82 GENERAL CONCEPTS 



optimum, ranging from a very acid one for pepsin to an alkaline one 

 for trypsin. The molecules of protein, fat and carbohydrate originally 

 present in the food are too large to pass through the wall of the digestive 

 tract; the digestive process converts these to amino acids, tatty acids, 

 glycerol and single sugars, which are able to be absorbed through the 

 wall of the digestive tract into the body. 



Herbivorous animals typically have a pouch in which the cellulose- 

 rich food is subjected to bacterial digestion, for the animal itself has no 

 enzyme to digest the cellulose walls of the plant cells. In the rabbit and 

 horse this pouch is the caecum, located at the junction of the small and 

 large intestine. The products of bacterial digestion are absorbed into 

 the blood stream. The cow and other ruminants have a large, complex 

 rumen between the esophagus and stomach in which the plants are 

 digested by bacteria and protozoa which were eaten along with the 

 plants. The bacteria convert cellulose to acetic acid, and a large part of 

 the cow's calories are absorbed as acetic acid directly from the rumen. 

 The bacteria further contribute to the cow's economy by synthesizing 

 vitamins and amino acids from the material ingested. 



The products of the digestive process are taken up into the proto- 

 plasm of the body. In those animals with intracellular digestion occvn- 

 ring within food vacuoles, the products of digestion are simply trans- 

 ported across the membrane of the food vacuole and are then available 

 for the many possible paths in cell metabolism. In animals with ex- 

 tracellular digestion, the products are generally taken through the cells 

 lining the digestive tract and on into the circulatory system for distribu- 

 tion to the cells of the body. In mammals, the amino acids and simple 

 sugars are absorbed in part by energy-requiring processes and in part by 

 simple diffusion. The cells lining the intestine comprise a semiperme- 

 able membrane which permits the passage of amino acids and simple 

 sugars but prevents the passage of intact proteins and complex sugars. 

 In many animals, the lining of the intestine is thrown into folds, which 

 increase the area available for absorption. Amino acids and sugars are 

 taken up by the blood stream for transport; in contrast, the products of 

 fat digestion in mammals cross the intestinal mucosa, are reformed into 

 fats and enter the lymph vessels (p. 556) to be carried to other parts of 

 the body. 



A discussion of the eventual fate of the absorbed food would involve 

 all the reactions of cell metabolism, some of which were discussed in 

 Chapter 4. The amino acids serve as raw materials for the synthesis of 

 cell proteins. Amino acids may undergo deamination (removal of the 

 amino group) and their carbon chains are then used to synthesize gly- 

 cogen and other carbohydrates, to synthesize fatty acids, or they are 

 metabolized in the Krebs citric acid cycle to yield energy. The amino 

 group is combined with carbon dioxide by yet another complex series 

 of enzymatic reactions to form urea. This waste product is synthesized 

 largely in the liver, carried in the blood to the kidneys, and excreted 

 in the urine. 



The sugars absorbed are converted into glycogen for storage pri- 



