414 BIOLOGICAL ENERGETICS 



a negative sign if heat is released when the reaction proceeds from left 

 to right. For example, in the case above, AH is —673,000 cal. 



The energy released from chemical processes which can be used for 

 doing useful work is called the free energy change of the reaction and is 

 represented by AF. For any particular process, \F may be either larger 

 or smaller than AH. If AF happens to have a larger negative value than 

 AH, more useful work can theoretically be obtained from the process 

 concerned than corresponds to the amount of heat liberated. This rather 

 surprising situation is caused by changes in entropy, which occur when- 

 ever chemical or physical transformations are carried out. The relation 

 between the three quantities, for processes occurring at constant tempera- 

 ture and pressure — as is approximately true in biological systems — is 

 given by the equation : 



af = ah-tas 



where AS is the entropy change, and T the absolute temperature at which 

 the process occurs. The entropy change, AS, is expressed in calories 

 per degree. It is difficult to define exactly, but may be thought of as 

 heat flowing into or out of the system from the surroundings while a 

 process is going on. For a more complete discussion of entropy consult 

 a textbook of physical chemistry or thermodynamics, such as those listed 

 at the end of the chapter. 



An illustration may help to make these abstract concepts more under- 

 standable. For the complete oxidation of glucose, AH is —673,000 cal. 

 as stated above, whereas AF is about — 683^,000 cal. The exact AH and 

 aF values depend on the state of the reactants and products, that is, 

 whether solid, liquid, or gaseous, and at what concentration, temperature, 

 pressure, etc. The value — 683,000 cal. is based on approximately physi- 

 ological concentrations, viz., glucose 0.05M, carbon dioxide 0.1 atmosphere, 

 oxygen 0.01 atmosphere, and on a temperature of 37°C. Useful work 

 equivalent to 683,000 cal., therefore, could be obtained from burning one 

 mole of glucose, if the machine or living tissue doing the work was able 

 to operate with 100 per cent efficiency. Actual machines and tissues, 

 of course, never reach this peak of perfection, but the efficiency which 

 they do achieve may be determined by comparison with the free energy 

 change of the process being used. 



The aF value is also an indication of whether a particular chemical 

 reaction can take place (if properly started) and to what extent it will 

 proceed. Reactions which release free energy (Ai^ negative) occur readily 

 and are called exergonic. The opposite type, called endergonic, will not 

 take place unless energy is supplied (AF positive). The change in free 

 energy is the driving force of the reaction, and the larger the amount of 

 free energy release, the more complete the reaction will be. 



The completeness of equilibrium reactions is represented mathematically 



