324 



ANIMAL METABOLISM 



of coenzymes and (or) activators in order to function properly (Chap. 

 10). This whole sequence of linked chemical changes forms a pathway 

 over which each molecule of carbohydrate passes, as need for energy 

 arises. 



All of these changes, and any others which carbohydrates undergo in 

 body tissues, are referred to collectively as intermediary carbohydrate 

 7netabolism. Although the major part of carbohydrate metabolism has 

 to do with breakdown into simpler substances, the process is in large 

 part reversible, and certain carbohydrates {e.g., glycogen, lactose) are 

 formed in the body from other carbohydrates or from intermediate break- 

 down products. The "building-up" aspects of metabolism are called 

 anabolis7n; break-down processes are termed catabolism. 



Interconversion of digested carbohydrates 



Formation of Glycogen. Carbohydrate metabolism starts when the 

 products of carbohydrate digestion pass through the intestinal wall and 

 enter the blood stream. These products, from a normal diet, are D-glu- 

 cose, D-fructose, and D-galactose. If mannose is eaten, it can also be 

 metabolized. All four sugars are interconvertible in the animal body 

 and give rise to glycogen by means of the metabolic reactions shown 

 in Fig. 13-1. 



At first, each sugar combines with a phosphate radical taken from 

 ATP (reaction I, Fig. 13-1).^ This is an irreversible- reaction catalyzed 

 by hexokina.se and Mg++ ions. It may be represented by the usual type 

 of equation. For example, 



„ , ■ , , m-r, (hexokinase) 



D-glucose + ATP — ^^ > D-glucose-6-phosphate + ADP 



or more concisely by the scheme used in Fig. 13-1: 



ADP 



D-glucose-6-phosphate 



The ATP here serves as the biological equivalent of a match used to 

 light a fire. ATP is a concentrated storehouse of chemical energy. 

 When one of its three phosphate radicals is transferred to the sugar, 

 some of the energy is transferred too. This activates the sugar so that 



^ Well established chemical reactions occurring in the animal body have been num- 

 bered for easy reference throughout the chapter. 



^ Although this reaction, as such, is irreversible, the glucose-6-phosphate can easily 

 be hydroly^ed back to free glucose (see p. 325). 



