146 NUTRITION AND METABOLISM. 



The process goes to a certain equilibrium and stops. If ester is mixed 

 with water, it gives acid plus alcohol, until the same equilibrium is 

 reached. If acid and alcohol are added to a system in equilibrium, more 

 ester will be formed. If ester is added, more alcohol and acetic acid will 

 be formed. The same is true with enzymes, at least with some enzymes. 

 Maltase will decompose maltose into two molecules of dextrose. In a 

 concentrated solution of dextrose, however, maltase will form maltose, or 

 a similar sugar, isomaltose. Lipase is able to produce fat from glycerin 

 and fatty acids. A solution of albumose with trypsin or pepsin gives a 

 precipitate of a body which is more complex than albumose and which 

 gives the protein reactions. It is believed by many physiologists that 

 pepsin and rennet are the same body. Under certain conditions, it has 

 a dissolving power, under other conditions it has the power to coagulate. 

 The reversibility of enzymic action has given rise to much speculation 

 about assimilation and growth. It seems reasonable to suppose that the 

 cell forms its protoplasm from amino-acids by the reversed action of 

 proteolytic enzymes. In the same way, cellulose may be formed from 

 dextrose, fat from glycerin and fatty acids. Nearly all phases of growth 

 can be accounted for in this way. This is nothing but theoretical specu- 

 lation, and the only fact to support it is the reversibility of certain enzymes. 

 The conditions under which chemical reactions take place inside of the 

 cell are very largely unknown. There are so many processes going on at 

 the same time that it is absolutely impossible at the present time to obtain 

 a perfect knowledge of all these reactions. Our experience is limited to 

 the very simplest changes, and even these are not perfectly understood 

 because the detailed conditions under which the changes take place are 

 unknown. Information obtained by experiments in vitro is practically 

 all that is available, and these experiments are conducted under very 

 simple conditions, where complications as they might occur in the cell, 

 are excluded. Thus, our knowledge of the processes of growth is largely 

 based upon analogy and speculation. 



