12 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



to ascribe more directly the final forces which induce changes in 

 living things to the same causes which activate the inorganic world. 

 This is the only hope for a mechanistic conception of life processes. 



The question naturally arises whether in metabolic processes 

 which involve the liberation of energy a continual decomposition of 

 protein serves the plant (1) to contribute to the total energy release 

 of decomposition by material other than carbohydrates and fats, 

 or (2) to provide material essential to the proper functioning of 

 enzymes and catalysists, or (3) whether such a continued decompo- 

 sition is an unavoidable accompaniment of metabolic energesis. 

 Concerning the first possibility, it is evident that the amount of 

 energy obtained by the decomposition of proteins would be very 

 small, for this is usually a cleavage into amino-acids, and, further- 

 more, the leaf is capable of re-forming these amino-acids into proteins 

 without the direct utilization of an extraneous source of energy, i. e., 

 through the chemo-sjmthetic energy derived from the oxidation or 

 the breakdown of sugars. ^ The total gain in energy for the plant 

 from the decomposition of proteins would therefore be nil; and unless 

 the decomposition of proteins yields chemical energy of a certain 

 form or at a particular rate, it is difficult to see what advantage 

 would result or what the mode would be of deriving energy by a 

 continual decomposition and subsequent resynthesizing of protein 

 material. Of the possible different forms of chemical energy referred 

 to we know at present practically nothing, so that this is quite 

 beyond the ken of physical-chemical reasoning. That the rate 

 of energy release from protein decomposition differs fundamentally 

 from that of carbohydrate-oxidation can not be assumed; judging, 

 however, from the rate of accumulation of protein-splitting products, 

 it would appear that the carbohydrate breakdown proceeds con- 

 siderably faster, mol for mol. 



As to the second possibility of the function of protein decom- 

 position — the providing of material essential to the functioning of 

 enzymes — the experiments hereinafter described may contribute some 

 information. This aspect is of interest on account of the stimulating 

 action which amino-acids exert on certain enzymes, as well as on 

 account of the fact that amino-acids, being amphoteric electrolytes, 

 i. e., capable of uniting with both acids and bases, have the power of 

 maintaining the hydrogen-ion concentration within definite limits. 



That amino-acids accumulate in seedlings left in the dark has 

 been known since the time of Hartig and Boussingault. This 



' It is quite well established that leaves are capable of synthesizing proteins from amino-acids 

 and even nitrates in the dark in the presence of an abundant carbohydrate supply. Protein 

 decomposition in the presence or absence of oxygen differs primarily in the proportion of 

 the various amino-acids, principally asparagine, tyrosine, and leucine. Suzuki, Bot. Centbl., 

 75, 289 (1898); Zalbski. W., Ber. d. deutsch. bot. Ges., 15, 536-542 (1897); Sapoznikow, W.. 

 Bot. Centbl., 63 (1893); Palladin, W., Ber. d. deutsch. bot. Ges., 6, 205-212, 296-304 (1888). 



