680 F. C. STEWARD AND R. G. S. BIDWELL 
endogenous carbohydrate and nitrogen drawn from the nitrogen-rich reserves in the 
the cells. The synthesis of protein, the use of sugar and the disappearance of soluble 
nitrogen were all affected favi passu by several variables. The conclusion was that 
the carbon skeleton from the erstwhile storage nitrogen compounds contributed via 
the Krebs’ cycle to the extra respiration which was so maintained. Such now nitrogen- 
free residues, which enter the Krebs’ cycle as keto acids, spare the use of sugars via 
glycolysis, and enter the cycle under such low concentrations of carbon dioxide and 
such efficient conditions of carbon dioxide removal that the carboxylation of pyruvate 
could well be limiting (STEWARD AND STREET®®, fc p. 482-486). 
RACUSSEN AND ARONOFF*®?, working with isolated soya bean leaves, produced 
somewhat similar evidence, which in this case pertained to the source of the carbon 
which entered into the protein. They showed that 1CO, entered into the protein in 
TABLE V 
RELATIVE INCREASES IN FRESH WEIGHT, TOTAL PROTEIN, TOTAL RESPIRATION, AND 
TOTAL RADIOACTIVITY OF THE PROTEIN DUE TO GROWTH; 12.€. RATIO OF THE QUANTITY 
IN THE FAST- TO THAT IN THE SLOW-GROWING CULTURES 
From STEWARD AND BIDWELL?*?. 



Ratio 
Fresh weight By 
Total protein 30 
Respiration 5 
M4C-Activity in amino acids of the protein 
Glutamic acid 
Eye) 
Aspartic acid 4°60 - 5° I average 
Threonine 5° 2 
Proline © 3 Omlie-e: rerag 
Hydroxyproline 2 Onifi SIE ee 

the light by the synthesis of new protein, even though the total protein in the leaf 
was on the decline. From data of this kind one may conclude that there is a normal 
process of breakdown and re-synthesis of protein in the leaf, and that the immediate 
products of photosynthesis supply in part the carbon used in that re-synthesis. (This 
idea has also been mentioned in the earlier consideration of the data of Table IV on 
corn seedlings. ) 
But the idea of a cyclical system of protein synthesis and breakdown as a normal 
concomitant of metabolism and respiration comes most readily from the consideration 
of cells which are actively growing, or which are about to embark on active growth 
and development. 
These concepts were necessary to explain the data which were obtained by the 
use of carrot tissue explants induced to grow under the stimulus of coconut milk 
under tissue culture conditions’! 4. It should be made clear, however, that in the 
first instance these experiments were not designed especially to this end, but their 
purpose was to study the fate of labeled sugars, glutamine and y-AB in the actively 
growing cells, with a view to comparing the metabolism of amides in growing and 
References p. 692/693 
