 mentations differ widely. As a rule, oxidations yield 

 the greatest quantities, while hydrolyses yield but very 

 little, if any. Only in one case, cells provide for their 

 energy by hydrolysis, namely in the urea fermentation. 

 Hydrolysis is very commonly caused by bacteria, but 

 in practically all cases, it raerely serves as a food- 

 preparing process. The enzymes causing these hydrol- 

 yses are mostly soluble. Table 5 (p. 23) shows the soluble 

 and the endo-enzymes arranged according to heat-units 

 produced from 1 gm. of substance. It is very evident 

 that the enzymes with small heat liberation are soluble 



