108 The Maximum Efficiency of Photosynthesis 



sion, and the spectral region is strongly absorbed by the cells, is a correct measure- 

 ment possible. Thus, as a compromise, diffuse white light whose intensity was not 

 measured was used to compensate or overcompensate the respiration, and then 

 we measured the efficiency of additional photosynthesis induced by a relatively 

 narrow beam of red light of measured intensity. 



This section may be concluded with an equation that clarifies the advantageous nature of the 

 intermittency of the illumination in rapidly shaken, dense cell suspensions. If cells are illuminated 



quanta 

 continuously by light of the intensity i , 



i x is given by the equation 



, then the equivalent intermittent intensity 

 minutes < cm. J 



In = ly 



Jfl+ Ati 



where .\t\ is the time of illumination and . Ifj is the time of darkness. — — j— may be called 



the "intermittency factor". It is the fraction of time the cells are illuminated in a rapidly shaken 

 dense cell Suspension. It is an oversimplification, of course, to assume that a given cell alternates 

 only between the füll incident intensity i x and complete darkness, whereas all the intermediate 

 intensities actually occur. 



The intermittency factor is the smaller, the smaller the illuminated volume v light of the cell 

 Suspension and the greater the dark volume v dark of the cell Suspension. Approximately, 



dt! _ v üght [4] 



Jfl + Jr 2 V H g ht + ^dark' 

 and when we insert [4] in [3] we obtain 



v light [5] 



Ix X 



1 light + V dark 



an equation which allows one to estimate i X) approximately, from magnitudes that can all be 

 easily determined experimentally. 



3. Compensating Light Intensity 



Chlorella in optically thin suspensions may be illuminated with such a light inten- 

 sity (which depends upon the temperature of the cells and the wavelength of the 

 light) that the 2 production of photosynthesis equals the O2 consumption of 

 respiration. This is the "compensating" light intensity for CK The same light 

 intensity compensates the CO2 production of respiration only if the respiratory 

 and the assimilatory quotients, CO2/O2 are equal. As a rule they are not exactly 

 equal and so the compensating light intensities for O2 and CO2 are not exactly 

 equal. This can easily be demonstrated in experiments with the twovessel method. 

 When the pressure change in one vessel is brought to zero by a certain light inten- 

 sity, the pressure change in the other vessel is as a rule not brought to zero, but 

 may be either somewhat positive or negative. Generally "compensating intensity" 

 in the following means compensating intensity with respect to O2. 



If the O2 consumption of a dense cell Suspension is compensated by light, it is 

 possible that in a part of the cells respiration is overcompensated and in another 

 part undercompensated. But if the Suspension is so rapidly shaken that the incident 

 light intensity is evenly distributed over all the cells, then when no O2 enters or 

 leaves the cell Suspension as a whole, no O2 enters or leaves any cell in any part of 

 the Suspension. This State is therefore a State of true, not of apparent compen- 



