SOURCES OF ENERGY AND AI ATE RIALS 



107 



action may even send substances against the diffusion gradient — that is, 

 cause them to go from places of lower to places of higher concentration. 

 Storage of Food. — Carbohydrates, in the form of glucose or other 

 simple sugars, are ordinarily present in the blood to the extent of less than 

 0.1 per cent. After a meal they may increase perceptibly, but when they 

 rise above 0.14 per cent they begin to be excreted by the kidneys and are 

 lost. Protoplasm contains some glucose, mostly in combination with 

 other compounds, and to that extent carbohydrates contribute to the 



CAVITY OF INTESTINE 



villus- 



capilla^^/ network 

 in villus 



artery- 



vein 



l^mph vessel 





peritoneum- 



FiG. 91. — Diagram of .section through wall of small intestine, showing two villi and their 

 enclosed blood and lymph vessels. {From Stover, "General Zoology") 



architecture of the living substance. Much carbohydrate material is 

 stored in the form of glycogen, which is made up of many molecules of 

 simple sugars combined. The main storehouse of glycogen is the hver, 

 from which it is withdrawn when current supplies are inadequate. 



Lipids enter into the construction of protoplasm, particularly at the 

 surfaces of cells, where they play an important role in determining perme- 

 ability of the cell membrane. Since the need of these materials is con- 

 tinuous, while the supply from digested food is intermittent, lipids must 

 be stored. The ones so deposited are chiefly fats. All cells store them to 

 some extent, but connective tissues between skin and muscles and among 



