334 



ANIMAL METABOLISM 



gen carriers, TPN, FMN, and cytochrome c. Note that the two hydrogen 

 atoms from one molecule of the metabolite are passed from one coenzyme 

 to another before they are finally combined with oxygen (heavy arrows). 

 This seems like an unwieldy and roundabout method of bringing hydrogen 

 and oxygen together. Apparently, the purpose of this procedure is to 

 release energy in small steps (p. 420) rather than in a sudden burst, which 

 probably would injure living tissues. Through the action of the cyto- 

 chrome system all the hydrogen released from pyruvic acid by the reac- 

 tions of the citric acid cycle is converted into water. 



Cytochrome c is an iron-containing protein (p. 279). The enzyme, 

 cytochrome oxidase, is poisoned by cyanide. The importance of the cyto- 

 chrome system to higher animals is apparent from the fact that cyanide 

 inhibits the respiration of animal tissue preparations to the extent of 

 80 per cent or more. 



Summary of carbohydrate metabolism. 



The conversation of food carbohydrates into glycogen and their oxida- 

 tion to carbon dioxide and water have been considered above under the 

 headings, glycolysis, citric acid cycle, and cytochrome system. It must 

 be emphasized that these phases of carbohydrate metabolism are not 

 in any way separate from each other but operate continuously and 

 simultaneously in the living animal. In order to gain a clearer over-all 

 picture, the result of these processes as applied to a molecule of glucose 

 may be summarized as follows: 



CfiHuOs * 2CH3COCOOH + 4(H) 



2CH3COCOOH + 6H2O > 20(H) + 6CO2 



24(H) + 6O2 > 12H2O 



Sum: CeHioOe + 6O2 *■ 6H2O + 6CO2 



At various stages of the process, energy is stored up by conversion of 

 ADP into ATP, and some energy is released as heat. These aspects will 

 be considered in Chap. 16. 



METABOLISM OF LIPIDES 



The fat which is poured into the blood stream by way of the lymph 

 system, following a fatty meal, can be used by the animal organism in 

 four different ways. These are storage, excretion, oxidation, or con- 

 version into essential lipides. 



