VAN KIEL'S THEORY: THIRTY YEARS AFTER 13 



hi/| hi/2 



X>685m/x X<685m/i 



hydrogen I / oxygen 



or [H] — Chi I -/*-"— Chi n— [oh] or 



/ \ ^ 



reduction \ oxidation 



[OH]or (+)-. [h] or (- 



Fig. 3. The basic mechanism of photosynthesis which 

 in a two pigment system produces overall what amounts 

 to a photolysis of water. 



about the final result, the photolysis of water. This final result is not 

 the appearance of positive holes on the one side, or a corresponding 

 stream of electrons ejected on the other, but the appearance of the 

 elements hydrogen and oxygen, or in their place such permanent 

 chemical changes as may be brought about metabolically by either 

 hydrogen or oxygen. 



To achieve present day photosynthesis, it was necessary in the 

 course of natural evolution to prevent as far as possible all types of 

 intermediary re-oxidations. Free oxygen had to be eliminated as a 

 waste product. And also the release of free hydrogen had to be pre- 

 vented in order that it could be used instead in intermediate forms 

 for synthetic reactions. 



Shall we believe now that the purple bacteria just have only one 

 half of the system and that nowhere water enters into their photo- 

 chemical mechanism? This would mean, if we think of evolution, that 

 the plants arose by doubling the arrangement of the purple bacteria. 

 If so, this would leave us with the dilemma of the eight quanta in 

 photo reduction mentioned above, Duysens, in desperation, believes the 

 quantum number to be an accident. Let me point out that the obligate 

 anaerobic phototrophic bacteria, as well 3.8 Chlamydobotrys oiVvings- 

 heim and Wiessner, have to grow while the light is shining. No fer- 

 mentation supports growth in the dark. In the green plants we keep 

 respiration and growth on a separate energy balance sheet, while with 

 the bacteria our energy measurements include everything. This may 

 equalize the energy requirements of plants and bacteria in an accidental 

 way. 



But a look at the regularity of Larsen's results in his beautiful 

 measurements of the quantum requirements in green and red bacteria 

 makes the chance hypothesis appear rather weak. Regardless of the 

 substrate, H2, H2S or thiosulfate, and the correspondingly different 



