180 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



and the ferrous-ferric end of the reaction cell. To make this measurement, 

 and thereby to measure AF, one might simply bubble hydrogen over a piece 

 of platinum (the metallic contact) in \N-ac\d soution; and attach the plati- 

 num through a voltmeter to another platinum piece sitting in equimolar fer- 

 rous and ferric salt solution. The two solutions must be connected if the 

 circuit is to be complete. The value measured in this case is 0.77 v, con- 

 sistent with a free energy of reaction of about 40 kcal per mole of hydrogen 

 consumed. The ferric end is positive to the voltmeter, the hydrogen negative. 



The concentrations may not be as stated, however, and we would expect, 

 and indeed find, that the voltage measured would then differ from 0.77. The 

 conditions specified in our example are arbitrarily chosen "standard state 

 conditions": unit (1) activities of reactants and products, 1 atm pressure, 

 25° C; and reversibility. We have already seen what a deviation from unit 

 activity ratio will do to AF. 



Purely as a matter of convenience and of convention, since the absolute 

 value of no redox system is known, the normal hydrogen electrode (NHE) 

 (1 atm pressure, normal acid, and H 2 on platinum) has been chosen as the 

 standard reference, and defined as zero volts. All other redox systems are 

 referred to this standard. In fact a table has been drawn up of known 

 standard redox potentials, F°'s, and is called the electromotive series. 

 However, a special table has been drawn up for biological redox systems. It 

 differs from the standard F°'s, referred to the NHE, in two ways: all the 

 redox reactions are measured against hydrogen at pH = 7, not zero; and 

 since the effective concentrations or activities are not usually known for bio- 

 logical molecules, measured concentrations are used instead; and the tabu- 

 lated values, E ml , refer to equal concentrations (midpoint, m) of oxidized and 

 reduced form (i.e., material 50 per cent oxidized). Table 7-4 lists some of 

 these. A very complete discussion of biological redox systems is given in the 

 remarkable book of W. Mansfield Clark, 2 who has spent a lifetime making a 

 systematic study of, and attempting to organize our knowledge of this 

 subject. 



Free Energy and Concentration. The Nernst Equation 



The free energy of reaction, and hence the emf, F, of reaction, varies with 

 the concentrations, as is evident from the relation between AF and K given 

 above. Insertion of nFE° for -AF°, and nFE for -AF, and rearrange- 

 ment gives the famous expression of the emf as a function of concentrations, 

 introduced just before the turn of the century by Walther Nernst: 



DT 



E = F°- —\ n (a m /a T J 



nb 



