CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 171 



reaction will determine a preponderance of this reaction over all other 

 possible ones. A fundamental question, therefore, in physiology must 

 relate to the manner in which the cell is able to influence the velocity 

 of some specific reaction. 



In spite of the enormous diversity of chemical reactions occurring 

 in the body, they may be divided into a relatively small number of 

 types. Nearly all the reactions are reversible. The chief types of 

 chemical change are as follows : 



(1) HYDROLYSIS. In most cases this involves the taking up of 

 water and a decomposition into smaller molecules. Thus the proteins 

 are broken down in the intestine into their constituent amino-acids. 

 The disaccharides, such as maltose or lactose, take up one molecule 

 of water and give rise to two molecules of rnonosaccharide. The fats 

 take up three molecules of water with the formation of fatty acid 

 and glycerin. Hippuric acid is broken down into benzoic acid and 

 glycine. The reverse change, that of dehydration, is also effected, 

 apparently with equal facility, by the living cell, the hexoses losing- 

 water and being converted into a complex starch or glycogen molecule, 

 while the amino-acids are built up first into polypeptides, and these 

 again into the complex proteins of the cell. Besides the reactions in 

 which there is a difference in the amount of free water on the two 

 sides of the equation, it seems probable that hydrolysis and simul- 

 taneous dehydrolysis at different parts of the molecule determine a 

 number of chemical transformations, which at first sight seem to 

 involve a simple splitting of the molecule. An example of such a 

 process is afforded by the conversion of glucose into lactic acid described 

 on p. 128. 



(-2) DE AMI NATION. This process involves the splitting off of an NH 2 

 group from an amino-acid as ammonia, and its replacement by H or OH. 

 Many tissues of the body appear to have this power. In most cases 

 the nature of the change in the remaining fatty moiety of the molecule 

 has not yet been ascertained. If, for instance, to a mass of liver cells 

 some amino-acid, such as glycine, alanine, or leucine, be added, ammonia 

 is set free in proportion to the amount of amino-acid which was added. 

 This ammonia is therefore assumed to be derived from the amino-acid, 

 and it has been suggested that here also the process of splitting off 

 ammonia is a hydrolytic one and that the NH 2 group is replaced by 

 OH. Thus- 



CH 3 



CH.NH 2 + H0 = CH.OH NH 



COOH COOH 



(alanine) 



