10 THE CARBOHYDRATE ECONOMY OF CACTI. 



For the problems of carbohydrate metabolism, and especially for glycoly- 

 sis, some recent investigations on the dissociation and molecular rearrange- 

 ments of the sugars under various conditions have been most suggestive. 

 !No application of these newer conceptions has been made in the interpreta- 

 tion of the metabolic processes in the plant. The more important principles, 

 therefore, are given here, as it seems to the writer that these ideas are among 

 the most valuable which chemistry has given to physiology. For the more 

 detailed exposition and proof, reference must be made naturally to the 

 rather voluminous original works. A thorough knowledge of the chemistry 

 of the substances concerned in these processes is, of course, essential to an 

 understanding of the physiology. The attention can be confined to the 

 reactions in aqueous solution, as it is in this state that the many trans- 

 formations occur in the living organism. Pure solutions of the mono- 

 saccharides in vitro are comparatively stable. In living matter they are 

 very unstable and capable of the most multifarious rearrangements and 

 disintegrations. Some insight can be gained into the causes of this insta- 

 bility of sugars in living matter from a consideration of the manner in 

 which these substances dissociate under the influence of various reagents 

 and the products which result from such reactions. 



Organic compounds containing one or more hydroxyl groups, such as 

 alcohol, glycerine, or glucose, exhibit some of the properties of exceedingly 

 weak acids. They form salts with metals and influence the electrical con- 

 ductivity of weak electrolytes as other weak acids do. The electrolytic 

 dissociation constants of several of these substances have been measured by 

 electro-chemical methods. Thus Michaelis and Rona 1 give the following 

 constants at 18 : 



Ethyl alcohol < 10-" 



Glycerine = 7 X 10-" 



Mannit = 3.4 X 10-" 



Dulcit = 3.5 X 10-" 



Galactose = 5.2 X 10-" 



Fructose = 9.0 X 10-" 



Mannose =1.09X10-" 



Glucose = 6.6 X 10~" 



Maltose =18.0X10-" 



Saccharose = 2.4 X 10-" 



Lactose = 6.0 X 10-" 



Raffinose = 1.8 X 10~" 



That sugars are very weak acids can be seen by comparison with the 

 dissociation constants of the following common organic acids : 



K for 18' 



Acetic acid 1.8 X 10-* 



Butyric acid 1.5 X 10-" 



Lactic acid 1.4 X 10~ 4 



Carbonic acid (HiHCO, 1 ) . . .3.04 X 10~ T 



Tartaric acid 1 X 10- 8 



Phenol 5.8 X 10- n 



Salts in which a metal replaces the hydrogen in the hydroxyl group of 

 the alcohols and sugars have been known for some time. Thus sodium 

 ethylate (C 2 H 5 O!Nra) can be readily obtained by the action of metallic 

 sodium on absolute alcohol. Glycerine also forms similar compounds, the 

 glycerates, with a large number of metals. Here there are three replaceable 



1 MicHAELis, Li., and P. RONA. Die Dissociationskonstanten einiger sehr schwacher 

 saeuren, insbesondere der Kohlenhydrate gemessen auf elecktrometrischem 

 wege. Biochem. Zeit., 49, 232-248, 1913. 



