CHLOROPHYLL PARTICIPATION IN THE PRIMARY PROCESS 551 



Two alternative pictures of the chemical participation of chlorophyll 

 in photosynthesis emerge from this discussion. In the first picture, 

 chlorophyll is bleached — or strongly changed in color — in the photo- 

 chemical forward reaction, and is restored by a thermal back reaction so 

 rapidly that no visible decolorization takes place in the stationary state 

 of photosynthesis. In the second picture, the color of chlorophyll is not 

 strongly changed by its photochemical transformation; the product of 

 this transformation may therefore accumulate, during photosynthesis, 

 until its concentration is about equal to that of the original form, without 

 changing the coloration of the cells; and the restoration of chlorophyll 

 may be achieved by a photochemical hack reaction. 



A choice between these two possibilities could be facilitated by a 

 closer investigation of the pigment spectrum during photosynthesis. 

 Visual observation is insufficient, because the eye is a poor instrument 

 for the estimation of extinction curves; it would hardly notice the dis- 

 appearance of even 20% of the pigment, not to speak of a mere shift in 

 the position of the absorption bands. The parallel alignment of chloro- 

 plasts in strong light was found by Schanderl and Kaempfert to increase 

 considerably the transmissivity of leaves (Chapter 22, Vol. II) ; and the 

 same authors have reported that leaves often grow more opaque during 

 prolonged illumination because of the deposition of assimilates. These 

 changes would hardly ever be noticed by a mere visual observation of 

 the leaves. 



In the first of the two pictures mentioned above, the photochemical 

 forward reaction may be either an oxidation or a reduction of chlorophyll ; 

 thus, we can classify the theories of the chemical function of chlorophyll 

 in photosynthesis in the following three groups (the first two of which 

 correspond to the mechanisms designated as /3i and ^2 in chapter 18, 

 page 514): 



(a) Photochemical oxidation of chlorophyll to a decolorized form; 

 nonphotochemical reduction; in this case, chlorophyll can be identified 

 with the primary reductant, HZ, in scheme 7.1. 



(b) Photochemical reduction of chlorophyll to a decolorized form; 

 nonphotochemical reoxidation; in this case, chlorophyll can be identified 

 with the primary oxidant, X, in scheme 7.1 ("decolorized" may mean 

 merely a form whose color is irrelevant for its transformation back into 

 chlorophyll). 



(c) Photochemical oxidation and photochemical reduction (in this case, 

 both the oxidized and the reduced form must be green). Chlorophjdl 

 can be identified with the intermediary oxidation-reduction system, Y, 

 in scheme 7.1. In case (c), the extracted chlorophyll may represent 

 either the oxidized or the reduced form. 



