323 



ANIMAL METABOLISM 



death, and even values only slightly below the normal range result in 

 feelings of listlessness and fatigue. 



Glycolysis. The catabolism of carbohydrate in the animal body may 

 be divided for purposes of study into two main phases, the anaerobic 

 and the aerobic. The anaerobic phase, which is called glycolysis, pre- 

 cedes the aerobic part and consists in the conversion of glycogen into 

 pyruvic acid and (or) lactic acid. The metabolic reactions which make 

 up glycolysis are shown in Figs. 13-1 and 13-3.^ The whole process is 

 often called the Embden-Meyerhof scheme. 



Starting with glucose, two moles of phosphoric acid or inorganic phos- 

 phate, are converted into "organic phosphate" (reaction 8, Fig. 13-3),^ 

 and two moles of ATP are used up, being converted into ADP (reactions 

 I and 4, Fig. 13-1). However, four moles of ATP are again formed 

 from ADP (reactions 9 and 12) so there is a net gain of two moles of 

 ATP for each mole of glucose used. In reaction 7, hydrogen is removed 

 from glyceraldehyde-3-phosphate and is held in the form of a reduced 

 coenzyme, DPN • Ho (p. 332). Four atoms of hydrogen are thus pro- 

 duced per mole of glucose. The net result of glycolysis, under condi- 

 tions of mild exercise, can be summarized by the equation: 



CgHioOe + 2H3PO4 + 2ADP + 2DPN -» 



2CH3COCOOH + 2ATP + 2H2O + 2DPN • H2 



During mild exercise, the hydrogen of the DPN • H2 is converted into 

 water by combining with oxygen via the cytochrome system (p. 332). 

 However, when exercise is very violent, oxygen cannot be carried by the 

 blood stream to the muscles quickly enough to reoxidize the DPN • Ho 

 as fast as it is formed. When this situation occurs, pyruvic acid is 

 reduced to lactic acid (reaction 13, Fig. 13-3) so that lactic acid becomes 

 the end product of anaerobic glycolysis. This process gives the organism 

 an extra burst of energy for a short time, but the muscles soon become 

 loaded Avith lactic acid, and exhaustion results. During rest, about four- 

 fifths of the lactic acid is converted back into glycogen, and the re- 

 mainder is oxidized to carbon dioxide and water. 



Lactic acid formation (reaction 13) therefore is essentially an offshoot 

 from tlie main line of carbohydrate metabolism. The main pathway 

 leads to pyruvic acid, ATP, and DPN • Ho, as given in the equation 

 above. These products are disposed of during the aerobic phase of car- 

 bohydrate metabolism. 



It should be noted that each carbon atom of the pyruvic acid comes 



• ^ The glyceraldehyde and glyceric acid phosphates, appearing in these charts, are 

 frecjuently called phospho-glyeeraldehydes and phospho-glyceric acids, respectively. 



= ()nly one molecule of HaPOi is shown in Fig. 13-3, but two C-3 fragments are 

 formed from each C-6 unit (reaction 5) so that the products shown subsequently 

 (reactions 7-13) represent only one-half of the molecules coming from one mole of 

 glucose. 



