94 The Maximum Efficiency of Photosynthesis : A Rediscovery 



7 cm. 3 of pyridine, with tank oxygen as the gas phase. The light absorption being 

 complete in the cell Suspension as well as in the ethylchlorophyllide Solution, the 

 quantum requirement per molecule oxygen was obtained by the very simple 



equation : 1 h • v O2 consumed by the actinometer, 



~ O2 O2 produced by the cells 



where the consumption and the production of the oxygen must be calculated for 

 equal time periods. 



Complete absorption is the only method so far devised by which the absorption of 

 the light and the action of the light can be measured manometrically under the 

 same conditions. Thus far all manometric determinations of quantum requirements 

 using incomplete absorption are contradictory and uncertain. On the other hand, 

 the main objection against the method of complete absorption was the existence 

 of too great a respiration relative to the measured light action; this objection 

 is no longer valid because today respiration can be eliminated by compensating 

 with white light or components thereof (see the following section). 



A New Principle 



Let us consider a Chlorella Suspension, shaken in a beam of red light that is absorbed 

 completely, to be a machine that transforms light energy into chemical energy. The 

 efficiency of this machine will be known if we know the light energy entering the 

 vessel and the amount of oxygen developed in it, one mole of oxygen being equi- 

 valent to the production of about 112,000 cal. Notheory ofthe mechanism of this 

 energy transformation can alter the observed result of such an efficiency determi- 



nation. 



Yet there are possible objections to be answered. When at low light intensities 

 respiration still exceeds photosynthesis, then our machine does not produce a 

 net gain of chemical energy, and it might be considered that the light merely inhi- 

 bits the loss of chemical energy. Since respiration is a catalytic process. conceivably 

 its inhibition by light could be merely anticatalytic, requiring no expenditure of 

 energy. Hence the calculation of the efficiency might be safe thermodynamically 

 only when the light intensities are so high that oxygen is in fact given off from the 

 cell suspensions into the gas space. 



To comply with this requirement we illuminated the cell suspensions with white 

 light (of nonmeasured intensity) over the greater part of the vessel surface, the 

 light intensity per unit area being relatively small, but the influx of light energy 

 through the total area ofthe vessel being sufficient to compensate or overcompen- 

 sate the respiration ofthe cell suspensions. As the compensating light intensity we 

 define here that light intensity which effects the result no oxygen enters or leaves 

 the cell Suspension. It is very likely that in this State the oxygen exchange of all 

 cells in the Suspension is zero, owing to the rapid motion ofthe cells. It is obvious 

 that our compensating light intensity varies with the amount of cells in the vessel 

 and is not to be confused with the compensating light intensity for very thin cell 

 suspensions. 



