BIOLOGICAL ENERGETICS 



419 



for the conversion of glucose to lactic acid is not known with certainty, 

 but is probably close to 40,000 cal. per mole. If this figure is correct, 

 the efficiency of glycolysis is nearly 60 per cent. 



It is perhaps not realistic to discuss the energy relationships of gly- 

 colysis in terms of the conversion of glucose to lactic acid, since this acid 

 represents only an offshoot from the main pathway of carbohydrate 

 metabolism and is not produced at all except during severe work (review 

 p. 328) . Even then, it is reconverted to pyruvic acid during rest. How- 

 ever, if carbohydrate breakdown is to be divided for purposes of study 

 into anaerobic and aerobic phases, the anaerobic part must be treated 

 as ending with lactic acid, even though discussion of the aerobic phase 

 begins with pyruvic acid. Allowance for the energy released in con- 

 verting lactic acid to pyruvic will be made below. 



Aerobic Metabolism. Complete combustion of glucose to carbon di- 

 oxide and water releases about 683,000 cal. under physiological conditions. 

 From the above figures it is obvious that only a small fraction of this 

 total appears during anaerobic glycolysis. Approximately 94 per cent 

 of the energy of the glucose remains to be released through the operation 

 of the citric acid cycle. It is of great interest to discover what portion 

 of this remaining energy becomes fixed in a biologically usable form (pre- 

 sumably ATP) , and to learn just how the reactions of the citric acid 

 cycle result in the formation of the necessary /^P bonds. During the 

 oxidation of one molecule of pyruvic acid by one "turn" of the cycle, 10 

 atoms of hydrogen are released (2 in each of five steps, namely, reactions 

 16, 20, 22, 23, and 25, Fig. 13-4). The energy from the whole cycle is 

 actually produced by the combination of these hydrogen atoms with the 

 oxygen of the inhaled air, and ^P bonds are evidently formed at the 

 same time. Before discussing this subject in greater detail, it seems 

 desirable to consider briefly the nature of oxidation and the quantitative 

 relations between oxidation and energy changes. 



Oxidation is often defined as addition of oxygen or removal of hydrogen, 

 but cases are also common in which oxidation occurs without either 

 oxygen or hydrogen being directly involved. The most exact and gen- 

 eral definition states that oxidation is a loss of electrons. For example, 

 Fe++-^Fe+ + + -f e, where e stands for an electron, the unit negative 

 charge of electricity. The tendency of substances to give up electrons 

 and become oxidized is expressed in terms of volts as an electrical po- 

 tential, called the oxidation-reduction or redox -potential. Strong oxi- 

 dizing agents have positive potentials ranging up to about -\-2 volts, 

 while reducing agents go down to about — 1 volt, and even lower in a 

 few cases. These relations provide a scale of oxidizing power, much 

 as the pH scale measures active acidity. When the oxidized and reduced 

 forms of an oxidizing agent are in equal concentrations, that is, when 

 the oxidizing agent is half reduced, its redox potential is called by defini- 



