THE CELL AND PROTOPLASM 



fundamental significance. It now appears 

 that these entities represent a margin of 

 animate nature beyond the limits of cellu- 

 lar organization as commonly understood, 

 yet they exhibit properties of organic syn- 

 thesis and reproduction characteristic of 

 the living cell. Evidently their size rela- 

 tions alone are not definitive since they are 

 larger than some well-known microbes but 

 several times smaller than some protein 

 molecules. Their apparently complete de- 

 pendence on a living cell for their repro- 

 duction would place them among obligate 

 parasites whose nutrient requirements are 

 highly specific and, at present, beyond ex- 

 perimental duplication. Their essential 

 nature, however, may have a counterpart 

 in the chromosomal genes of the cell nu- 

 cleus or in other known protoplasmic con- 

 stituents such as the enzymes — a relation- 

 ship which would obviously carry funda- 

 mental implications. 



Some of the major advances in modern 

 researches on the cell have had to do with 

 its active protoplasmic constituents. Re- 

 sumes of some recent results are presented 

 in the three articles that follow on "En- 

 zymes" by Theorell, on "Plant Hormones" 

 by Went, and on "Vitamins" by Szent- 

 Gyorgyi. 



The common theme of these discussions 

 demonstrates the essential relations be- 

 tween these several active constituents. 

 The common role of enzymes in the forma- 

 tion or release of linkages within the car- 

 bon chain is referable initially to the 

 prosthetic groups. And for a number of 

 well-known enzymes, the vitamin nature of 

 their active groups is now established. 

 Thus, Theorell has isolated the prosthetic 

 groups of the "yellow enzymes" from the 

 protein component by means of electro- 

 phoresis and has identified this active 

 group with Vitamin B2. It is now known 

 also that Vitamin Bi, including its pyro- 

 phosphate derivative, is identical with the 

 prosthetic group of the enzyme carbox- 

 ylase, and that the antipellagric vitamin is 

 identical with the nicotinic acid amide 

 which is the essential part of the prosthetic 

 group of the dehydrogenases. 



Enzyme specificity, however, is evidently 



not due to the prosthetic group but to its 

 associated protein molecule, thus denoting 

 a relationship between activating and re- 

 acting components of the cell which may 

 come to account fundamentally for all bio- 

 logical specificity. According to "Went, 

 therefore, the more generalized activity of 

 the growth hormones can be attributed to 

 their identity with prosthetic groups. This 

 was well illustrated by the multiple effects 

 of auxin in cell elongation, bud inhibition, 

 root formation, and probably other func- 

 tions inside the plant. The initiation of 

 these growth processes, or their inhibition, 

 is traceable to the effect of diffusing or free- 

 moving auxin on the translocation of other 

 essential growth factors. But the specificity 

 of this effect inheres in the co-growth fac- 

 tors of the reacting tissues. The produc- 

 tion of these essential active groups by 

 some cells, such as those of the growing tip 

 of a coleoptile, and the transport of these 

 groups to other cells of the plant, which 

 through cellular differentiation have lost 

 this producing capacity, afford an excellent 

 illustration of the interdependence of cells 

 and the functional integration of the vari- 

 ous differentiated organs. These relation- 

 ships obviously underlie a unity in the 

 plant organism that is entirely comparable 

 with that in the animal. 



These considerations of enzymes and 

 growth hormones clearly indicate the essen- 

 tial nature of the vitamins. As re-empha- 

 sized by Szent-Gyorgyi, a vitamin is to be 

 identified with the prosthetic group of en- 

 zymes and it differs from a hormone, chiefly 

 through the accident of nomenclature, ac- 

 cording to the source of its production. 

 Thus for rats or plants, ascorbic acid is not 

 a vitamin since they themselves are able to 

 synthesize it. In the same sense, thiamin is 

 a vitamin for animals, a hormone in some 

 plants, and a vitamin for other plants, de- 

 pending only on their powers of synthesis. 

 Obviously these relationships give further 

 evidence of the fundamental unity in the 

 plant and animal kingdoms, and in terms 

 of the enzyme concept noted above, the 

 vitamins constitute an important key to a 

 better understanding of the essential na- 

 ture of protoplasm and the cell. 



