424 



Birgit Vennesland 



cell. Users of this method assume that the isotope ratio in the external gas 

 phase reflects the isotope ratio at the site of O^ generation and 0_ reutili- 

 zation inside the cell. This assumption is unjusTified. There are bound to 

 be concentration gradients, and there is no way of calculating how large or 

 small these concentration gradients will be. 



Personally, I regard the light-induced respiration as a genuine experi- 

 mental phenomenon. I have seen it. And I think Warburg's experiments 

 prove that the fundamental mechanism of photosynthesis should be repre- 

 sented as an oxygen elimination from a CO^ derivative, followed by a re- 

 oxidation of a part of the reduction product. The stoichlometry of the re- 

 oxidation is calculated from the quantum requirement. For an overall quan- 

 tum requirement of three, 2/3 of the reduced carbon must be reoxidized; an 

 overall quantum requirement of four, suggests that 3/4 are reoxidized. When 

 photosynthesis proceeds at low efficiencies, this may be because a larger 

 fraction of the reduction product Is reoxidized, or because much of the 

 chlorophyll is not associated with photolyt under the conditions used for the 

 measurements. 



All of the preceding points are preliminary to a consideration of some 

 experiments with isolated chloroplasts. In Chicago, we work mainly with 

 chloroplasts from leaves, particularly spinach leaves, and I have described 

 experiments done mainly with Chlorella. This is because It hasn't yet been 

 possible to provide a direct demonstration of the formation and function of 

 CO^ in leaves. But it seems reasonable that oxygen-generating photosyn- 

 thesis should have the same fundamental mechanism wherever it occurs in 

 nature, and we assume this is so. The assumption has received support from 

 the recent demonstration, by Warburg and his associates, that small young 

 lettuce leaves can perform photosynthesis with a quantum requirement not 

 greater than 4.5 per molecule of O,^ released^"'. 



THE STIMULATION OF THE HILL REACTION BY CO2 



For many years, the occurrence of the Hill reaction in isolated chloro- 

 plasts or grana seemed to show that the CO^-fixing part of photosynthesis 

 could be separated completely from the O^-evolvIng part. This is how Hill 

 Interpreted his Initial observations, and this is how most of you still Inter- 

 pret the Hill reaction. In fact. It is the existence of the Hill reaction with 

 its associated photophosphorylation which provides the best evidence for the 

 statement that the grana function mainly to provide ATP and reducing power 



