EXPERIENTIA VOL. Vni/12. 1952 - p. 445 



VERLAG BIRKHAUSER, BASEL/SCHWEIZ 



The Path of Carbon in Photosynthesis' 



(XX. The Steady State) 

 By M. Calvin and P. Massini^ Berkeley, Cal. 



Photosynthesis, the process by which green plants 

 are able to capture electromagnetic energy in the form 

 of sunlight and transform it into stored chemical energy 

 in the form of a wide variety of reduced (relative to 

 carbon dioxide) carbon compounds provides the only 

 major soufce of energy for the maintenance and pro- 

 pagation of all life. For this and other reasons, the 

 study of the nature of this process has been a very 

 attractive area for many years and a wide variety of 

 scientific interest and backgrounds have been brought 

 to bear upon it. These range from the purely biological 

 to the strictly physical with the biochemical and phy- 

 sicochemical area lying between. Important contri- 

 butions to the understanding of the phenomenon have 

 come from all these areas, but in spite of the enormous 

 amount of work and study that has gone into the prob- 

 lem, relatively little is known, or rather understood, 

 about the fundamental character of the process even 

 today. It is perhaps pardonable that one engaged in 

 studies in this area tends to the conclusion that most of 

 the knowledge has been acquired in the relatively recent 

 past. Discounting that tendency, it still seems fair to 

 say that we have only just begun in the last decade or 

 so to gain some understanding of the intimate details 

 by which the basic process represented in the overall 

 reaction 



CO, + H,0 



-Hhti 



-> 0,+ {CH,0), 



- Energy 



has come to be understood. The recognition of this 

 overall reaction as written, to represent the basic nature 

 of the process of photosynthesis, and, further, that its 

 reversal represents the basic reaction of respiration is, 

 of course, an old one. 



As a result of more recent study, it has been possible 

 to separate the process of photosynthesis into two dis- 

 tinct and separate parts. The general features of this 



^ The work described in this paper was sponsored by the U.S. 

 Atomic Energy Commission. 



■ Radiation Laboratory and Department of Chemistry, Univer* 

 sity of California, Berkeley. Fellow of the Swiss Foundation, 

 iStiftung fiir Stipendien auf dem Gebiete der Chemie», 1951-1952. 



separation may be represented in the following chart 

 (Fig. 1). The essential feature of the separation is the 

 independence of the photochemical part of photosyn- 

 thesis from the carbon dioxide reduction part. We shall 

 not here even try to outline all of the various forms of 

 evidence which have been adduced in support of such 

 a scheme but only to point out additional bits which 

 have been added in recent years and particularly those 

 which stem from our own work'. 



ICHgO) 



C02 



L. 



Fig. 1. 



The scheme itself is an outgrowth of proposals of 

 some fifteen years ago by Van Niel* resulting from his 

 studies of the comparative biochemistry of photosyn- 

 thesis. More recently, the photochemical apparatus has 

 been shown to be separable from the rest of the plant 

 by the experiments of Hill'. 



He was able to make preparations of chloroplasts 

 and chloroplastic fragments which, upon illumination 

 in the presence of suitable oxidizing agents other than 

 carbon dioxide, were able to evolve molecular oxygen. 

 Still more recently, Ochoa an others* were able to 

 demonstrate that these same preparations were capable 

 of using coenzyme I and II (D.P.N, and T.P.N.) as 



' M. Calvin and A. A. Benson, Science 107, 476 (1948). - A. A. 

 Benson and M. Calvin, Cold Spring Harbor Symp. quant. Biol. tS, 

 6 (1948). - M. Calvin and A. A. Benson, Science lOS, 140 (1949). 



» C. 3. Van Niel, P*o/osy>i(A«si5 in P/anb, Chapter 22 (Iowa State 

 College Press, Ames, Iowa, 1949), pp. 437-495. 



' R. Hill, Nature 139, 881 (1947); Proc. roy. Soc. (London) [BJ 

 127, 192 (1939).- R. Hill and R. Scarisbrick, Nature H6. 61 (1940). 



• W. VisHNlAC and S. Ochoa, J. Biol. Chem. 19S, 75 (1952). - 

 D. I. Arnon, Nature H7, 1008 (1951). - L. J. Tolmach, Arch, Bio- 

 chem. Biophys. 33, 120 (1951). 



79 



