STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 19 



are more effective in this respect than any of the other substances 

 tried, i. e., sodium acetate, acetamide, glycerine, oleine, and dextrose. 



It must be reaUzed, of course, that the evidence regarding the 

 behavior of amino-acids toward enzymes, here introduced, does not 

 serve as a direct explanation of the influence of amino-acids on the 

 rate of respiratory activity nor of the specific dynamic action of 

 proteins. However, in the present very incomplete state of our 

 knowledge of this subject, the information of these relations may 

 serve as a valuable guide in developing further our conceptions of 

 the respiratory processes. So, while there is as yet no simple expla- 

 nation of the role of amino-acids in respiration, it seems also at 

 present exceedingly doubtful whether such a process is amenable 

 to simple treatment. 



We are obliged to recognize more and more that not only are the 

 various activities of a living organism intimately interrelated, 

 but that the proper functioning of any one process depends upon 

 the coordination of various enzymatic activities. Thus in the oxy- 

 biosis of carbohydrates there are naturally a great many steps be- 

 tween the stored starch and the formation of carbon dioxid and 

 water. Plant physiologists have very generally fallen into the 

 habit of overemphasizing and reasoning as to the modus vivendi 

 of reactions in living things from evidence of the substances found in 

 such organisms. It must be borne in mind that, applying the prin- 

 ciples of the kinetics of step reactions, the rate of the whole reaction 

 is determined by the rate of the step progressing with the lowest 

 speed, and the more rapidly a total reaction progresses the fewer 

 intermediate products are there possible to detect. Thus, often 

 the most important and reactive products are not detectable by our 

 present chemical methods. 



Fundamentally a better understanding of respiration depends 

 upon a knowledge of the nature of enzyme activity, and this, of 

 course, brings us to the very frontier of scientific thought. While 

 the activity or mode of action of enzymes has its analogy in more or 

 less well-defined behavior of catalysis, the composition of the enzymes 

 themselves is an open question. Briefly, there appear to be two 

 schools — the one conceiving of an enzyme as a definite complex 

 substance with catalytic properties, the other regarding enzymes 

 not as chemical individuals but rather as mixtures or complex 

 systems. Although the former conception has received very little 

 advancement through the efforts of synthetic chemistry, it has its 

 strong adherents, as, for instance, Willstaetter,^ who hopes by means 

 of preparation work to obtain constantly purer products until their 

 chemical nature can be established. On the other hand, following 

 the idea of enzymes as complex mixtures, some very suggestive 



» WiLLSTAETTEB, R. Zeitsch. f. atigew. chem., 33, 209 (1920). 



