Cellular oxidations and the synthesis of amino-acids and amides in plants 



of the tissues shows a fairly steady rate of incorporation of 15 N, which is much greater 

 than can be accounted for by the net synthesis of protein in the roots. It seems prob- 

 able, therefore, that the proteins of the cells are maintained in dynamic equilibrium 

 with soluble nitrogenous constituents by means of exchange or other reactions. 



Much other work, reviewed by Chibnall (1939), by Steward and Street (1947) 

 and by Virtanen and Rautenen (1952) converges with that discussed above in 

 showing that the amides, asparagine and glutamine, may be readily formed from 

 ammonia in plant cells. The more active role of glutamine in protein metabolism is 

 indicated by earlier experiments of Yemm (1937, 1949, 1950), Steward and Street 

 (1946), Rautenen (1948) with higher plants, and by those of Roine (1947) and 

 Virtanen, Csarky and Rautenen (1949) with yeast. Vickery, Pucher, Schoenheimer 

 and Rittenberg (1940) and Mac Vicar and Burris (1948), using isotopic nitrogen, 

 have shown that glutamic acid and glutamine are highly active in the metabolism of 

 proteins in plants. 



THE BREAKDOWN OF CARBOHYDRATES IN RELATION TO RESPIRATION 

 AND THE SYNTHESIS OF AMINO-ACIDS 



As already indicated, a rapid depletion of carbohydrate accompanies the high rate 

 of respiration during the assimilation of nitrogen by the cells. In most of the experi- 

 ments, analytical data were obtained from which it is possible to estimate the losses 

 of readily available carbohydrates. As no external supplies of carbohydrate were 

 provided, these losses can be related to respiration and the synthesis of nitrogenous 

 constituents. For this purpose balance sheets for carbon have been drawn up, in 

 which the production of respiratory CO a and the synthesis of amino-acids are 

 balanced against the breakdown of carbohydrates. An example of the data from a 

 typical experiment with barley roots is given in Table I. It is evident that the break- 

 down of carbohydrates, mainly hexoses and sucrose in these tissues, is adequate to 



Table I 



Carbon balance sheet for barley roots 



Roots excised from 10-day-old seedlings and allowed to 

 assimilate for 18 hours in 0*0025 M NH 4 H 2 P0 4 at 22 -5° G. 



Products mg. c/100 roots 



(1) Respiratory C0 2 36 -6 



(2) Synthesis of Glutamine 159 



(3) Synthesis of Asparagine 2-8 



Total (1), (2), (3) 55-3 



Loss 



Carbohydrates 61 7 



meet the needs for both the synthesis of amides and the production of C0 2 . The losses 

 of carbohydrates during the eighteen hours of assimilation are in fact slightly greater 



57 



