150 - The Cell 



But a second activation is also required. 

 Quickly, in the presence of appropriate en- 

 zymes, the structural arrangement of glucose- 

 6-phosphate changes to that of fructose-6- 

 phosphate and a second phosphorylation, 

 mediated by fructokinase, yields the very 

 active compound fructose diphosphate (Fig. 

 8-5). Initially, therefore, the energy from 2 

 molecules of ATP is sacrificed for the initia- 

 tion of carbohydrate catabolism. However, 

 in the end, when this catabolism has run to 

 completion, a generous recompense has oc- 

 curred. In fact, the high-energy phosphate 

 (ATP) reserves are considerably increased 

 (Fig. 8-5). _ 



In passing it should be noted that ATP 

 also provides energy when the catabolism of 

 fatty acids is initiated. In this case, however, 

 the reactions are very complex. Suffice it to 

 say that the long carbon chain of a fatty acid is 

 made shorter and shorter by the breaking off 

 of a series of 2-carbon (acetyl) units. These 

 acetyl units are then passed on, one at a 

 time, to the Krebs cycle (Fig. 8-5), for final 

 oxidation. 



Glycogen Synthesis and Glycolysis. In 

 many cells, particularly in animals, some 

 glucose is converted to glycogen, which rep- 

 resents a stable reserve of carbohydrate fuel. 

 Each glycogen molecule is formed from 

 many (up to 20,000) glucose units, which 

 become chemically bonded into a single 

 macromolecular structure, as is indicated in 

 Figure 8-6. 



In building glycogen, the cell starts with 

 phosphorylated glucose. Many of the chem- 

 ical bonds of the glycogen molecule (the 1-4 

 bonds of Fig. 8-6) are formed by dephos- 

 phorylation; the other (1-6) bonds are gen- 

 erated by the dehydration type of synthesis 

 (p. 79). 



Glycolysis may be defined as the partial 

 catabolism of glycogen — down to the level of 

 pyruvic acid or (in some cells) lactic acid 

 (Fig. 8-5). This series of reactions generates 

 only about 20 percent of the useful energy 

 available from the total breakdown of carbo- 

 hydrate. But no oxygen is required for gly- 



CH 2 0H CH 2 0H CH 2 0H 



0>H C 

 I ~ 

 H0-P-0H 

 II 

 

 PH0SPH0RYLASE 



CH 2 0H 



C 

 I 



C 

 I ^ 



c 

 I 



C 



I II 



C-0-P-0H 

 I 

 OH 



GLYCOGEN 





 H I 

 HC-O-P-OH 

 I I 

 C OH 



I 



C 

 I 



c 



4*° 



Ku 



GLUC0SE-1-PH0SPHATE GLUC0SE-6-PH0SPHATE 



Fig. 8-6. Part of the macromolecular structure of 

 glycogen; relation of glycogen to glucose phosphate. 

 Note that the 1-4 bonds of glycogen are ruptured by 

 phosphorolysis; whereas the 1-6 bonds are broken 

 by hydrolysis. 



colysis. Consequently cells can utilize this 

 source of energy under anaerobic conditions, 

 in which oxygen is scarce or lacking. A hard- 

 working muscle, for example, expends its 

 high-energy phosphate reserves at a very 

 rapid rate. Eventually, if adequate oxygen is 

 available, the muscle uses oxidative metab- 



