68 GENERAL BIOCHEMISTRY 



lose may be represented by a long chain of glucose rings connected as 

 shown for its associated carbohydrate. The hexagons represent the 

 usual configurations of the first five carbons of the a-o-glucose sub- 

 units. 



Glycogen 



Animal starch is the principal reserve carbohydrate of animals and 

 is most abundant in muscle and liver. The same or a very similar 

 carbohydrate occurs in yeast and certain fungi. Glycogen resembles 

 amylopectin rather closely in both physical and chemical properties 

 but is not identical with it. Similar linkages involving a-D-glucose 

 occur in both carbohydrates, although the number and length of the 

 branches may ditfer somewhat. The schematic structure of glycogen 

 would be like that of amylopectin shown above. 



Cellulose 



This polysaccharide is also based on glucose, and the linkages are 

 1-4. However, the glucose is in the /3 form as chains of glucose units. 

 Thus the structure of cellulose might be indicated by 



CH2OH 



representing a chain of great length. Ihese rather linear arrange- 

 ments are packed together to form the fibers of cotton and to serve as 

 the structural materials in plants. 



Besides carbon dioxide (free and in solution), mineral carbonates, 

 coal, and perhaps petroleimi, cellulose is probably the most abundant 

 organic material. In fact, it is estimated that the quantity of carbon 

 dioxide fixed as cellulose is about half the weight of this gas in the 

 atmosphere. From this relationship it is clear that cellulose must be 

 steadily degraded by bacteria and fungi, and the carbon dioxide re- 

 turned to the atmosphere in order to maintain the carbon cycle. 



Celhdose is resistant to the amylases attacking starch. The presence 

 of the /3 form of glucose introduces a different enzyme specificity, and 

 animals ordinarily do not possess enzymes capable of breaking cel- 

 lulose into soluble fragments. However, the bacterial flora of certain 

 animals do hydrolyze part of the cellulose, allowing the host to utilize 

 it. The ruminants in particular gain an appreciable part of their 

 energy from cellulose in this way. In most animals, the undigested 



