Respiration and Metabolism 269 



The oxidation of lactic acid to carbon dioxide and water occurs only in the 

 presence oii free oxygen. Thus the process of anaerobic glycolysis proceeds as 

 far as the formation of lactic acid, and this substance is the common accumu- 

 lation product of anaerobiosis, piling up in the tissues and flooding the blood 

 stream during excessive activity, incurring an oxygen debt, and awaiting an 

 adequate return of oxygen to balance the oxidation budget. 



Glycogen as an energy source is common among invertebrates, according 

 to determinations made by direct analysis and measurement of the R.Q. Some 

 tapeworms consist of as much as 60 per cent dry weight of glycogen; when 

 raised in a glucose-free medium these worms may undergo a decrease in glyco- 

 gen to 6 per cent of the original level.'^'^'' Nematodes can synthesize glycogen 

 if glucose is present in the culture medium. In some free-living planarians 

 seasonal changes occur in glycogen content with a minimum in summer, cor- 

 responding to a reduced carbohydrate metabolism. Free-living and parasitic 

 forms alike undergo considerable anaerobic glycolysis, with a tendency natural- 

 ly for extreme anaerobic adaptation in the endoparasitic animals.^^'' Indeed, 

 moderate oxygen tensions are claimed to be poisonous to some anaerobes. 



The over-all pattern of anaerobic glycolysis is a complex system of reactions 

 in which phosphorylated esters are formed under enzymatic control with 

 energy shifts of relatively high levels, brought about through phosphate high- 

 energy double bonds.-'^"' -•"' •^•'^ Although certain exceptional cases have been 

 cited,-^° the work at present indicates remarkable consistency and uniformity 

 in the pathway of glycolysis (Fig. 62).i-' ''^ Phosphorylation appears to be 

 mandatory for glycogen breakdown. It may be noted that all of these reactions 

 are theoretically reversible, but ordinarily move in the direction of degradation. 

 The important thermodynamic aspects of this series of reactions involve the 

 transfer of energy through the adenosine triphosphate (ATP) system between 

 steps four and five, the conversion of fructose-6-phosphate to fructose- 1,6- 

 diphosphate, and the liberation of energy in the oxidation of the glyceralde- 

 hyde diphosphate to diphosphoglyceric acid. 



The conversion of pyruvic acid may be through a number of alternate 

 routes: reduction to lactic acid, decarboxylation, or oxidation by way of the 

 famihar tricarboxylic acid cycle (Fig. 63). The latter oxidative transfer system 

 was established in vitro by Krebs and his co-workers,-"" and involves the suc- 

 cinate-fumarate-malate-oxaloacetate dehydrogenase systems. Pennoit-de Coo- 

 man^^*^ demonstrated in the cestode, Cysticerctis pisifonnis, the presence of 

 succinase, fumarase, and lactic acid and acetic acid dehydrogenases. 



This much detail has been considered in relation to glycolysis, inasmuch as 

 glycogen is present in considerable quantities in practically all organisms and 

 is the most readily mobilizable, and conveniently stored, source of energy. 

 Simpler carbohydrates, glucose and fructose, for example, are readily avail- 

 able for glycolysis, but generally are not stored in sizable quantities. 



Carbohydrate Conversion. An effective metabolic adaptation which makes 

 possible energy stores at an oxygen saving is the well established conversion 

 of carbohydrate to fat. The synthesis of fat from partially non-lipoidal sources 

 has been experimentally demonstrated, utilizing deuterium as a tracer, feed- 

 ing it to mice in drinking water. •^'•'* Organisms on lipoid-free diets may synthe- 

 size fat, and the "hardening" of fats of livestock by feeding carbohydrate and 

 protein illustrates the conversion that must take place. The general saving in 



