EXPERIMENTS WITH CHLOROPHYLL PREPARATIONS 67 



in photosynthesis takes place outside the chloroplasts. This may explain 

 why isolated chloroplasts are unable to use carbon dioxide as oxidant, 

 even if they are capable of oxidizing water in light. 



Attempts to take the photosynthetic mechanism apart in order to 

 find out how it works have been for a long time as unsuccessful as the 

 proverbial farmer's attempt to get at the source of golden eggs by killing 

 the goose which laid them. Hill's experiments represent the first step 

 forward in this direction, and their continuation appears of great interest. 

 (A confirmation of his results was contributed by French and coworkers, 

 1942.) They support the conclusion, derived by van Niel and Gaffron 

 from experiments with bacteria and anaerobically treated algae (c/. Chap- 

 ters 5 and 6), that the two partial processes of oxygen evolution and car- 

 bon dioxide reduction are largely independent, and can be investigated 

 separately. While van Niel and Gaffron showed that it is possible to 

 substitute in photosynthesis other redudants for water, Hill's observa- 

 tions indicate that ferric salts can be substituted for carbon dioxide as 

 oxidants in this process. 



Hill's original experiments with leaf and yeast extracts, as well as 

 the earlier qualitative observations of Friedel and Molisch (in which, 

 too, leaf extracts in glycerol or water were found necessary to bring 

 about the evolution of oxygen by leaf powders) make it seem probable 

 that these extracts contain organic oxidants which can be used to oxidize 

 water, in the presence of illuminated chloroplasts, instead of ferric 

 oxalate. In the case of leaf extracts, the oxidants may well be identical 

 with the intermediate hydrogen acceptors in true photosynthesis. It 

 would be important to identify these oxidants by systematic analysis. 



One can ask whether Hill's reaction is similar to true photosynthesis 

 from the point of view of conversion of light into chemical energy. Ferric 

 salts are much stronger oxidants than carbon dioxide. At pH 8, where 

 Hill's reaction proceeds most easily, the potential of an oxygen electrode 

 is — 0.75 volt and thus almost equal to the normal potential of the 

 nonassociated ferri-ferro system ( — 0.77 volt). However, ferric oxalate 

 solutions are strongly associated, and their potential is therefore much 

 less negative. The reduction of potassium ferricyanide by ferrous oxa- 

 late, cf. page 64, proves it to be > —0.49 volt. Therefore, the photo- 

 chemical oxidation of water by ferric oxalate must lead to the conversion 

 of a considerable amount of light energy into chemical energy — even if 

 this amount is much smaller than that utilized in true photosynthesis. 



2. Experiments with Chlorophyll Preparations 



Of all chemical components of plants, the only one which is clearly 

 indispensable for photosynthesis is the green pigment chlorophyll. 

 Ingen-Housz, in 1779, established that only green parts of plants improve 



