170 



A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



is evident that the heat capacities of reactants and products help to deter- 

 mine the position of equilibrium. Thus if a product is formed which has 

 more degrees of freedom (i.e., modes of vibrations, translations, rotations) 

 than the reactant, the product can store more energy as kinetic energy (as 

 energy unavailable for work); then AFis less than H, and 7 is less than 1. In 

 other words the products, once formed, have to be heated up to the same 

 temperature as the environment, and are heated by an energy which could 

 have performed useful work were this not necessary. On the other hand if 

 the products can store less heat energy at 37° than the reactants, then A b 

 is greater than A// and Y is greater than 1. The unavailable energy in a 

 process depends upon the temperature and upon the heat capacities of reac- 

 tants and products. This special heat capacity, S cal/deg C. mole, is called 

 entropy. A list of different types of energy and their factors is given in Table 

 7-2. Note that heat energy is the only one listed for which the intensive 

 factor does not have the dimensions of a force. Perhaps it should be listed 

 as d(TS)/d.x. 



TABLE 7-2. Factors of Several Kinds of Energy. 



Type of Energy 



Intensive (Force) Factor 



Extensive (Capacity) 

 Factor (s) 



Electrical (joules) dvjdx (volts/cm) 



Mechanical (ergs) F' (dynes) 



P (dynes/cm 2 ) 

 Chemical (cal) dF/d£ (cal/mole • cm) 



Thermal (cal) T(deg) 



q coulombs x .v(cm) 



<7(cm) 



!'(cm 3 ) 



£(cm) x rz(moles) 



■S'(cal/deg. mole) x n 



Explanation: 



£ = reaction path length. F is free energy, Mechanical force, above, is given the symbol !■' (in this Table 



< »i 1 1 s ). 



For the reaction or process under consideration, 



a = ts 2 



TS, 



where 2 and 1 refer to final and initial states. Then 



d = TAS 



Substitution in A// = AF+ Q., gives 



AH = AF + TAS 



which is the algebraic statement of the Second Law. 



Table 7-3 lists values obtained experimentally for AH, AF and AS. An 

 example of particular biochemical and physiological importance is the hy- 

 drolysis of adenosine triphosphate, ATP. At pH = 7 and 37°C: 



