FREE NITROGEN COMPOUNDS IN PLANTS 077 
citric acid in the cells, one which is concerned with the simple storage of citric acid 
and the other associated with the rapid metabolism of citric acid as in respiration, 
There is also to be found in work from the laboratory of Dr. H. PoRTER a similar 
indication in the carbohydrate metabolism of tobacco leaves that there are geo- 
graphical areas or pools of sucrose which are distinct??. 
Therefore, in both the relation of nitrogen compounds to protein synthesis and of 
carbohydrates and organic acids to respiration, we are led to the view that different 
phases or compartments in the cells exist, and in these the same substances may be 
associated with different degrees and kinds of metabolic activity. 
A possible method whereby amino acids could be formed in the soluble phase of 
a cell from newly incorporated carbon and then passed directly to the protein syn- 
thesizing system without mixing with the soluble amino acids has been indicated by 
HANFORD AND Davies!®. An enzyme from pea hypocotyls forms phosphoserine via 
phosphohydroxypyruvate from phosphoglyceric acid, a compound which plays a 
direct part in both photosynthesis and respiration. The phosphoserine so formed 
could easily be incorporated into protein without ever mixing its carbon with the 
free serine which occurs in the cell. 
In the work of Roperts et al.8> on micro-organisms there is also evidence that all 
the free amino acids in the cell are not equally accessible. For example, certain 
acids (notably threonine and lysine) when supplied exogenously to Escherichia coli 
enter completely different sequences of reactions than do the threonine and the 
lysine which are already within the cells. 
In later work by CowlE AND WALTON!? on Torulopsis, reference is made to distinc- 
tive pools of amino acids which are here regarded as being segregated by binding on 
more complex compounds. In the view of Cowlk e¢ al., it is these latter amino acids 
that are more immediately accessible to protein synthesis than those which exist freely. 
(This idea has been amplified in another contribution to this Symposium, see p. 633.) 
Thus, in several examples which have been cited, one has to consider the location 
of a given metabolite in the cell in order to understand its metabolic role. However, 
these considerations obviously do not apply to experiments which are carried out 
with cell free preparations which lack the organization of the intact cell. For example, 
RaBsON AND NoveELLt*! used particulate cell free preparations from Zea mays to 
show that these tend to behave like animal systems, for they incorporate amino 
acids into protein in a manner essentially comparable to the mammalian cell free 
systems which have been studied. This sort of work, including the results of WEBSTER” 
with cell free preparations, contrasts with the work done on intact cells of bigher 
plants, for the latter have conspicuous vacuoles and extensive soluble fractions, and 
to these systems additional and somewhat different considerations must apply. 
Stress has been laid upon the ability of exogenously supplied sugar to furnish 
carbon directly to protein in the synthesis of protein in growing carrot tissue cultures. 
Some recent and hitherto unpublished work by HELLEBUST AND BIDWELL with corn 
seedlings and corn root tips also bears on this question. The soluble compounds in 
the leaves of corn seedlings became strongly radioactive during 2h of photosyn- 
thesis in 4CO,, and the protein amino acids also acquired “C. This gave a high level 
of activity in the sugars and in the soluble nitrogen compounds. Subsequently the 
plants were allowed to continue photosynthesis for 64 h on a 16-h day and to meta- 
bolize, but only in the presence of non-radioactive carbon dioxide. During this 
References p. 692/693 
