ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 165 



and other groups but the magnitudes are uncertain. In most cases the 

 efficiency seems to be surprisingly high. 



A typical animal converts glucose to carbon dioxide and water dur- 

 ing its normal respiration in the presence of oxygen according to the 

 overall reaction: 



C6H12O2 + 6O2 -^ 6CO2 + 6H2O AF = -688,000 cal. 



At various individual steps in this process, sizable transfers of free 

 energy occur. Some of these form ATP directly from ADP. Other 

 reactions yield DPNH and the like, which later transfer their energy 

 to ADP, forming ATP. There are believed to be about 40 molecules 

 of ATP thus formed per molecule of glucose completely oxidized. 

 However, in two early reactions ATP is required, leaving a net gain of 

 38 molecules. Each of these molecules has available about 10,500 cal. 

 for the needs of the animal, representing a total of approximately 

 400,000 cal. or about 60 per cent of that theoretically available from 

 the glucose. 



Some of the remaining energy is converted to heat and lost to the 

 environment. However, a part may be converted to heat used in 

 driving some of the processes requiring low-level energy. This latter 

 probability thus raises the efficiency still higher. On the other hand, 

 some of the energy funneled through ATP is lost as heat during sub- 

 sequent reactions and reduces the efficiency. These effects cannot 

 be evaluated at present, so the overall efficiency cannot be estimated. 

 Nevertheless, it is expected to be fairly high. 



Under other conditions some organisms metabolize glucose incom- 

 pletely by a process called fermentation: 



CeHiaOe -^ 2CO2 + 2C2H5OH AF = - 50,000 cal. 



One or more compounds other than ethanol may be formed as by- 

 products, depending upon the species and the environment. While 

 the nature of the products alters the free-energy change, the energies 

 available from the reactions of this type are but a fraction of the 

 energy derived from the complete oxidation of glucose. In the in- 

 stance above a net gain of only two high-energy molecules equivalent 

 to about 21,000 cal. is obsei-ved. 



On this basis then, 15 to 20 times as much glucose must be metab- 

 olized by a fermentation mechanism to provide energy equal to 

 that from the respiration of glucose. Therefore, given equal oppor- 

 tunities otherwise, fermenting species would be hard put to compete 

 with respiring species. This expectation seems to be fulfilled. How- 



