ENERGY EXCHANGE IN PHOTOREACTIONS 59 



enzymes and depends on the transient transfer of electrons to or from 

 the protein. Only surface groups of the protein are considered to be 

 involved in catalysis. Another interesting mechanism would depend on 

 the folding and unfolding of the protein, during which the substrate 

 molecule is literally torn apart (Lumry and Eyring, 1953). Although 

 this idea stems in various forms from'the early days of protein chemistry, 

 it has recently been given quantitative support by Casey and Laidler 

 (1950). An interesting mechanism containing both types of effects has 

 been proposed by Smith (1949). 



There is abundant evidence of unusual energy migration in photo- 

 synthesizing organisms. Modern evidence indicates that the over-all 

 reaction of photosynthesis may be broken into two major subdivisions 

 thus : 



Over-all: 6CO2 + GH.O -^ CeHi.Oe + 6O2 (l-48a) 



CO2 fixation: 6CO2 + 24H+ + 21<? -^ CeHi-Oe + 6H.,0 (1-486) 



H2O oxidation: 2H2O -^ 4H+ + 4e + O-, (l-48c) 



From the viewpoint of photochemistry the oxidation of water, Eq. (l-48c), 

 is one of the most interesting of all photoreactions not only because of its 

 high efficiency but also because there may be as many as three different 

 intermediate oxygen-hydrogen compounds, all produced by the action of 

 the same light-absorbing system. The substances that carry out this 

 complex reaction are confined to small granular bodies whose orientation 

 and functional structure are not understood. These bodies contain a 

 varied array of proteins, lipoids, and pigment molecules. A number of 

 pigment molecules have been shown to participate in photosynthesis, 

 presumably following transfer to the chemical entity that actually com- 

 bines with water (Button et al., 1941; Emerson and Arnold, 1931 1932; 

 Haxo and Blinks, 1950; Duysens, 1951). There must exist some means 

 for direct transfer of electronic energy among various absorbing pigments, 

 since neither collisions of the second kind nor reemission and reabsorption 

 can explain the observations. This conclusion is well buttressed by the 

 fact that the in vivo fluorescence of chlorophyll is sensitized in high yield 

 by several of its companion pigments (Button et al., 1943; Wassink and 

 Kersten, 1946). Chemical coupling, i.e., compound formation between 

 pigments, is not indicated. The spectral differences between in vitro 

 and in vivo pigments are slight. Forster (1947), Arnold and Oppenheimer 

 (1950), Buysens (1951), and others have attributed the energy transfer 

 to the interaction of dipole fields, as proposed by Forster (Sect. 4-3) (but 

 see Franck and Livingston, 1949). The distances over which trans- 

 fer must occur do not, however, need to be especially large, since the 

 concentrations of pigments are high. Chlorophyll, the major pigment, 

 is present in the granules to the extent of about 0.1 M (Rabinowitch, 



