422 BIOLOGICAL ENERGETICS 



Esterification of Inorganic Phosphate. Another line of evidence has 

 been uncovered which bears on this question of the number of ~P bonds 

 formed during the aerobic phase of carbohydrate metabohsm. Ground-up 

 preparations from tissues such as liver and kidney are able to take up 

 molecular oxygen and use it to oxidize pyruvic acid, or any other acid 

 involved in the citric acid cycle, to carbon dioxide and water. Inorganic 

 phosphate is needed for this oxidation, and as the oxidation proceeds, 

 some of the phosphorus becomes esterified, that is, united with organic 

 substances. Lehninger has demonstrated with the aid of the isotope, 

 P^-, that the newly formed organic phosphate has the properties of ATP. 

 It is very probable that one molecule of ATP is produced for every atom 

 of phosphorus esterified during the oxidation. The number of P atoms 

 taken up for each atom of oxygen used is difficult to measure accurately 

 because of side reactions which break down the new '— -P bonds even as 

 others are being formed. The best results, however, show values ap- 

 proaching those given in the last column of Table 16-3. This evidence, 

 then, also tends to indicate that phosphorylations occur and '^P bonds 

 are formed each time hydrogen atoms, from whatever source, are passed 

 from one hydrogen carrier to the next. In fact, this hydrogen transport 

 system is almost certainly the chief energy transformer of aerobic 

 organisms. 



Efficiency of Energy Metabolism. The above discussion deliberately 

 goes somewhat beyond the bounds of present well-established knowledge 

 in order to estimate the efficiency of energy metabolism in animals. If 

 38 -^P bonds are produced during the conversion of lactic acid to carbon 

 dioxide and water and two more are produced during glycolysis, a total 

 of 40 moles of ATP could be formed from the metabolism of one mole 

 of glucose. If these figures are correct, the efficiency of the overall process 

 would be 



40X11,500 ^ 

 683,000 

 This is a very high value in comparison with other types of machines. 

 The maximum efficiency of a steam engine, for example, is around 25 

 per cent and that of a diesel engine about 40 per cent. As a matter of 

 fact, direct work measurements show that animals also have maximum 

 efficiencies of about 30-40 per cent, but usually work at only about 15-20 

 per cent efficiency. This is not surprising since the above value of 67 

 per cent applies only to ATP formation. No information is available 

 regarding the efficiency with which the chemical energy of ATP can be 

 converted into muscular work by the animal. 



Physiological Fuel Value of Foods. Until quite recently the study 

 of energy metabolism in animals and man was conducted almost ex- 

 clusively from the standpoint of the total heat produced by combustion 

 of various foods and the total energy needs of metabolism under various 

 ponditions. Although thi§ sort of knowledge does not provide as much 



