154 PHYSIOLOGY 



mental 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 monosaccharide. 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 dehydra- 

 tion, 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 simultaneous dehydrolysis 

 at different parts of the molecule determine a number of chemical transforma- 

 tions, 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. 116. 



(2) DEAMINATION. 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 sug- 

 gested 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 3 



CH.NH 2 + H 2 = CH.OH + NH 3 



COOH COOH 



(alanine) 



Recent work by Neubauer tends to show that deamination is accompanied 

 in the first place by oxidation, so that the first intermediate product formed 

 is not an a oxy-acid, but an oc ketonic acid. A second atom of oxygen is 

 then taken up, and carbon dioxide is split off, with the production of the next 

 lower acid of the series. 



