CARBOHYDRATE METABOLISM 445 



free energy is conserved and use of ATP avoided in l^riiiging the 

 glucose into the metabolic pathways providing energy and intermedi- 

 ates. Since glycogen is a branched molecule, it will be recalled (page 

 220) that enzymatic provision is necessary in the formation and re- 

 moval of the branches. Glucose-1 -phosphate not used in the liver cells 

 themselves is apparently converted to glucose-6-phosphate and hy- 

 drolyzed by a phosphatase. The resulting glucose is taken by the 

 blood to other cells needing it. 



Glycogen in Muscle 



Muscle contains only about 0.5 to I per cent glycogen, much less 

 than is usual in liver. Nevertheless, the total (juantily of glycogen 

 stored in muscle is large because this tissue is abundant (25 to .'?0 kg. 

 in human beings). Starvation does not deplete the glycogen of muscle 

 so rapidly and completely as it does that of liver. However, the severe 

 stresses of muscle convulsions can reduce muscle glycogen to zero. 



When the hormone epinephrine is administered to well-fed animals, 

 the liver rapidly loses its glycogen, as mentioned above. However, 

 when a fasting animal is given epinephrine, glycogen is lost from the 

 muscle and appears in the liver. The sequence of processes goes some- 

 ■ivhat as follows: 



The hormone activates phosphorylase, which converts glycogen to 

 glucose-1-phosphate, and phosphoglucomutase converts this intermedi- 

 ate to glucose-6-phosphate. Muscle cells apparently do not contain a 

 phosphatase for conversion of either glucose phosphate to glucose. 

 Hence, glucose-6-phosphate is carried down the glycolytic pathway 

 (page 168) to pyruvate and lactate. These intermediates diffuse into 

 the blood stream and are transported to the liver, where they are 

 converted to glycogen which is retained at least temporarily. 



Although muscle cannot convert glycogen to glucose, it can use 

 glucose in making glycogen and takes this sugar from the blood for 

 such a purpose. The reactions are the same as those of the liver. Rare 

 inherited diseases impair glycogen utilization in either the liver or 

 muscle, leading to excessive accumulation of the carbohydrate in the 

 tissue affected and reducing its availability as an energy reserve. 



Glucose Levels in Blood 



Most animal cells depend on glucose from the blood for the energy 

 they need for synthesis, repair, and mechanical activity. Once in- 

 gested food has been digested, the reserves of glycogen are mobilized 

 to supply needed glucose to the circulatory system. A continual sup- 

 ply is essential since the central nervous system can store very little 



