OXIDATION CHAIN 



93 



and ethyl alcohol are all part of the glycolytic cycle in plants as well as other organisms. 

 Zymohexase, carboxylase and a number of dehydrogenases in plants have been 

 characterised. 



OXIDATION CHAIN 



As the breakdown of carbohydrate proceeds through the various stages detailed 

 in previous pages the systems pass from the highly reduced reactants related to 

 glucose to the completely oxidised carbon dioxide and water. This is reflected in the 

 oxidation-reduction potentials as indicated in the following table : — 



TABLE 22 

 Approximate Eq^ Values 



It will be recalled that the change in free energy associated with the formation 

 or breaking of an energy-rich phosphate bond is about 12,000 calories corresponding 

 to a change in potential of 0-25 volt. Allowing for a 60-70 per cent, yield of energy 

 the complete breakdown cycle would allow of 3 phosphate bonds being formed in 

 passing from the reduced substrates to the completely oxidised products. 



In considering steps in the breakdown of nutrients that are concerned with 

 oxidation-reduction changes it will be seen that in the initial stages hydrogen transfers 

 are of importance and we have in the previous chapter dealt with a number of 

 hydrogen-carriers . 



Wieland (1913) attributes to the enzyme concerned in biological oxidations a 

 hydrogen-activating effect, whereby the hydrogen may be transferred from the 

 oxidisable substance (AHg) to the oxidising agent or hydrogen acceptor (B). In the 

 presence of the appropriate specific enzyme the hydrogen hnkings are weakened and 

 may be transferred to any available hydrogen acceptor, thus : — 



A.H2 + B -> A + BH2 

 When oxygen itself acts as the hydrogen acceptor or oxidising agent, hydrogen 

 peroxide may be formed : — 



A.H2 + O2 -> A -f HPa 

 This reaction is of importance in bacteriology and will be discussed later. In 

 the presence of catalase the hydrogen peroxide is broken down to water and oxygen : — 



2 H2O2 -^ 2 H2O + O2 



