Corbohydrote metabolism 



18 



Much information directly pertinent to the metabolism of carbohy- 

 drates by animals has been already presented. Chapter 7, Energy 

 Transfers and Biological Oxidations, and Chapter 9, Plant Metab- 

 olism, summarize much that is important here. Since many of the 

 metabolic reactions take place in cells of all kinds, work done with 

 plants and microorganisms has direct application to animals. The 

 occasional exceptions make it necessary to repeat experiments with 

 several different species but seldom render it impossible to write 

 schematic summaries covering large groups of metabolic reactions for 

 many living forms. 



Generally speaking, carbohydrate metabolism in animals begins 

 with the ingestion of food. The first step thus becomes digestion, the 

 subject of Chapter 17. Digestion involves two overall processes. First, 

 the large molecules are partially degraded by hydrolytic enzymes, and, 

 second, the small molecules and the intermediate hydrolytic fragments 

 are absorbed by the intestinal tract. The details of these processes 

 have been covered in the preceding chapter and need not be presented 

 again here. 



After digestion has occurred, the metabolites thus provided are 

 distributed by the circulatory system to the organs and tissues utilizing 

 them. Some of the compounds concerned are transported as simple 

 solutes in solution, others in complexes with proteins. Often the sup- 

 ply of metabolites from digestion and the demands for energy, growth, 

 and tissue repair differ. Therefore, excessive supplies must be stored 

 temporarily until a need arises. 



This arrangement is so common it arouses little curiosity, for it 



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