Mode of Action of the Photocatalytic System in Organisms 615 



the pigment system had the result that the active products of the photoprocess 

 were hnked to formerly existing biocatalytic systems. The further development 

 of catalytic pigment and systems led to the more rational utilization of light 

 energy. 



Energy Transfer 



Within the frame of the present report it is impossible to dwell at length on 

 the prestmiptive mechanisms of energy transfer in biological systems (see 

 reviews [iS, 19, and 32]). The following point, however, should be made. 

 'Resonance' transmission of energy from the chlorophyll excited molecule to 

 enzyme systems requires definite conditions, discussed above. This mode of 

 energy transfer to enzymic systems is not very plausible in the light of existing 

 data, since the biocatalytic systems which probably take part in photosynthesis 

 usually do not exhibit pronounced absorption maxima in the red region of the 

 spectnmi (with the exception of iron porphyrins). It is more probable that there 

 is an energy transfer at the triplet state level, provided the conditions defined 

 by Terenin & Ermolaev [23] are fulfilled. Such an energy transfer at the triplet 

 state level may, in fact, be interpreted as the transfer of an unpaired electron in 

 a conjugated system [41]. 



It seems probable that in ancient organisms conjugation was effected by the 

 transfer of energy (electron) from magnesium porphyrins to iron porphyrins. 

 In our model experiments we demonstrated the transformations of cytochrome c 

 under the action of Ught absorbed by chlorophyll [33, 4]. 



DEVELOPMENT OF HYDROGEN (ELECTRON) 

 DONOR UTILIZATION 



Photosynthetic bacteria have not yet 'learned the trick' of hberating mole- 

 cular oxygen from water, and utilize an energetically less expensive type of 

 photosynthesis. Van Niel [34] has assumed that in all autotrophic organisms, 

 including photosynthetic bacteria, there takes place photolysis of water (HOH 

 ^- H + OH). The initial hydrogen donor reacts with the OH radicals, while 

 the (H) is used to reduce the carbon dioxide. In line with this view, it has been 

 shown [35] that, regardless of the AF of the overall reaction (with diiferent 

 hydrogen donors), the quantum efficiency of bacterial photosynthesis is constant 

 (roughly 8 hv per CO2 molecule). These results suggest that a uniform type of 

 elementary photoprocesses is involved in bacterial photosynthesis, irrespective 

 of the nature of the initial electron donor. 



The work of our laboratory revealed that bacteriochlorophyll in solution can 

 eflfect the photosensitized transfer of electrons from hydrogen sulphide or 

 ascorbic acid to flavins and other hydrogen acceptors [36], The participation of 

 water in the photoprocess, in our opinion, stands in need of further substan- 

 tiation. It appears more probable that photosynthetic bacteria utilize the electron 

 of a donor molecule (e.g., SH', S2O3", organic acids) and the protons (hydrogen 

 ions) of water. The ability to 'take hold' of the electrons of water or OH'-ions 

 only arose in the process of further evolution of the autotrophic organisms. It is 



