BACTERIAL RESPIRATION 



189 



As we have already seen in Chapter iV the reactions 

 catalysed by these enzymes involve only relatively small 

 energy changes, whilst, on the other hand, those reactions 

 brought about by the endo-enzymes, inside the cell, 

 where the energy liberated is of real value to the organism, 

 involve large energy changes. This is illustrated in 

 Table 15, w^hich shows the energy liberated from 1 gram 

 of the appropriate substrata by the action of different 

 enzymes. 



Table 15 



An exception to the general rule that hydrolytic 

 reactions involve little energy change is the case of the 

 breakdown of urea, w^hich is used as energy source by the 

 urea bacteria. This is only an apparent exception because 

 the reaction takes place in two stages, first the hydrolysis 

 of urea to ammonia and carbon dioxide : — • 



.XHo 



CO 



^H.,0 



JXHa -fC02+ about Cab 



XHo 



which involves practically no liberation of energy. The 

 second step is the formation of ammonium carbonate 

 from the ammonia and carbon dioxide : — 



2XH3 - CO, + H2O > (XH4)2C03 + 12 Cals. 



and it is this part of the reaction which supplies the 



