128 VERTEBRATE RESPIRATION 



The presence of cytochromes in the cells of many organisms 

 has been demonstrated spectroscopically, for they have very 

 characteristic absorption spectra when in the reduced form. The 

 oxidation of reduced cytochrome can readily be demonstrated 

 in suspensions of yeast. When oxygen is bubbled through such a 

 suspension there is no absorption spectrum visible but when it 

 is stopped and nitrogen bubbled through instead, the spectrum 

 of the reduced cytochrome immediately shows itself. The re- 

 admission of oxygen to the suspension immediately removes the 

 reduced cytochrome. It is clear, therefore, that the yeast cells 

 contain systems which are not only able to reduce cytochrome 

 but also others whereby it can be reoxidised by molecular 

 oxygen. 



If we now enquire into the nature of the substrate AH 2 we 

 find that usually it is not a simple organic degradation product 

 of glucose, but a relatively complex molecule of the group of 

 substances known asflavoproteins. These are also carriers which 

 receive the hydrogen from earlier stages in the long sequence of 

 coupled reactions which occur during the degradation of a 

 single glucose molecule. The large number of these separate 

 steps is one of the remarkable features of the biochemistry of 

 cells. 



(b) ENERGY RELATIONSHIPS 



Even the system which we have summarised above for the 

 mechanism of cytochrome c and cytochrome oxidase is a very 

 much abbreviated form of the mechanisms now known. It is not 

 unreasonable to ask, therefore, why such a complicated chain of 



* DPN = diphosphopyridine nucleotide. 



