METABOLISM 55 



only the end-product of a long series of changes. 

 The carbohydrate food in many plants is more often 

 in the form of a dilute solution of sugar, which, of 

 course, is much less easily demonstrated than the 

 solid starch. 



Production of Fats and Proteins. We know of no 

 other food than the starch that is synthesized from 

 such simple chemical compounds, 1 and it has been 

 shown that the fats and proteins are produced from 

 the starches as a foundation. The spontaneous pro- 

 duction of fatty oils in seeds containing starch has 

 been directly observed, and, since the fats contain 

 no elements not also found in the carbohydrates, 

 such transformation involves no more than a re- 

 arrangement of these elements in the molecule. It is 

 known that water and CO 2 are produced as the result 

 of such a change, although we have still much to 

 learn of the intermediate steps in the process. 



The explanation of the origin of the proteins is much 

 more difficult, for proteins possess, in addition to the 

 carbon, hydrogen, and oxygen of starch, a high percent- 

 age of nitrogen, as well as sulphur, and often phos- 

 phorus. These latter elements are obtained from the 

 soil in the form of nitrates (or nitrites), sulphates, and 

 phosphates, and are absorbed through the roots of the 

 plant in solution. The nitrates (sodium or potassium) 

 probably enter into union with the carbohydrate radicle 

 to produce some simple amino-acid such as asparagin 

 (C^HgNoOs). By a succession of syntheses involving 



1 Other organisms recently discovered can apparently utilize carbon 

 monoxide (coal gas), while still others may use methane (marsh gas) as 

 their only source of carbon. 



