The Maximum Efficiency of Photosynthesis 115 



10. The Measarement of Quantum Requirements 



In each of two rectangular vessels of unequal total volume, bearing capillary vents, were placed 

 300 cu. mm. of Chlorella, suspended in 7 ml. of culture medium at pH 4.9 — 5.1. The vessels were 

 connected to simple Haldane-Barcroft blood-gas manometers (capillary diameter 0.8 mm.) and 

 aerated in the thermostat, usually at 20C, with 5",, COj in air, with a low shaking rate at first. 

 The shaking motion was horizontal with an excursion of 2 cm. After equilibration was reached and 

 the vents closed, the shaking was speeded up to 150 min. and some 15 min. later the first mano- 

 metric readings were taken. Foaming never occurred, even in experiments of more than 24 hr. 

 duration. Splashing occurred only when by accident a manometer was too loosely fastened and 

 jerked suddenly during horizontal motion. 



In all experiments the measured light, the light increment \J, was of the spectral region 

 630 — 660 vn.fi, isolated by the monochromator. The beam of this red light entered the thermostat 

 in a horizontal direction through a lateral double glass window (Thermopane, to eliminate moisture 

 condensation), and was reflected by a 45 mirror vertically into a manometer vessel. The mirror 

 could be moved horizontally in the thermostat, reflecting the light alternately into the one or the 

 other vessel. In both positions of the mirror the intensities of the light beam at the bottoms of the 

 vessels were sufficiently equal, if the water in the thermostat was clean. Small inequalities were 

 eliminated, if necessary, by interchanging the positions of the manometers. 



The subsequent procedure was different depending on whether the experiments were carried 

 out with uncompensated or compensated respiration. In all experiments with uncompensated 

 respiration the mirror was shifted, and thus the gas exchange in both vessels was obtained without 

 interruption, the one vessel being in the dark when the other was in the light. The periods of dark 

 and light varied in these experiments from 5 — 60 min. and the duration of the entire experiments 

 did not exceed several hours after which the light action usually began to decrease along with the 

 respiration. Owing to the rapid shaking, physical manometric transition effects of equilibration 

 were not ordinarily observed when dark cells were illuminated, and vice versa, even when the 

 readings were taken every 1 or 2 instead of 5 or 10 min. 



In the experiments with compensated respiration the method of shifting the mirror was not 

 usually applied. In a thermostat diffusely illuminated by the compensating white light, shifting 

 of the mirror could alter the distribution of the white light, and the two vessels even if equally 

 illuminated by J before the shifting might not be after a shifting. Thus only one position in the 

 thermostat could be used for the measurement of the \J light action, if essentially equal J values 

 for each vessel were desired.* Hence, first one vessel was illuminated alternately with the intensities 

 J and J 4- 1 J, and in succession the second vessel was brought to the same position and illuminated 

 in the same manner. In the meantime the vessel not receiving \J was shaken in compensating 

 white light (J), but not necessarily read manometrically. These experiments with compensated or 

 overcompensated respiration could be extended for a long time without decrease of efficiency. 

 If after 24 hr. of continuous shaking and illuminating with J the efficiency began to decrease, transfer 

 of the cells to fresh culture medium raised the efficiency again to the initial value. 



When in the efficiency experiments, with or without compensated respiration, the light intensity 

 AJ effected in t minutes the pressure changes H in the one vessel and H' in the other, vo 2 and 

 yco%, the gas exchanges effected by \J t were calculated from H and H' by eqs. 9 and 10 and 

 obtained in cu. mm. of gas. \J was then determined manometrically. The manometer vessels, 

 containing the cell Suspension, were replaced by a similar vessel containing, instead of the 7 ml. 

 of cell Suspension, 7 ml. of the actinometric Solution. Two such vessels may be used, the one to be 

 placed in the red light beam and the other as a control in the "dark." When in t minutes of Illumi- 

 nation \J effected the pressure change h, \J was calculated by Eq. 12 and obtained in cu. mm. of 

 O^/min. 



The quotient of the two oxygen exchanges, computed for the same time, is the 

 quantum requirement per molecule of oxygen: 



1 Oj absorbed by the actinometer \J x t 



O2 produced by the cells .V02 



2 



[14] 



* Since experience eventually showed that the action produced by \J was independent of J over 

 wide limits, it was not strictly necessary for J to be equal for both vessels, and in certain types of 

 experiments shifting of the mirror from one vessel to the other was permissible, if distribution 

 of the white light was not significantly altered. 



