VOL. 12 (1953) ENERGY CYCLE AND QUANTUM YIELD IN PHOTOSYNTHESIS 353 



curve already referred to on page 349, and its variable slope in the ascending portion 

 is merely an incidental expression of the slightly uneven rate of addition of the oxygen 

 from the calibrated microburette. Curve 5 is a partial recording of the fall in oxygen 

 pressure that was taken when the compensating white light was turned off, between 

 Curves 2 and 3, in order to return the oxygen pressure in the medium to ca. 0.15% 

 of saturation at i atm. O,. 



Qtiantum requirements ivith short illuminations 



As already indicated, the quantitative aspects of the photosynthetic cycle can in 

 general best be studied by using relatively short illumination periods. In Fig. 5 is shown 

 an experiment in which the quantum requirement observed was 1.8 per molecule of 

 O2 produced as positive pressure alone, uncorrected for any back oxidation reaction 

 occurring, during a 5 second illumination period with 5 /xl of red light (A = 644 m[x). 

 A conventional correction for back oxidation, made by adding algebraically the back 

 reaction observed in the first 5 seconds after turning off the red light, yielded a quantum 

 requirement of 1.45. The quantum requirement for the net gain of O 2 in the entire cycle 

 (5 seconds illumination with red light, followed by 20 seconds without red light, until 

 re-establishment of compensation) was 3.4 hvjO^. Similar experiments were also per- 

 formed for 10 and 30 second illumination periods of red light, with results summarized 

 as follows: 



Quantum requirement (hv/Og) 



Time of illumination Light reaction Light reaction Cycle 



(seconds) (Positive O, alone) Corrected for back (Net gain after return 



reaction) to compensation) 



30 2.35 2.15 3.4 



10 2.0 1.7 3.3 



5 1-8 1.45 3.4 



These results are a confirmation of the manometric results of 1950"*, with the 

 difference that the back reaction is here somewhat faster, with Chlorella cultures grown 

 somewhat differently. With the electrochemical method employed, one can fortunately 

 measure rapid photo and back reactions not possible with manometry. 



From these quantitative data, it is again evident that, for a given and appropriate 

 light intensity, the quantum requirement for the positive production of oxygen in the 

 light reaction becomes less as the time period of illumination is shortened, and falls 

 below the conventionally assigned thermodynamic limit of 2.7 ^ ; the shorter the 

 illumination period the less will be the relative effect of the back reaction in reducing 

 the observed yield of O2 during the actual illumination period itself. The net gain of 

 oxygen in the entire cycle of light and dark reactions was, however, independent of 

 the period of illumination here given. 



The rate of back oxygen consumption for the first 10 seconds after turning off the 

 red light in the foregoing experiments was of the order of three times the cell volume 

 per hour, or five times the normal rate of respiration observed in total darkness just 

 before the photosynthetic experiments described were carried out. 



References p. 355. 



