184 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



electrode. If the redox indicator (KI, for example) is present to an amount 

 much less than the redox systems in the solution to which it is added, it can 

 exchange electrons (KI — * I 2 ) until its potential (determined by a x /a KX ) is 

 the same as that of the solution. 



The third trick is really a combination of the first two. If a solution con- 

 tains two reactants, such as glucose and oxygen, which can react together 

 spontaneously (negative AF), the reaction will be extremely slow unless the 

 solution contains mediators. Consider one step in the over-all process, for 

 example succinate added to pyruvate in a test tube. Although these two ions 

 can exchange electrons (and hydrogen atoms), with the liberation of free 

 energy, they don V unless a redox system such as cytochrome-C is present as 

 a mediator. Its job is to couple with succinate and reduce it to fumarate, 

 then (itself now oxidized) to oxidize pyruvate. In other words it provides a 

 path by which the over-all reaction can go in two steps, via the mediator, 

 whereas it could not go at all in one. The whole respiratory enzyme sys- 

 tem is a system of mediators, permitting the complete, controlled oxidation 

 of glucose by oxygen to go in discrete - steps, the free energy of each step 

 being thus made readily available to recharge ATP, for example, and there- 

 fore to be usable elsewhere in the system. 



There seem to be no generic differences among electrochemical catalysts, 

 redox indicators, and mediators. The name used depends upon one's point 

 of view. Indeed, in his classical work on the succinate-fumarate system, 

 Lehman (1930) called succinic dehydrogenase the catalyst and methylene 

 blue the mediator. 



MEASUREMENT OF AH, AF, AND TAS 



The simplest way to measure all three energies is in an electrochemical 

 redox cell, described in the previous section, if indeed the reaction is an 

 oxidation-reduction reaction. Thus AF is directly related to the voltage on 

 the NHE scale by -AF = nFE, and A S is directly related to the rate of 

 change of AF with temperature through the relationships' 



^1=-A<>; and AS = nF d -^- 

 dT dT 



Since AFand AS can be so determined, AH can be obtained from the Sec- 

 ond Law: 



AH = AF + TAS 



However, AH, the heat of reaction, is itself hard not to measure! If no 

 work at all is extracted in a calorimeter experiment, as a process is allowed 

 to go spontaneously to equilibrium, all the free energy is wasted away into 

 heat, and A His the quantity of heat measured in the experiment. 



