6g0 EF. C. STEWARD AND R. G. S. BIDWELL 
SECTION VII. THE IMPORTANCE OF ORGANIZATION IN RELATION TO PROTEIN IN AMINO 
ACID METABOLISM 
The purpose of this concluding section is to engender a respect for the organization of 
cells and of the plant body in the interpretation of the amino acid and nitrogenous 
metabolism of plants. 
It has been shown that amino acid and protein metabolism are not fully under- 
stood without appreciating their interactions with carbohydrate and organic acid 
metabolism as well as with respiration; these interactions go even further because 
they include a variety of other physiological functions that are linked to protein 
synthesis and to growth, as for example water and ion intake in the growing cells. 
Thus, important as individual enzyme reactions are, as in the present trend toward 
the understanding of activating enzymes which are specific for each amino acid, 
these considerations alone cannot properly yield a full understanding of the protein 
metabolism of even a single cell, which is a far too heterogenous system to be under- 
stood solely in this way. Although it is of great interest to see how much, or how little, 
synthesis may be achieved by particles in cell free preparations, this also is too 
attenuated a system to represent a whole cell, because it does not include the variety 
of organelles and loses everything gained by their interactions. Moreover, in the sub- 
division of the cells the essential features of the growing system are invariably lost. 
(An example is that isolated mitochondrial particles from carrot will incorporate 
(4C|proline readily, but they do not form hydroxyproline from it®. 
Even isolated organs of the plant body also give but a partial view. Rarely can 
isolated leaves be induced to grow, and when they are excised and fully mature 
their nitrogen metabolism leans heavily toward protein breakdown. The attached 
leaf on the contrary, controlled by the factors which maintain integration in the 
plant body, preserves its greater ability to maintain itself in nitrogen balance. 
Throughout development each organ, and in particular each leaf primordium, traces 
out a sequential series of nitrogenous changes, the full significance of which in terms 
of both total protein synthesis and the kinds of proteins synthesized are still largely 
to be understood. One may, however, anticipate from observations already made 
that the kinds of protein metabolized in the phase of active cell division may be 
different from those that are largely involved during growth which is predominantly 
by cell enlargement. 
Difficult as it is to discern these sequential changes by reference only to the relative 
amino acid composition of the total soluble pool and of the total protein hydrolyzate, 
nevertheless certain observations have been made on the shoot growing points of a 
flowering plant, Lupinus, and of a fern, Adiantum®. Briefly, and not unexpectedly, 
the metabolism of the dividing cells seems to stress the basic amino acids more than 
the metabolism of the cells that only enlarge. It is interesting to note that changes 
which are characteristic of the transition from the dividing to the vacuolated state 
in cells were detected, and it was the growing point of Adiantum that gave the first 
evidence of the presence in large amount of a then undiscovered amino. acid, which 
later turned out to be y-methyl-y-hydroxyglutamic acid®. 
During development, however, there is an apparently brief phase in which the 
young leaf is most active in exporting its synthesized materials to other parts of the 
plant body and, by analogy with the distribution of other solutes, such as salts, and 
References p. 692/693 
