ENZYMES 



61 



evolved and the amount of oxygen absorbed and of carbon dioxide evolved 

 were measured. In the absence of a nitrogen source the synthesis of 

 protoplasm was avoided. The rate at which heat was evolved was con- 

 stant until all the glucose was consumed (exogenous respiration), after 

 which the rate of heat formation decreased (endogenous respiration) (see 

 Fig. 13). 



20 



50 



60 



30 40 



Time in minutes 

 Fig. 13. Heat produced from glucose oxidation by yeast in the absence of a nitrogen 

 source; 10, 20, and 30 mg. of glucose were added at zero time in curves I, II, and III, 

 respectively. In all cases, only 26 per cent of the expected amount of heat was 

 evolved before the endogenous respiration rate was resumed. (Courtesy of Winzler 

 and Baumberger, Jour. Cellular Comp. Physiol. 12: 199, 1938. Published by per- 

 mission of Wistar Institute of Anatomy and Biology.) 



In this experiment the theoretical amount of heat could be calculated 

 for the amounts of glucose used. Only 26 per cent of the theoretical heat 

 was produced before endogenous respiration set in. The volume of oxy- 

 gen used was equal to the volume of carbon dioxide evolved, i.e.j the R.Q. 

 was 1. These data may be interpreted as follows: For every molecule of 

 glucose oxidized to carbon dioxide and water, three molecules were syn- 

 thesized into a carbohydrate, presumably glycogen. When sodium 

 acetate was the substrate, about 59 per cent of the theoretical heat was 

 evolved, but in the presence of dinitrophenol the theoretical amount of 

 heat was evolved. This inhibitor, therefore, blocked the assimilative 

 mechanism but not the oxidative processes. 



Within recent years it has been discovered that certain phosphate esters 

 may play a very important role in energy transfer. The student is 

 referred to the review of Lipmann (1941) for further information on this 

 subject. 



The utilization of energy derived from degradation reactions depends 

 upon such energy-yielding reactions being coupled with energy-requiring 

 reactions. Degradation reactions which are not so coupled (blocked) 



