126 VERTEBRATE RESPIRATION 



The precise mechanisms whereby the oxygen is utiHsed by the 

 organism and the CO 2 evolved and, what is more important, 

 how energy is made available by the degradation of the large 

 molecules of the animal, are beyond the scope of this book. 

 Nevertheless, it seems inappropriate to leave the oxygen just at 

 the point where its most significant effects are to take place. The 

 parts of the cellular mechanisms which use molecular oxygen will 

 first be discussed before going on to consider the energy- 

 releasing processes. In some ways this is a reversal of the normal 

 view presented by biochemists, who usually begin with the large 

 molecules and discuss their progressive degradation and later 

 consider oxidation of the end products by molecular oxygen. 

 Equally of course, many important processes of cellular respira- 

 tion can take place in the complete absence of oxygen, and is 

 the rule in some organisms. Anaerobic respiration provides far 

 less energy, however, and is certainly not the normal mechanism 

 found among vertebrates. Following a period of exercise extra 

 O2 is taken up in order to complete the breakdown of respira- 

 tory substrates, but some fishes (e.g. carp) do not show this after 

 periods when their Og supply has been reduced to almost zero. 



(a) THE CYTOCHROME SYSTEM 



In all aerobic respiration the molecular oxygen functions as 

 the final agent which takes part in the removal of hydrogen from 

 a substrate which is therefore oxidised. In fact it is mainly con- 

 cerned to oxidise one particular substance, namely cytochrome. 

 This is one of a series of chromo-proteins which are very similar 

 in structure to haemoglobin, for they are made up of a haem- 

 prosthetic group and protein portions with molecular weights of 

 about 13,000. The vital importance of the cytochrome system, 

 rediscovered and named by D. Keihn, is indicated by the pro- 

 found effect of hydrogen cyanide on nearly all living organisms. 

 This poisons the cytochrome system and stops between 80 and 

 90 % of aerobic respiration. It may be concluded therefore that 

 at least this proportion of the oxygen which enters the blood 

 stream of vertebrates is utilised in the tissues by this route. The 



