36 GROWTH OF THE PLANT 



is made from carbohydrates — probably dextrose. Since 

 oils contain less oxygen than dextrose or other carbohydrates, 

 the process must be one of reduction — possibly a result of 

 intermolecular respiration. 



30. Manufacture of Protein. — By far the most important 

 chemical work done by the plant is the making of proteins, 

 the most complex compounds known. Just how these 

 compounds are manufactured from raw materials is not 

 thoroughly understood. Starting with carbohydrates — 

 probably dextrose — nitrates, sulphates, and sometimes 

 phosphates, the cells of plants build up a compound which 

 in no way resembles its constituents. The nitrogen and 

 sulphur are no longer in the oxidized condition, in fact, 

 just the reverse. In other words, the nitrates and sulphates 

 are reduced, possibly to ammonia and free sulphur. Then 

 they are combined with carbon, hydrogen, and oxygen from 

 carbohydrates to gradually build up the complex protein. 

 One of the first nitrogen compounds to be formed seems to 

 be an amino-acid. It has been suggested that nitrates and 

 sulphates are reduced by carbohydrates, as a result of 

 intermolecular respiration, whereby the carbohydrates are 

 oxidized in part to oxalic acid. This acid unites with the 

 bases of the reduced nitrates or sulphates, namely, calcium, 

 potassium, or sodium. Later the soluble oxalates of potas- 

 sium and sodium are changed to insoluble calcium oxalate. 

 (Fig. 3, d). This excess of calcium may possibly be furnished 

 by calcium phosphate, since the phosphoric acid radicle is 

 used in making some proteins. The remainder of the oxi- 

 dized carbohydrates, in the form of some other acids, may 

 unite with the ammonia to form amino-acids. A number 

 of these amino-acids uniting together with sulphur and 

 sometimes phosphorus gradually construct a protein. 



Proteins are made to a great extent in leaves, but their 

 formation is not restricted to the leaves. Other parts of the 

 plant make these compounds from the raw materials. Light 

 is not directly essential for their formation. The necessary 

 energy is derived from oxidation of carbohydrates and is a 

 chemosynthesis as in the case of oils. The presence of 

 carbohydrates is, of course, required since they are the source 



