QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1 107 



Background Light. Because only 5% of all cells were illuminated at 

 any given moment, even the very high incident intensity of the red light 

 used in the Warburg-Burk measurements did not prevent the respiration 

 correction from being equal to or larger than the photochemical gas ex- 

 change. It has been suggested (this hypothesis will be discussed later in 

 this chapter), that the effect of light below the compensation point could 

 consist in reversing respiration midway (and not after it has led to the 

 ultimate products, CO2 and H2O). To check this hypothesis, Warburg, 

 Burk and co-workers made experiments in which the net gas exchange in 

 light was made positive by substituting for dark periods, periods of dif- 

 fuse illumination of the reaction vessels by white "background light" of 

 such intensity that photosynthesis equalled or exceeded respiration. 

 This background illumination was maintained also during the "light" 

 period (when a measured beam of red light was added to it), so that the 

 "light effect," from which the quantum yield was calculated, was the 

 increment of gas exchange caused by an increment of illumination. War- 

 burg and Burk argued that the quantum yields obtained in this way must 

 be those of true photosynthesis, with the storage of 112 kcal. chemical 

 energy per mole of liberated oxygen, and could not be those of a partial 

 reversal of respiration (with an unknown, and possibly small, conversion 

 of light energy into chemical energy), since this reversal, if at all possible, 

 should be accomplished already by the background illumination. 



It will be noted that this argument is tied up with the assumption of 

 uniform photosynthetic activity of all cells — those that are momentarily 

 illuminated by the flash as well as those that are momentarily in darkness. 

 If only the actually illuminated cells (or cells <0.01 sec. out of the illumina- 

 tion zone) can contribute significantly to photosynthesis, then only the 

 part of the background light that falls on these particular cells is of im- 

 portance. This part is insignificant if the background light falls from above 

 and is absorbed in the top layer of the suspension — while measured red light 

 enters the vessel from below and is absorbed in a thin bottom layer of the 

 suspension. Warburg and Burk (1950) described a single experiment in 

 which the background light, similarly to the measured light, was thrown 

 on the vessel from below. This light was so strong as to overcompensate 

 respiration about fivefold; nevertheless, the addition of the measured light 

 produced an increment of oxygen production equivalent to a quantum re- 

 quirement as low as 2.8. It is unfortunate that this particularly important 

 experiment was carried out with a particularly unsatisfactory time schedule 

 — three 5-min. light-dark cycles in one vessel, followed by two 10-min. 

 light-dark cycles in the other vessel. 



Quantum Yield in Carbonate Bufifers. The same cells which gave, in 

 Warburg and Burk's experiment, high quantum yields at pH 5 (culture 



