394 PLANT METABOLISM 



would liberate small amounts of oxygen upon illumination. Substitution 

 of potassium ferric oxalate solution for the leaf extract increased the 

 rate of oxygen evolution, and later workers have further improved the 

 system by substituing p-benzoquinone. The potassium ferric oxalate or 

 p-benzoquinone serves in place of carbon dioxide, which is the oxidant 

 in normal photosynthesis. The Hill reaction with p-benzoquinone is: 



O OH 



II I 



9 



II +2H.0 ,, "„^,, > 2 I II +0.-52Cal. 



chlorophist 

 suspension 



HCv. ^CH suspension HC^vv^ ^CH 



O OH 



Benzoquinone Hydroquinone 



Oxygen is liberated, the energy of light is used in the reduction of the 

 quinone, and energy is stored in the hydroquinone formed. The Hill 

 reaction should more properly be called the Hill reactions, for both 

 photochemical and dark reactions apparently are involved. This isolated 

 system again emphasizes the close relationship of the photochemical and 

 oxygen-liberating reactions of photosynthesis. 



Success in carrying out light-sensitive partial reactions concerned in 

 the reduction of carbon dioxide without the use of intact cells was not 

 achieved until 1951, when Vishniac and Ochoa reported that light energy 

 could be used in the reductive carboxylation of pyruvic acid to malic 

 acid (reaction 15, Fig. 13-4). Grana from spinach leaves used the 

 energy captured from light for the reduction of TPN. The reduced TPN, 

 in the presence of the proper enzymes, then caused the reductive car- 

 boxylation of pyruvic to malic acid. In a similar manner DPN, reduced 

 in the light, could effect the reduction of pyruvic to lactic acid (reaction 

 13, Fig. 13-3). The overall reactions did not occur in the dark, but it 

 should be noted that the reactions directly involving the carbon dioxide 

 were dark reactions. Illuminated chloroplasts, or grana, which we al- 

 ready have seen can liberate Oo, also can transfer the hydrogen pro- 

 duced in the photolysis of water to oxidized DPN or TPN. The co- 

 enzymes in turn can reduce pyruvate to lactate or function in the reductive 

 carboxylation of pyruvate to malate. These observations have been con- 

 firmed by Tolmach, and Arnon has reconstructed a system for the light- 

 dependent reductive carboxylation of pyruvate to malate, in which all the 

 parts (except TPN) were of plant origin. 



It is justifiable to say that the Hill reaction represents a true partial 

 reaction of photosynthesis, but there is some question whether or not 

 this can be said about the reactions described by Vishniac and Ochoa. 



