PHOTOCHEMISTRY OF LIVE CELLS 1617 



However, back reactions (reoxidation of ferrous salts by free oxygen) 

 caused the oxygen production to come to an early stop. Better oxygen 

 yields could be obtained with various organic compounds containing a car- 

 boxyl group, particularly hcnzaldehjde and acetaldehyde, as well as para- 

 banic acid and nitrourea. Numerous other aldehydes, oximes, acids, quin- 

 onoid dyestuffs, carbohydrates, and urea and its derivatives (for a list, 

 see Vol. I, page 542) were tried but gave negative results. The greatest 

 oxygen production was observed with benzaldehyde, but the interpretation 

 of this reaction was somewhat uncertain, since a considerable dark reac- 

 tion was noted which produced carbon dioxide. Conceivably, this reac- 

 tion could be accelerated in light, and benzaldehyde, instead of participat- 

 ing directly in the oxygen-liberating reaction, could be first photoxidized to 

 carbon dioxide (or dismuted to reduced compounds, such as benzalcohol, 

 and carbon dioxide) and the latter assimilated in the normal way. True, 

 Fan et al. were unable to trap any free carbon dioxide by alkali ; but this 

 evidence is never completely convincing because rapid intercellular reutiliza- 

 tion of carbon dioxide may make its trapping by extracellular absorbers 

 impossible. 



The highest observed rate of liberation of oxygen with benzaldehyde 

 as oxidant was about 10% of the rate of photosynthesis. 



Warburg and Liittgens (1946; cf. Warburg, 1948) obtained the first 

 reliable results with o-henzoquinone. In this case, too, a dark reaction oc- 

 curred, which produced carbon dioxide; but its rate (about 0.02 cc. CO2 per 

 1 cc. of cells in 5 min.) was only about 2% of the rate of liberation of oxygen 

 in light (0.5 cc. O2 per 1 cc. of cells in 5 min.). 



Aronoff (1946^ repeated Warburg's experiments with Scenedesmus in a 

 nitrogen atmosphere (0.1% O2). He observed only slow oxygen evolution 

 in light, and thought it to be limited by the rate of penetration of quinone 

 into the cells. 



Clendenning and Ehrmantraut (1951) made manometric studies of the 

 oxygen production by Chlorella with o-benzoquinone or Hill's mixture as 

 oxidants. (No oxygen liberation was obtained with ferricyanide alone or 

 with phenol-indophenol.) 



The manometer vessel, containing C02-free cells and the substitute 

 oxidant, could be placed in homogeneous light side by side with a similar 

 vessel containing identical cells in carbonate buffer; or the two vessels 

 could be used alternately in the same position. In this way the yield of the 

 Hill reaction could be measured in relation to the yield of photosynthesis 

 under the same conditions (except for the difference in pH). Several con- 

 clusions emerged from this comparison : 



Constancy of Rate, and Total Yield. With 10 mm.^ of cells and 1 mg. of 

 quinone in 3 cc. (about 3 X 10"* mole/liter), the initial rate of oxygen 

 evolution in continuous light was maintained for 30-40 min.; a total 



