HO The Maximum Efficiency of Photosynthesis 



experiment were definitely positive, then the efficiency of photosynthesis remained 

 constant for even more than 24 hr. Compensation of respiration therefore made 

 long experiments possible. 



2. Compensation does not eliminate respiration chemically; but it eliminates 

 respiration manometrically, and thus removes many of the difficulties of efficiency 

 determinations. Positive pressure changes can be produced in the desired ränge. 

 They are more constant than negative pressure changes and when the light actions 

 are obtained as differences of two positive pressure changes at two intensities, the 

 accuracy of the manometric method is improved. 



3. Although it has been shown that light does not inhibit respiration, the theore- 

 tical possibility still remained that the process of photosynthesis might inhibit res- 

 piration. Thus the old problem, of whether respiration in the dark and during 

 photosynthesis is the same, was still important in experiments below the compen- 

 sation point. But the problem no longer exists when efficiencies are determined as 

 differences of two positive pressure changes at two light intensities. For then, at 

 the lower intensity photosynthesis has already had occasion to interfere with res- 

 piration, leaving no further respiration for the higher intensity to interfere with. 



4. Thermodynamically, efficiency determinations below the compensation point 

 were uncertain because below this point chemical energy is not gained, but only 

 the loss of chemical energy is inhibited. But when in experiments above the com- 

 pensation point we obtain the same quantum requirement as below this point, it is 

 proved that one molecule of Oz produced or one less molecule of O2 consumed are 

 thermodynamically equivalent. 



5. Let us consider a Chlorella Suspension to be a machine that transforms light 

 energy into chemical energy. The maintenance of this machine requires an expen- 

 diture of energy that is defrayed by respiration and amounts to 112,000 cal./mole 

 of O2 consumed. If we substract this amount from our gain of energy, we obtain 

 what may be termed the "economic efficiency," E', as distinguished from the 

 "thermodynamic efficiency," E. 



When, for example, the quantum requirement is 4, the thermodynamic efficiency in red light is 



112,000 



E = X 100 = 65%, 



4 43,000 



but the economic efficiency is , ^ 



E' = Ex ■ 



where/is the "compensation factor", 



oxygen developed in light 



/ = 



oxygen consumed in the dark 



At the compensation point we have/ = 1, and therefore E' = 0. 



At 6 times the compensation point (/ = 6), where we still obtain a thermodynamic efficiency 



of 65%, the economic efficiency is 



/— 1 5 



E' = 65 —— = 65 x - = 54%. 

 / 6 



At 10 times the compensation point (f = 10), if the thermodynamic efficiency were still 65%, 



the economic efficiency would be 



/— 1 9 



E' = 65 -— - = 65 x — = 59%. 



