L.N.M. Duysens 



thesis in both beams applied together is foiuid to be greater 

 than the sum of the rates in each beam separately. In light 1, 

 system 1 is producing oxidized intermediates at a higher rate 

 than system 2, and in light 2, system 2 is producing reduced 

 intermediates at the higher rate. If both beams are applied 

 simultaneously, the oxidized and reduced intermediates formed 

 in excessin each beam separately react with each other, which 

 results in an enhanced electron transport or an enhanced rate 

 of photosynthesis. 



Cytochrome reactions 



Purple bacteria. Illumination of PB^P?)®iV^9$®f4^ causes the 

 oxidation of one or more cytochromes^ * * * * • In purple 

 bacteria presumably only one photochemical system, analogous 

 but not /identical to system 1 in algae and higher plants is 

 present^ * . Oxidized cytochrome may be reduced either by 

 a so-called hydrogen donor, which has a function analogous to 

 H2O in algae, or by a reductant, e.g. by XH, formed in the 

 light. In/ ihe, latter reaction so-called cyclic phosphorylation 

 may occur ' . The sequence of the reactions in purple bac- 

 teria between the various cytochromes present, has not yet been 

 definitely established. Olson and Chance^ 'made an extensive 

 investigation of cytochromes in the purple bacteria Chromatium . 

 They concluded that at least four cytochromes were present, dis- 

 tinguished by different time courses and difference spectra. 

 One of these cytochromes, G 423,5 (with a difference spectrum 

 having a soret maximum ^*/4S^»5 uni) is still oxidized at the 

 temperature of liquid air . Oxidation of other cytochromes 

 was not observed at this temperature, but it was suggested that 

 the oxidation of these cytochromes could not be observed, be- 

 cause these cytochromes were oxidized spontaneously in darkness 

 during cooling. 



The rate of oxidation of C 423.5 was the same at -I96 as at 

 20 C. Cytochrome photooxidation ie an extremely efficient reac- 

 tion. Assuming that the/Sgecific absorption difference at 420 

 m|i is 62 /(mM»cm),01son calculated that the number of quanta 

 required for the oxidation of one cytochrome molecule (the 

 quantum requirement) was about 1 within rather^large limits of 

 error. Making similar assumptions, Vredenberg calculated a 

 minimum requirement of 0.6, which suggests that the true quantum 

 requirement is 1 and that the true specific absorbancy diffe- 

 rence at 423 mji is about 100 /(mM'cm). The equality of the rates 

 (and thus that of the quantum requirements of cytochrome oxidation) 

 at 20 and at -I70 C was confirmed. However, it was found that 



