72 INTRODUCTION TO GENERAL PHYSIOLOGY 



upon the relative concentration of the reagents, it will readily be 

 seen that if one of them is insoluble, it is deposited out of solution 

 as fast as it is produced, and takes no further part in the equilibrium. 

 The result of this is that more of it continues to be formed in 

 order to supply that constituent of the system necessary for 

 equilibrium. In this way, even if the equilibrium position is such 

 that a very small amount of the synthetic product suffices to 

 maintain it, yet a notable quantity may be formed even in a 

 moderate time, if it is removed from the reacting system in any 

 way. Thus it may be removed by its insolubility, or by com- 

 bining with some other substance, or again by being carried away 

 by diffusion into the blood stream. 



Similar considerations apply to the deposition of starch from 

 the sugar formed in the photo-chemical assimilation of carbon 

 in the leaf of the green plant, and to its removal to other parts 

 of the plant, stem, or root during darkness. 



But the capacity of the liver and muscles to store glycogen 

 is somewhat limited, and there is another way in which sugar not 

 needed for use at once is stored. This is as fat. The fact is well 

 known to those who grow cattle or pigs for food, although the 

 chemical changes involved are unknown, except in a general way 

 (p., p. 278). A similar change occurs in plants, as shown by fatty 

 seeds, such as linseed. 



Although we have hitherto spoken only of starch, it is familiar 

 to the reader that all our carbohydrate food is not in this form. 

 We take cane sugar, maltose, and lactose (milk sugar), which are 

 compounds of two molecules of simple hexoses. These must be 

 split by hydrolysis before they are of use to the cells. We have 

 seen that there is a maltase in the intestine, and appropriate 

 enzymes are also produced there for the other two sugars. Inver- 

 tase causes the rapid hydrolysis of cane sugar to glucose and 

 fructose ; lactase hydrolyses milk sugar to glucose and galactose. 



The final destiny of all these sugars is to be burned up to give 

 energy. This takes place chiefly in the muscles, because, as we 

 shall see later, this is the situation where most supply of energy 

 is wanted. The machinery by which this combustion is effected 

 is not completely known, but there is reason to believe that the 

 sugar does not become chemically combined with the actual 

 protoplasmic molecules. We may call to mind that, in a petrol 

 motor, the fuel does not become a constituent part of the mechanism, 

 but is burned up in such a way in relation to this mechanism that 

 its energy becomes available for use. 



Although this is what finally happens to glucose and other 

 foods used for energy purposes, there are a number of intermediate 

 products formed before they become carbon dioxide and water. 



