i62 ELEMENTS OF BIOLOGY 



slightly alkaline saliva, acts upon starches. The reaction is a com- 

 plicated hydrolysis (p. 12); its details do not concern us here. The 

 process may be summarized as the conversion o£ a fairly large starch 

 molecule into a simple sugar : 



CgHioOo (as an aggregate consisting o£ n molecules) + H2O 

 (n molecules) :^ n CgHi206 (maltose). 



The process is only started in the mouth; it is considered certain 

 that it is continued in the stomach. 



Digestion in the Stomach. The digestion o£ proteins begins 

 in the stomach. The fluid in the stomach, known as gastric juice, 

 contains two main ingredients, the proteolytic enzyme pepsin, and 

 0,5 per cent HCl. The action o£ these substances is largely to initiate 

 the conversion o£ the proteins; the processes are completed by the 

 juices in the intestine. In the stomach the complex, large protein 

 molecules, or at least some o£ them, are converted into simpler and 

 more soluble substances. Hydrolyses (p. 12) are involved and the 

 reaction occurs in several steps. It was shown in an earlier chapter 

 that protein molecules are composed of combinations of amino- 

 acids (p. 14). In the ordinary diet of Man a very considerable va- 

 riety of proteins is taken into the system but by the action of 

 the proteolytic, or protein-digesting enzymes they may all be re- 

 duced to some simpler combinations of their essential amino- 

 acids. In the stomach the proteins are first changed to proteoses, 

 a type that may be thought of as the first intermediate between 

 proteins and amino-acids. Proteoses do not diffuse readily through 

 animal membranes and hence are sparingly absorbed by the walls 

 of the stomach. Proteoses are next split up into molecules that 

 are still smaller, although not yet single amino-acids, known as 

 PEPTONES. Peptones diffuse readily through animal membranes and 

 consequently are absorbed by the walls of the digestive system. 

 When the diet contains milk the digestion of its proteins is pre- 

 ceded by the curdling of the milk due to the action of another 

 enzyme, rennin. Calcium must be present, else no curd forms. The 



