396 Photosynthesis 



Table 3. Geiger counter impulses per minute by chromatographed heated 

 extracts of 10 mm :! of Chlorella cells. 



by A//10,000 benzoquinone. This is the CO2 that is used in light and taken up in 

 the dark 8 . 



Remarks 



1) Quantum requirement in the United States. In the years 1938 to 1948 the quantum 

 requirement of photosynthesis was measured in various institutes in the United 

 States with the result that 12 to 20 quanta were found to be required by Chlorella 

 for the development of one molecule of O2. The average value was 16. The value of 

 12 was regarded as the optimum value. A few, including Dean Burk and van 

 Niel, dissented, but the Interpreters and advocates of the high quantum numbers, 

 James Franck and Eugene Rabinowitch, maintained the upper hand. In 1941 

 Franck and Gaffron 10 wrote, " We know novo that the high efficiency is only apparent 

 and that the true efficiency is probably only a third of it, namely 12 quanta per mole- 

 cule CO-2 reduced" (italics added). The "photosynthesic unit" in Urbana, Emerson 

 and Rabinowitch, stayed with the high quantum numbers until at least 1952 11 . 

 Later, under the influence of the Dahlem investigations, the quantum numbers 

 reported in the United States sank, and today approach the Dahlem number of 

 4 to 312. 



2) Light reaction and dark reaction. According to the equations of the light and 

 dark reactions, which can be separated in point of time, CO> is taken up anew in 

 the light reaction, so that the ratio CO2/O2 is about — 1 in the light reaction per se 

 (although the CO? from which the oxygen is developed may not be the CO2 that 

 is taken up). However, this holds only for optimally cultured cells whose quantum 

 requirement in the balance is 3 to 4. Other cells, for example those grown at a 

 south window with added constant artificial illumination, may take up CO2 more 

 slowly, so that the ratio CO2/O2 in the light reaction is not — 1 but lies between 

 — 1 and 0. In the latter extreme the new CO2 is first taken up in the dark reaction 

 following cessation of illumination. Thus there are two reactions of CO2 to be 

 distinguished : the binding of CO2, and the transformation of bound CO2 into the 

 photolyte. In the case of optimally cultured cells, these two reactions of COj may 

 be completely separated, whereas the O2 development and the binding of CO2 

 take place simultaneously and equally. In the less active cells the two reactions of 



