Chapter 13 



ANIMAL METABOLISM 



METABOLISM OF CARBOHYDRATES 



The chief function of carbohydrate in the animal body is to provide 

 energy in a form which the animal can use. Like all fuels, it must be 

 burned, or oxidized, for the energy to be released. The end result of 

 the burning process is the conversion of the sugar into carbon dioxide 

 and water, which is the reverse of photosynthesis: 



CgHioOe + 60o -^ 6CO2 + 6H0O + 683 Cal. 



When carried out by living organisms, this process is called respiration. 

 This term is often used in a broader sense to include all metabolic proc- 

 esses by which gaseous oxygen is used to oxidize organic matter chiefly 

 to carbon dioxide and water. This type of metabolism is most pro- 

 nounced in animals, but is also carried out by plants and by a few 

 microorganisms. 



If the carbohydrate is burned directly in a flame, all the energy is 

 released in the form of heat. Some heat is produced also in the animal 

 body, but much of the energy released is stored up in the form of certain 

 chemical by-products, particularly adenosine triphosphate (ATP) , which 

 can later be used for muscle contraction or other useful purposes. Fur- 

 thermore, the energy is released in small portions and at temperatures 

 low enough so the living tissues are not injured. Direct burning of 

 carbohydrate material {e.g., wood, paper) of course never takes place 

 except at a temperature fatally high to all living things ; yet the same net 

 result is accomplished rapidly and continuously in all living animals. 

 Obviously, nature must have devised some very special and effective 

 method of "low-temperature, biological burning." 



After a very great deal of painstaking research, many of the details 

 of this complicated process have now been discovered. In brief, what 

 happens is that the carbohydrate undergoes a long series of chemical 

 changes, each altering it slightly, so that it is gradually converted into 

 the final end products, carbon dioxide and water. Each chemical reaction 

 involved in the process is catalyzed by a particular enzyme, without 

 which the reaction will not proceed fast enough to be of any use to the 

 organism. Many of these essential enzymes in turn require the presence 



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