CARBOHYDRATE METABOLISM 443 



must occur with each meal, since higher animals all eal more or less 

 intermittently. Yet the control ol' storage processes is complex, partly 

 because the osmotic concentrations ot blood, interstitial fluid, and 

 cells must be kept fairly constant. During and immediately alter a 

 meal, the concentration of small metabolites rises in the blood. This 

 increase is controlled to prevent osmotic damage by converting the 

 material not immediately needed into forms having little or no effect 

 on the osmotic concentration. Obviously this process must be reversi- 

 ble to meet demands arising before the next feeding period. 



Osmotic concentration depends, of course, only on the numbers of 

 particles and not on their sizes. Therefore, avoidance of excessive 

 osmotic concentration becomes simply a matter of polymerizing the 

 small molecules into very large ones or converting them into insoluble 

 substances. Both schemes are employed, leading to polysaccharides 

 and lipides, respectively. 



Animals convert the excess of digested carbohydrates into insoluble 

 lipides by the reactions of pages 457 to 460. This energy reserve then 

 becomes available for mobilization whenever the food supply does 

 not meet the energy requirement. Since the lipide is stored in special 

 depots and is not very soluble, fat is not rapidly available and is not 

 adequate for the demands raised by sudden bursts of activity. 



To provide for major physical exertion between eating periods, a 

 more readily available energy reserve is needed. The polysaccharide 

 called animal starch or glycogen fulfills this role. Glycogen is stored 

 in cells throughout the animal body, but its great molecular size 

 allows the accumulation of cjuite useful weights without much effect 

 on the osmotic concentrations of intracellular fluid. Since fat is 

 more highly reduced than carbohydrate, it is a more condensed form 

 in which to store energy. Thus fat serves as the major supply of 

 stored energy while glycogen acts as the immediate reserve, sufficing 

 for some hours without food intake. 



GLYCOLYSIS AND GLYCOGENESIS 



Glycogen in Liver 



Although, as mentioned above, glycogen is known to occur through- 

 out an animal, even in certain areas of the depot fat, it is most abun- 

 dant in skeletal muscle and liver. Most of the biochemistry of gly- 

 cogen has been based on work with these two tissues. 



In mammalian species the liver is normally about 2 to 8 per cent 

 glycogen, undergoing a rapid turnover. A starvation of 24 hours al- 



