SEPARATION OF THE EFFECTS OF TWO PHOTOCHEMICAL 

 REACTIONS BY STUDIES OF OXYGEN EXCHANGE 



C. S. French 



SUMMARY 



In the red alga Porphyridium red light, absorbed by chlorophyll 

 a, produces some substance that is an intermediate in the process 

 of O2 evolution. This material is used up by green light which is 

 absorbed by phycocyanin while O2 is produced. Its presence after 

 red illumination may be detected either by the enhanced photosyn- 

 thesis from a short exposure to green light or by the increased 

 rate of Go uptake which it causes. No evidence could be found for 

 the post-illumination survival of a comparable green light prod- 

 uct. Some other investigations of photosynthesis in flashing mon- 

 ochromatic light are described. 



INTRODUCTION 



(1 2 3) 



By recording rates of O2 evolution polarographically ' ' it 



is possible to see some of the consequences of interactions ^^'^^ 

 between the two photochemical processes in whole cells or in 

 chloroplasts . Perhaps the most striking evidence for a complex 

 interlocking of photochemical and dark reactions is in the time- 

 course curves for O2 evolution during the induction period of 

 photosynthesis. These curves have an extraordinary range of vari- 

 ation, yet they are obviously composed of underlying basic charac- 

 teristics--an initial sharp spike, a later rounded spike, then a 

 dip and a slow rise, perhaps followed by a slow decline. Various 

 inf luences--wavelength of light, temperature, added chemicals, hy- 

 drostatic pressure, and previous treatment of the material, all 

 produce systematic changes of the specific features of the rate 

 curves during the induction period. 



Learning to read the meaning of these curves in terms of two or 

 more photochemical reactions acting on limited pool sizes of in- 

 termediate substances is still a distant objective. Intuition 

 backed up by the application of mathematical expressions for rates 

 of interrelated chemical reactions of a hypothetical nature^ -' is 

 only a beginning. Eventually the biochemical transformations and 

 the kinetic observation on rates of O2 evolution must be explain- 

 able in the same terms. 



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