268 Comparative Animal Physiology 



centers in low concentration becomes the predominant and delicate controlling 

 factor in the respiratory pattern of the organism, relegating the factor of oxygen 

 want to a minor role except in anoxic and emergency conditions. 



METABOLIC ADAPTATIONS 



Oxygen Requirements and Energy Conversion. The significance of oxygen 

 rests on its importance as the ultimate oxidizing agent in the manifold series 

 of energy-releasing metabolic processes in the organism. As such its utilization 

 is a direct measure of the degree of activity, food conversion, heat production, 

 and protoplasmic reorganization occurring within the cells and tissues— a 

 function of those processes dependent on or indicated by free energy release. 

 To deny organisms oxygen, however, is not necessarily to deprive them of 

 the means of survival. Fortunately there are other energy-yielding reactions 

 which do not depend on atmospheric oxygen— in anaerobic environments in 

 which free oxygen is not a characteristic component. These metabolic adapta- 

 tions bear an important and universal place in the oxidative economy of organ- 

 isms and permit life under conditions of little or no oxygen, either temporarily 

 or for indefinite periods of time. 



Oxidation which depends on the combination of substrate with atmospheric 

 oxygen, forming carbon dioxide and water, is but one of many processes in- 

 volving redox energy shifts. Oxidation may, for example, be just as truly a 

 process of dehydrogenation or electron loss as one consisting of the addition of 

 oxygen. For reviews of the increasing tide of oxidation-reduction literature a 

 number of important contributions are available.^-- ^^' ^•^^' ^^'■^' ^"" The break- 

 down of food substances, the release of pent-up energy, and the electronic 

 shifts characteristic of particular oxidation-reduction systems may occur in the 

 presence of specific substrates and enzymes and often in the absence of oxygen. 

 These anaerobic reactions may be of temporary and topical occurrence, as 

 muscle glycolysis, on which contraction depends, and which in severe exercise 

 or during submergence of diving mammals is accompanied by pronounced 

 oxygen debt. Or they may be more permanently anaerobic and non-localized 

 processes affecting the entire animal, as, for example, in an oxygen-deprived 

 nematode worm, which has yielded the advantages of an oxygen environment 

 in favor of endoparasitic anaerobiosis. 



Glycolysis. Oxygen requirements and energy yields from various types of 

 organic food have been cited previously (p. 237). The steps of degradation 

 of these substrates in the animal are important phases in intermediary metab- 

 olism, of which carbohydrate breakdown is probably the most common and 

 certainly the best known. Glycolysis, the catabolism of glycogen to lactic acid, 

 occurs under both anaerobic and aerobic conditions; glycolysis under aerobic 

 conditions makes available more energy for the amount of carbohydrate 

 utilized, and may thus be considered more efficient.'^'' Calculated on a basis 

 of energy made available to the system per mol of hexose sugar utilized, the 

 relative elHciencies of the various processes'- are as follows: 



oxidation CoH^O,, + 6 O. -^ 6 CO. + 6 H,0 + 686 kcal./mol 



Icrmcntation C„M,.0„ -> 2 CIH.-.CH.OH + 2 CO. + 50 kcal./mol 



glycolysis C0H..O,; -> 2 CH:,CHCOH)COOH + 36 kcal./mol 



