GENERAL ZOOLOGY 



ALL CELLS 

 Fig. 3.22. Transport, storage, and use of carbohydrates. 



submucosa; the mechanism of their movement remains to be discovered. 

 About a third of the dietary Hpid is completely digested to form fatty acids 

 and glycerol. Glycerol is absorbed by difTusion and enters the blood stream. 

 The fatty acids form a complex with certain bile salts. This complex is 

 soluble and passes by diffusion into the epithelial cells. There it is freed from 

 the bile salt and converted by synthetic metabolism into a phospholipid; this 

 lipid then passes into the blood stream. Absorption of the fat soluble vita- 

 mins, A, D, and K, is also dependent on the presence of bile salts. In the 

 absence of bile, deficiency of these vitamins may occur (p. 31) 



The large intestine is the chief site of water absorption. Pyridoxin, folic 

 acid, and vitamin B,^ (p- 34), which are synthesized by the bacteria of the 

 large intestine, are also absorbed there. 



Transport and Storage of Food 



Animals eat at intervals, as food is available. The products of the inter- 

 mittent meals are handled in such a way as to provide for the continuous 

 metabolic requirements of the cells of the body. This involves the main- 

 tenance of suitable levels of nutrients in the blood at all times. In the 

 case of carbohydrates and lipids, the organism stores some of the abundant 

 supplies available after eating; later these can be returned to the blood to 

 satisfy metabolic demands. It is relatively easy to determine the general 

 pattern of movement, storage, and reconversion of the absorbed carbohydrate 

 and lipid. In the case of the amino acids, it was not until it became 

 possible to tag them with radioactive isotopes of carbon and nitrogen that 

 we became aware of their almost ceaseless journeys in the body. 



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