1964 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



reaction" in minute light-minute dark (or minute bright-minute dim) 

 experiments; these seem to indicate that respiration, far from being in- 

 hibited, proceeds, during intense photosynthesis, at up to 12 times its steady 

 rate in the dark. 



Franck has endeavored to show that these experiments, too, can be 

 plausibly explained by assuming that respiration intermediates can serve as 

 alternative substrates of photochemical reduction. 



(1) The experiment on respiration in C02-free medium (p. 901) was 

 made with a dense suspension, and a narrow pencil of light. Therefore, 

 at any given moment, only about 5% of algae received light. The experi- 

 ment thus merely proved that irradiation of 5% of algae in a suspension 

 cannot affect significantly the total respiration of all of them. (The 

 method is not sensitive enough to disclose a 5% decline in respiration!) 

 Furthermore, Franck had deduced from other observations {cf. below) that 

 a high concentration of carbon dioxide favors the entry of respiration inter- 

 mediates into the photochemical process, while effective removal of CO2 

 (as achieved in Warburg's experiment) makes this substitution improbable. 



In contrast to Warburg, others who have attempted to study respiration 

 in hght by making the medium free of carbon dioxide, have not succeeded in 

 preventing partial compensation of respiration in light. Franck attributed 

 this to a more diffuse illumination. 



Whenever hght is found to have an effect on gas exchange in a COa-free 

 medium, one is uncertain whether the compensation of respiration occurs by 

 re-utihzation of respiratory CO2 before it has had time to escape from the 

 cell, or by chemical reversal of an intermediate step in respiration, such as 

 the oxidation of triose to glycerate (in other words, by the utilization of 

 gly cerate as a substitute reduction substrate in the C02-denuded photo- 

 synthetic mechanism). Franck considered the second mechanism as 

 indicated in the experiments of Kok (1951), in which a very thin (1 mm. 

 thick) layer of a cell suspension was used, permitting very efficient carbon 

 dioxide removal. Despite this efficiency, Kok found that the yield of 

 photosynthesis (according to Franck, one should say "anti-respiration") 

 was quite as high as when carbon dioxide was present, even close to com- 

 pensation. 



(^) The experiments illustrated by fig. 37D.25 seem to be quite con- 

 vincing, since, in them, considerable net amounts of oxygen were produced, 

 over periods of time of the order of an hour, with an average quantum 

 requirement of 3 (or less). However, it must be kept in mind that the 

 quantum yield was calculated in these experiments by assuming that the 

 "background" process (which is a combination of dark respiration with 

 approximately compensating photosynthesis caused by the steady back- 

 ground light), is not affected by the superimposed alternating illumina- 



