BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 101 



This thermal energy is then used for work or chemical syntheses by 

 transforming it into mechanical, electrical, or chemical energy. This 

 transformation of heat, a "degraded" form of energy, is always inefficient 

 from the standpoint of thermodynamics, and involves other wasteful 

 losses due to practical difficulties. In cells, where only small changes in 

 temperature can be tolerated, heat cannot generally be used to transfer 

 the energy derived from one metabolic reaction to another process in 

 which it will be utilized for locomotion, establishment of electrical 

 potentials, absorption against osmotic pressure, or chemical synthesis. 

 Energy-producing processes must be carried out in a carefully controlled, 

 stepwise manner to prevent appreciable rises in temperature and to per- 

 mit the liberated energy to be stored and utilized as needed. 



It has been found that a relatively simple device, though somewhat 

 unique from the standpoint of classical physics and chemistry, is em- 

 ployed in most if not all living systems — that of conserving the chemical 

 energy arising from the degradation and oxidation of organic compounds 

 (and perhaps from other sources available to some organisms) by con- 

 verting it into another type of chemical energy, popularly termed "high- 

 energy phosphate bonds." This conversion may be more accurately 

 described by saying that the energy-producing metabolic reactions result 

 in the formation of acid anhydrides of phosphoric acid, compounds which 

 are most versatile in their reactions and from which the chemical energy 

 inherent in the acid anhydride linkages can be readily utilized by 

 biological systems. 



Although the B vitamins themselves are not the substances which act 

 as transporting agents for these high-energy phosphate units, they are 

 usually involved in the reactions by which these agents are formed and 

 often in subsequent processes wherein they are utilized. Quantitative 

 values for the amount of free energy (the energy available for useful 

 work) liberated or absorbed during a reaction (AF) enable one to predict 

 which metabolic reactions can be used to create the high-energy units 

 and which processes will necessarily require expenditure of some of the 

 cell's reserve of these energy units. This same thermodynamic informa- 

 tion also enables one to calculate the relative concentrations of the 

 reactants and products of a reaction at equilibrium. From this it is 

 possible to determine the direction in which an enzymatic reaction will 

 proceed under any given set of conditions. 



The problem of the "reversibility of a reaction" involves the question 

 whether a reaction is theoretically capable of proceeding in either direc- 

 tion. Reversibility implies that a reaction and the reverse transformation 

 are taking place simultaneously, although the conditions may be such as 



