[Chop. XVI SYNTHESIS OF FATS AND PROTEINS 143 



amino group (NHir). Then by chemical condensation a large number 

 of these amino acids become joined together as one large molecule of 

 protein. The materials used and the more obvious products formed may 

 be represented briefly by the accompanying diagram. 



+ XPO4 



Sugar + XNO3 > ^JJJI"'' > Proteins + H2O 



/ 

 Sugar + XNO3 + XSO4 ^ 



(oxidation-reduction) (condensation) 



In this diagram, X, as in XNO:{, represents some basic ion such as 

 potassium, calcium, or sodium. Sugar may be replaced in this process by 

 some of its derivatives, such as organic acids, but nevertheless sugar is 

 the primary substance used. Of the 20 known amino acids, only 2 con- 

 tain sulfur, and none contains phosphorus. 



Synthesis of amino acids. The svnthesis of amino acids from sugar 

 and the inorganic salts is the most unique part of the whole process. 

 This synthesis may occur in practicalh all plants ( green and non-green ) , 

 but apparentlv not in animals, at least not in adequate amounts. The 

 data of numerous experiments show that animals mav die, fail to grow, 

 fail to reproduce, or fail in other wavs merely because of the deficiency 

 of certain amino acids in their diet. These failures in development 

 begin to disappear immediately after the particular amino acids — or 

 the proteins containing them — are added to the diet. Some bacteria 

 also are unable to synthesize amino acids. 



The svnthesis of amino acids ma\^ occur in roots or any part of a 

 green plant in which the cells are not too acid and all other conditions 

 are suitable. These conditions seem to be most suitable in the phloem 

 and in young tissues. Not onlv is there some reduction of carbon during 

 amino acid synthesis, but the nitrogen and sulfur are also chemically 

 reduced. The energy necessary for the process may be obtained entirely 

 from the oxidation of sugar or of some of the derivatives of sugar in 

 the cells. The process apparently depends upon a complex set of condi- 

 tions and enzymes that occur all together only in plant cells. It may go on 

 in the absence of light and chlorophyll if sugar is present, as for instance, 

 in roots and fungi. However, it has not yet been proved that all the 

 amino acids can be made in all fungi and in the roots of all plants. 

 There probably are exceptions to the general rule, as tliere are in most 

 biological processes. 



