872 LIGHT AND LIFE 



by the accessory pigments) is very susceptible to four conditions which 

 also specifically inhibit the evolution of oxygen: heating, aging, and 

 the action of hydroxylamine and of 3 (3,4-dichlorophenyl) -1,1-dimethyl 

 urea (DCMU) . This reduction of P 700 by light, through electrons 

 derived from the primary photoreductant, XH, completes the cycle. 

 Moreover, the cyclic photophosphorylation with phenazine metho- 

 sulfate, which bypasses the oxygen-evolving phase, is maximal at 710 

 m/A, a wavelength that is quite ineffective in driving complete photo- 

 synthesis or in causing the reduction of P 700 by light. 



In any event, it is clear that the assumption, made in other papers 

 in the Symposium, that all the absorption above about 660 xnjx may 

 be ascribed to "chlorophyll «" in its various forms has to be modified 

 in view of the data of Kok and Hoch showing that absorption at 

 700 ni/x is attributable to a photosynthetic pigment of distinctive 

 functions. 



The Cytochromes in Photosynthesis 



R. Hill, in 1939, in his now-classic study of photosynthesis, sug- 

 gested that a "respiratory pigment" might play an important part in 

 a chemosynthetic system linked to the quantum absorption process. 

 It is by now quite clear that this is so. The chloroplasts of the 

 higher plants contain cytochromes, and photosynthetic bacteria like- 

 wise possess cytochromes linked to the photosynthetic system. 



Two cytochromes, / and 6^, have been found to be characteristic of 

 chloroplasts. Neither of these cytochromes is identical with any of 

 the mitochondrial or microsomal cytochromes, which, unlike the 

 chloroplast cytochromes, are denatured by 80% acetone. In the light 

 cytochromes / and ^,) are present in the oxidized form. The oxida- 

 tion-reduction potential (£'„) of cytochrome / is +0.365, of cytochrome 

 bf,, —0.03. Upon extraction with acetone, cytochrome / becomes re- 

 duced but cytochrome b,., remains fully oxidized. In spinach chloro- 

 plasts the ratio of bj^ was found to be 1.3. In low-temperature spectra 

 of whole chloroplasts, in comparison with the acetone-extracted prep- 

 arations, the absorption bands are shifted to slightly longer wave- 

 lengths. Illuminated chloroplasts perform the reaction: 



R + H.O -» RH., + i/oOo 



a reaction the reverse in direction of the mitochondrial oxidations. 

 A variety of hydrogen-acceptors, either two-electron acceptors or one- 

 electron acceptors, will serve. The cytochromes of the illuminated 

 chloroplast may function as a hydrogen transport system acting 



