PHOTOSYNTHETIC MECHANISM OF PURPLE BACTERIA I 7 1 



Duysens : The difference spectra of Forp/ii/ridiutn measured so far do not show a 

 change in that region. Neither do the difference spectra of Chlorella. These experi- 

 ments do not exclude that cytochrome b is participating in photosynthesis, since 

 the changes in the difference si)ectrum may have been too small to be observable 

 under the present experimental conditions. 



Becker: WTiat is the present precision in measuring these differences? 



Duysens: The highest precision obtained is 2 X 10~^ unit in optical density, 

 when everything is all right. 



With the verj^ dense suspensions the precision is less because, at lower intensi- 

 ties, the signal-to-noise ratio is lower. I think that, with the present apparatus, 

 no greater precision than that mentioned can be obtained. It is not only the noise 

 of the primary phototube current but also other changes which cause variations 

 of the indicating apparatus. 



Shibata: We have done experiments just on temperature-produced changes in 

 absorption. With Chlorella, we observed some changes like those caused by 

 illumination, for instance, in the 525 m^u band. W^e have also recorded temperature 

 difference spectra for Rhodospirillum. With this bacterium, there appeared to be a 

 slight change between 820 and 880 m/x, which in some ways is comparable to that 

 observed in light-produced difference spectra. I do not wish to say that the differ- 

 ence spectrum produced by illumination is actually a temperature effect, but I 

 cannot exclude that a part may actually be caused by temperature. In our experi- 

 ments, we maintained one sample at 10°C. and the other at 40°C. For the infra- 

 red measurements, we used a sulfide detector. For the work in the visible, we 

 modified a model 11 Gary spectrophotometer. I would like to ask Dr. Duysens 

 whether he noticed anj' temperature effects similar to ours. 



Duysens: In our experiments, there was no general increase in temperature 

 exceeding 5° or 6°C. even after hours of periodic illumination. Our illumination- 

 produced difference spectra changes were quite rapid. Thej' were also reversible 

 within a few seconds. 



Shibata : I do not implj^ that your entire effects are due to temperature changes, 

 but I do not see that you have eliminated temperature effects. Bear in mind that 

 the temperature within the grana during illumination may be considerabty above 

 that of the surrounding medium. 



Duysens : I can only give indirect evidence that the temperature increase during 

 my experiments had little influence, if any. If there was a pronounced effect, one 

 should certainly have seen an increasing change during illumination, because the 

 temperature would be changing during that time. 



Benson: The grana would warm up immediatel3^ They would then transfer the 

 heat to the solution. I do not see that the temperature of the grana should rise 

 continually. 



Bassham : One might reach a steady state of temperature where there is a warm 

 area around the grana and the heat is conducted away at a steady rate. It is not 

 inconceivable to me that there would be an initial warming up followed by the 

 attainment of a lower steady-state temperature. Much light energy is being 

 converted into heat within the small volume of the grana. 



Duysens : This distribution of the heat will take place, I believe, in 0.01 second 

 or so. If this is true, then j'ou would expect changes in the spectrum to talsc i)lace 



