ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 161 



Such a quantity of energy converted to heat would be immediately 

 destructive. However, if this reaction can be carried out in a sufficient 

 number of steps to reduce the energy yields at each to safe values, 

 glucose would serve as a useful source of chemical energy. Evolution 

 has provided the mechanism for such a reaction pathway. 



Even though the oxidation of glucose by cells takes place by a 

 number of steps, some of the AF values are still fairly large, over 

 -10,000 cal. Moreover, reactions in the sequence require energies in 

 this range, and use of heat intense enough for the purpose is undesir- 

 able. At such points in the system, reactions convert the free energy 

 of one compound into that of another used later to drive a second 

 reaction requiring a large input of free energy. These high-energy 

 reactions are thus linked together not by heat but by the free energy 

 of these special energy-transferring compounds. Such substances are 

 called high-energy compounds from their metabolic function of trans- 

 ferring sizable quantities of free energy. These compounds are illus- 

 trated and discussed in the next section. Some of the metabolic 

 reactions in which they participate appear in all the subsequent 

 material devoted to metabolism. 



High-energy Compounds 



The biological substances usually placed in this category undergo 

 hydrolysis with AF = -6,000 to -15,000 cal. They are found in and 

 participate in many of the reactions of all living cells. Indeed, they 

 have been called a common denominator of life. During normal cell 

 function, the energy inherent in these high-energy compounds is not 

 set free by hydrolysis but is transferred by reaction to products which 

 then undergo subsequent reactions along the metabolic pathway. In 

 general terms the action of these versatile substances may be illustrated 

 with 



A -f B->C -f D* 

 D* + L->B + M 



where the asterisk denotes a high-energy compound converted by the 

 second reaction into its original form. 



Adenosine triphosphate, ATP, is one of the most widely studied of 

 the compounds whose role is primarily that of energy transfer. Its 

 structure is shown on page 141. This compound is formed by a 

 variety of metabolic steps, and the free energy thus stored is available 

 for subsequent processes. The following specific example occurring 

 during the utilization of glucose by common plants and animals shows 

 how ATP participates in energy transfers. 



