VAN NIEL'S THEORY: TfflRTY YEARS AFTER 7 



not have arisen. On the other hand, as I pointed out, the wrong theory 

 exerts perhaps an even stronger inhibitory influence. 



And then there is costly apparatus— it too can hinder progress when 

 you believe that because it was acquired on a special government grant 

 it has to be used for its money's worth. It made a great impression on 

 me when a new-fangled very expensive light source (I believe specially 

 built by Siemens) had arrived at Warburg's laboratory. The great man 

 and his assistants spent an afternoon trying to make it work. When it 

 turned out that the thing was a disappointment, Warburg coolly said, 

 "Negelein— store it in the attic," and never looked at it again. 



To return to thephotolysisof water. What! have been able to under- 

 stand about our experiments on adapted algae during the past twenty 

 years I managed on the basis of the idea of internal back reaction 

 involving water. That is, internal oxido- reductions involving hydrogen 

 donors have to be assumed. 



As you know, during recentyears the question concerning the nature 

 of the primary process has been progressively restricted to a smaller 

 and smaller field of enquiry (Compare Fig. 1). The entire carbon 

 assimilation mechanism has been cleared away as a typical example 

 of orthodox biochemistry upon which plants and photosynthetic bacteria 

 have no exclusive property right. And a look at our program will tell 

 you that the discussion has narrowed to the question of how much 

 phosphorylation, TPN reduction, and oxygen evolution have to do with 

 the light energy conversion process. Thus the difference between bac- 

 teria and plants brings us to the question: What happens in the pigment 

 complex of bacteriochlorophyll which distinguishes the end result from 

 that we find in the green plants? 



Let us enumerate what purple bacteria and plants have in common. 

 1) A chlorophyll a type pigment. The bacterial form contains two more 

 hydrogens and handles light quanta at a discount of ten kilocalories 

 per mole quanta as compared with the green chlorophyll, because the 

 singlet absorption band lies around X. 890 mfi. 2) Different percentages 

 of one chlorophyll a are distributed among several binding sites, as 

 attested to by the various humps in the main absorption bands seen in 

 living cells. 3) Plants and bacteria contain carotenes and quinones, 

 not quite identical chemically but very likely serving the same func- 

 tions. 4) Not only the aerobic plants as shown by Hill, but also the 

 obligate anaerobic purple bacteria have, as Kamenandhis co-workers 

 discovered, cytochromes— not one, but at least two, and with oxidation- 

 reduction potentials that are 0.2 volts apart. This is true for plants as 

 well as bacteria, and we ought to remember this when we come to dis- 

 cuss the role of cytochromes, 5) Photophosphorylation, The observa- 

 tion of a light-induced phosphate turnover in intact purple and green 

 cells preceded by several years the demonstration that a respiratory 

 type of phosphorylation occurs in cell-free preparations. Light and 

 water replace, as van Niel would say, the role which a hydrogen donor 



