STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 83 



osmotic energy are, of course, of enormous importance in the life 

 of the plant. Similarly, the phenomena dependent upon inhibition 

 of water by the colloidal material and surface-tension energy are 

 of great significance to the economy of the plant and are capable of 

 many external manifestations. Nevertheless, their relation to the 

 energy transformations of food material can be considered at most 

 to be only an indirect one. 



In general, photosynthesis and respiration bear an intimate 

 association, not only on the basis of direct observation, but because 

 oxidation and reduction actions in the living organism are intimately 

 connected and apparently dependent upon the same or very closely 

 allied agents. The two processes of photosynthesis and respiration, 

 proceeding in opposite directions, may be related either on the basis 

 that the energy released in respiration actually aids or is essential 

 for one of the steps of the reduction process, or the relation may be 

 based upon the action of an enzyme which functions in both reactions. 

 Under no circumstances, of course, can all the energy for the reduc- 

 tion of the carbon-dioxid come from the oxidation of the carbohy- 

 drates. An extraneous source of energy is essential. 



Thermodynamically, the contribution which respiration could 

 make in the photosynthetic process would naturally be relatively 

 small, so that this amount of energy could at best serve only as a 

 partial source. 



For a molecular relationship recourse must be taken to our modern 

 conceptions of the nature of carbohydrate breakdown or glycosis. 

 According to this view, as has been elaborated in an earher publi- 

 cation,^ precursory to oxidation there must take place a cleavage 

 or dissociation of the molecule. This action has as its result the 

 formation of a very large number of enormously reactive substances. 

 These pieces, the products of dissociation, either rearrange, react 

 with each other, or react with some other substance present in the 

 medium. Now, these pieces, on account of their enormous reactivity, 

 can not be isolated. It is, in fact, only from the products formed 

 by their reaction or rearrangement that they can be known. They 

 are, nevertheless, of foremost importance in the chemical reactions 

 involved in carbohydrate breakdown. A molecular relation of 

 photosynthesis and respiration would depend upon the activity 

 of these intermediary products of sugar catabolism. Just as these 

 products serve as the building-blocks from which other compounds 

 of higher potential energy may be formed in the cell, it is conceiv- 

 able that they also may react with carbon dioxid or with some of 

 the primary products of the photochemical breakdown of carbon 

 dioxid.^ 



» Spoehr, H. A. Carnegie Inst. Wash. Pub. No. 287, 5-24 (1919). 

 "Siegfried, M., and S. Howwjanz. Zeit. f, Physiol. Chem, 59, 376 (1909). 



