CELLULAR RESPIRATION 127 



action of the cytochrome system may be indicated in the 

 diagrams below which show how a substrate to be oxidised 

 (AH 2) reduces the cytochrome and is itself oxidised in the 

 presence of a specific dehydrogenase enzyme. 



AH2 cytochrome 



dehydro- oxidase 



genase 



The reduced cytochrome so produced has its iron in the ferrous 

 condition and it is oxidised to the ferric state by combination 

 with molecular oxygen, the latter reaction being catalysed by 

 another member (a 3) of the cytochrome system — cytochrome 

 oxidase. This enzyme is the cyanide-sensitive component. Cyto- 

 chrome therefore functions as an electron carrier which is able 

 to pass on the hydrogen to molecular oxygen and is then restored 

 to its original condition. In doing this the two hydrogen atoms 

 attached to the AH 2 can be thought of as two protons and two 

 electrons. The latter reduce the ferric cytochrome c to the ferrous 

 condition, but this is immediately returned to the oxidised state 

 because cytochrome oxidase takes the electrons and causes them 

 to unite with the two protons and oxygen to form water. By this 

 cyclical mechanism small amounts of the cytochrome system can 

 participate in the oxidation of enormous amounts of materials 

 in the cells. 



I 



cytochrome 

 oxidase 



Proton \ / Proton 



. H-A-H . ^ 



Velectron/ \electron/ 2 Protons AO, 



Z^JZ 



H,0 



