1108 THK LIGHT FACTOR. IT. QUANTUM YIELD CHAP. 29 



medium) gave 2-3 times lower yields (7 = 0.10-0.09) in carbonate buffers 

 (pH ^^ 9), both with and without compensating white light (last section 

 of table 29. IV). Yields of 0.12 or less were obtained also in bicarbonate 

 solutions equilibrated with 5% carbon dioxide in air (pH 7-8). It thus 

 appears that for Warburg and Burk's cells (grown in acid medium) even 

 neutral solutions were "unphysiological." 



Respiration in Light. The question whether respiration is affected 

 by light is crucial for the measurement of the rale of photosynthesis in 

 weak light. Different indirect methods (Vol. I, Chapter 20, and Chapter 

 36) have been used to answer it, and have given contradictory answers 

 (including all three alternatives: "no change," "stimulation," and "in- 

 hibition"). The simplest approach to this prol)lem is to remove (e.g., 

 by absorption in alkali) all carbon dioxide (including that produced by 

 respiration) and to measure the oxygen consumption in light unobscured 

 by photosynthesis. This procedure was attempted repeatedly, but with- 

 out success, because immediate photosynthetic reutilization of respiratory 

 carbon dioxide competed too effectively with its absorption by the com- 

 paratively remote external absorber. In fact, it proved difficult to reduce 

 photosynthesis in this way much below the compensation point. War- 

 burg et.al. (1949^) reported, however, that Avith the increased frequency 

 of shaking, they were now able to absorb respiratory carbon dioxide 

 in an alkali-filled side arm of the reaction vessel so effectively that the 

 rate of oxygen uptake by a Chlorella suspension in light was exactly the 

 same as in the dark. They saw in this experiment the proof that respira- 

 tion as such is quite unaffected by (red) light, and refutation of all hy- 

 potheses which postulate an exchange of intermediates between photosyn- 

 thesis and respiration. 



Complete prevention of photosynthetic reutilization of respiratoiy 

 carbon dioxide by absorption of the latter in an external absorber (although 

 reutilization must be possible even before the carbon dioxide had escaped 

 from the cell into the medium), is a remarkable achievement. A possible 

 reason why Warburg and co-workers were successful where others have 

 failed is intermittent illumination. For 95% of the "light period" each in- 

 dividual cell is practically in darkness. Respiration goes on during all this 

 time; all, or at least a large part of the carbon dioxide produced while the 

 cell is in the shade may be able to escape into the medium before the cell 

 had moved into the illuminated zone. Once a carbon dioxide molecule is 

 in the medium, it may have a much greater chance to diffuse into the gas 

 space than to diffuse into the small illuminated volume. In this way, 80 

 or 90% of respiratory carbon dioxide produced during the "light period" 

 could perhaps escape re-utilization by the cells and reach the external ab- 

 sorber. 



