CHLOROPHYLL CONTENT AND YIELD IN FLASHING LIGHT 1279 



where A^^, is the rate of incidence of Hght quanta per second per square 

 centimeter. The highest quantum yields of photosynthesis have been ob- 

 served in light of the order of 100-1000 lux, corresponding to A^^, = 10 1'^ 

 10^^; with a ^ 10\ this means n = 4 X 10-» to 4 X 10"^, i. e., each chloro- 

 phyll molecule absorbs a quantum every 25-250 seconds, jf a carbon di- 

 oxide molecule were to remain anchored, during the whole reduction process, 

 at one chlorophyll molecule, waiting for 4 (or 8) quanta to be absorbed by 

 the latter, this should take, in the light of the above-mentioned intensity 

 (100-1000 lux), from 100 to 1000 sec. for 4, and from 200 to 2000 sec. (i. e., 

 up to one half hour) for 8 quanta. In completely absorbing, dense cell 

 suspensions, such as were used by Warburg and Negelein, Emerson and 

 Lewis, and others, in quantum yield determinations, the average light inten- 

 sity was lower than 100 lux, and the frequency of absorption acts by a single 

 chlorophyll molecule must have been only one every 10 or 20 minutes, cor- 

 responding to 1.5 to 3 hours for 8 quanta! 



If one would start, after a dark period, with chlorophyll deprived of all 

 intermediates, and all chlorophyll molecules associated with ACO2 mole- 

 cules, over 1 hr. of illumination would thus be required, under the assumed 

 conditions, for the uptake of new carbon dioxide to reach the steady rate. 

 The Hberation of oxygen, on the other hand, could become steady after a 

 quarter or an eighth of this period, because, in the most plausible reaction 

 mechanisms, the final oxidized photoproduct (ROH in scheme 7.VA and 

 A'OH in scheme 28.1, etc.) is obtained in consequence of a single photo- 

 chemical step, and is converted to molecular oxygen entirely by dark reac- 

 tions (e. g., combination and dismutation of radicals, such as 2 [OH] -^ 



[H2O2]— [H20] + 02). 



Experiments have shown, however, that not only the liberation of oxy- 

 gen, but also consumption of carbon dioxide begins very quickly upon il- 

 lumination after a dark period, even in very concentrated suspensions. This 

 fact can be explained, from the point of view adopted in this section (z. e., 

 without recourse to the hypothesis of a "photosynthetic unit") in two ways, 

 depending on whether one adopts Franck and Herzf eld's scheme, (7.VA), 

 in which the reduction of carbon dioxide is achieved by four consecutive 

 photochemical steps, or chooses one of the reaction schemes (e. g., scheme 

 28. lA) in which a single photochemical reduction step is combined wdth 

 repeated catalytic dismutations. 



In the first case, one has to assume either that the intermediate reduc- 

 tion products, AHCO2, AH2CO2 . . . , are so stable that they can survive 

 a very prolonged period of darkness, or that the stock of these intermedi- 

 ates is continuously replenished in darkness in consequence of slow reversal 

 of photosynthesis by an autoxidation process, different from normal respira- 

 tions, occurring within the chloroplasts. In either of these two ways, the 



