REDOX SYSTEMS; ELECTRON TRANSFER PROCESSES 179 



The ATP-ADP system is one of a class of oxidation-reduction (redox) or 

 electron-transfer systems operating in the living being. There are many 

 others. 



REDOX SYSTEMS; ELECTRON TRANSFER PROCESSES 



Equivalence of Electrical and Chemical Energy 



Oxidation-reduction reactions have very wide exemplification in living 

 systems: They bring about energy-producing oxidations of food; electro- 

 chemical reactions in the brain and nerve; hydrogenation of oils and dehy- 

 drogenation of fats and sugars, etc. Some are simple electron-transfer re- 

 actions, the reaction 



Fe+ 2 __» p e +3 + g - 



for example. The free energy of this /W/-reaction (There must be a place 

 for the electron to go!) can be trapped as un-neutralized electrons — i.e., as 

 electrical energy. In fact if a metallic or molecular electron-acceptor is 

 present at the site, such as 



H + + e~ — 1/2 H 2 



the chemical free energy of the total reaction 



1/2 FT + Fe +3 — H + + Fe +2 



can be drained off as electrical energy. This transformation is almost re- 

 versible (and therefore highly efficient), even at fairly high speed. The free 

 energy of oxidation of foodstuffs is guided by a series of redox enzymes 

 through a particular reaction scheme, in which each step of the process is a 

 fairly efficient redox process. Most of the free energy of each step is trapped 

 as an electron per molecule, and then passed on at the site where it can 

 be used. 



Equivalence of electrical and chemical energy is a requirement of the First 

 Law. Thus AF calories/ mole of reaction must be equal to the electrical 

 energy derived per mole of reaction. Now Faraday showed about 1830 that 

 96,500 coulombs (amperes x seconds) are required to oxidize or reduce one 

 equivalent weight of redox substance; and one equivalent weight is defined 

 as the weight which will transfer one electron per molecule. Hence if the 

 number of electrons transferred per mole, or the number of equivalents per 

 mole, is n, and if 96,500 cou/equiv is abbreviated to F, then the product nF 

 is the number of coulombs required to oxidize or reduce 1 mole. But elec- 

 trical energy in joules is volts x coulombs. Therefore 



-AF = nF E 



What voltage is E? It is the voltage measured between the hydrogen end 



