360 Mineral Nutrition of Plants 



sented as having a multiple source, namely, breakdown of hexose, of 

 aliphatic acids, and of amino acids. 



The consensus of opinion hitherto has been that the protoplasmic 

 proteins remained unchanged after formation, that is, only the storage 

 proteins were labile. Recent studies by Vickery, Pucher, Schoenheimer, 

 and Rittenhouse (56) disclosed that isotopic nitrogen supplied to the 



fProU-'i 



Amide 4. 

 v Z 



,<7 



inino 

 Cud 



v NH y 



__lE= 



\ 



Amino 

 acid 



ED 

 T 



Carbon 

 "^ residues 



Undetermined, 

 carbohydrate 



_^ Oraan'tc acids'^- 



3 Carbon 

 compounds 



CO, 



I 



, V 



I Sucrose 



It 

 Hexose 



• 



, 2 Carbon 

 Compounds 



+ CO* 



1 



Figure i. Diagram from Gregory and Sen (14) illustrating the probable 

 course of protein and carbohydrate metabolism in plants. 



tobacco plant as ammonium salt appeared in its proteins within four 

 hours. Calvin and Benson (5) could detect the presence of radiocarbon 

 in the amino acids (alanine and serine) of an alga after a period of 

 thirty seconds of photosythesis with C*O l> . The extreme rapidity of 

 these reactions is a striking demonstration of the manifold chemical 

 reactions taking place in the hitherto presumably static protoplasmic 

 proteins of the cell. 



This is one reason that the amino acid-protein cycle depicted in the 

 above scheme seems to the writer to be the weakest part of the picture. 

 Alternate condensation and liberation of amino acids would seem a 

 rather futile procedure; especially so since there is some evidence that 

 protein may participate in the formation of respiratory carbon dioxide. 

 The writer is of the opinion that many of the metabolites of the cell, 

 including other proteins and enzymes, are formed and liberated 

 through cleavage from the permanent or protoplasmic proteins. Only 

 the debris of these reactions would undergo complete oxidation to 



