FOOD DIGESTION AND RESPIRATION 75 



Proteins 



We have learned that these compounds are the form in which 

 our nitrogen supply is provided. They also serve as a source of 

 energy. 



In order that the tissue cells may be able to make use of these 

 substances, they must first be split up into their constituent amino- 

 acids. The first step is taken in the stomach, where the gastric 

 juice contains an enzyme, pepsin, which acts only in a fairly acid 

 medium. The acid present in the stomach is hydrochloric acid, 

 which is secreted by the glands in the walls of the stomach, but not 

 by the same cells that produce the pepsin. Pepsin does not, how- 

 ever, carry the hydrolysis as far as amino-acids, but only to certain 

 polypeptides, called peptones, which still consist of several amino- 

 acid molecules. Peptones are not absorbed in the stomach, but are 

 passed on to the small intestine where the " tryjDsin " of the pan- 

 creatic juice converts them almost entirely "into amino-acids, 

 although some of the dipeptides, formed near the end, are difficult 

 to hydrolyse. The operation is completed by another enzyme, 

 "erepsin," which is produced by the cells lining the intestine. 

 Trypsin acts in a faintly alkaline solution, resulting from the 

 mixture of the acid products from the stomach with the alkaline 

 pancreatic juice (E., p. 193). 



Although we can only follow the series of changes in the 

 vertebrates, there are indications that the process is common to all 

 animals, even including the amoeba. There is a preliminary action 

 of an enzyme acting in acid solution, followed by another in faintly 

 alkaline solution. Since trypsin can act upon the original protein, 

 it is not quite clear why there is a preliminary action by pepsin. 

 It is doubtless a means of hastening the process, because the 

 products of gastric digestion are more rapidly hydrolysed by 

 trypsin than if the process had not already been partially 

 performed. 



The amino-acids thus formed are absorbed by the blood 

 vessels of the small intestine, and carried to the liver. The greater 

 part are subjected there to a chemical change, which will be 

 described presently. The smaller part passes on to the tissue 

 cells, which select the particular amino-acids which they require 

 for the repair of their structure, or for growing new structures. 



We have already seen that proteins can be burned and used 

 as sources of energy. The process, however, as it takes place 

 in the living organism, is incomplete. The NH 2 groups are not 

 oxidised, and are lost as urea. How are these groups split off from 

 the amino-acids? So far as we know, what takes place is as 

 follows. There is an enzyme or enzymes in the liver which 



