ENERGY EFFICIENCY IN PHOTOSYNTHESIS 287 



changes in techniques. They continue to report 4 photons and sometimes 

 even 3 photons per molecule. Most of their recent work has been done 

 with the excellent ethyl chlorophyllide actinometer, which simplifies the 

 experiments and gives a reUable check on the absolute measurements of 

 the light intensity. Using a transmission actinometer in the form of a 

 vessel of ethyl chlorophylhde solution surrounding the reaction cell, they 

 have been able to use thinner algal suspensions that transmit some of 

 the light. These conditions are much better than those of their earlier 

 experiments in which thick algal suspensions were used so as to absorb 

 all the light. In the thick suspensions the conditions of exposure range 

 all the way from full light intensity to complete darkness, and the shaking 

 gives rise to the complication of unknown, intermittent exposures to dark 

 and light. Moreover, in the thick suspensions, respiration overbalances 

 photosynthesis. 



Another change involves the use of a constant, unmeasured light 

 exposure to offset respiration and the superposition of monochromatic 

 Ught of measured intensity. This use of two light beams was tried also 

 by Moore and Duggar (1949). 



In this way the extra molecules of oxygen evolved per photon of extra 

 light absorbed permit a calculation of the quantum yield. The intensity 

 of the background light and the quantum requirement in this light are 

 unknown, and it must be assumed that they fall in the region of linearity 

 on the Blackman curve. In the dark-light experiments it must be 

 assumed that respiration continues in the fight at the same rate as in 

 the dark. In the light-brighter light experiments it must be assumed 

 that respiration and the quantum requirement of photosynthesis both 

 remain constant. 



A new development in the laboratories of Warburg and of Burk is the 

 reporting of evidence for a 1-quantum process. Certainly, everyone 

 would agree that the primary photoactivation process must be a 1-photon 

 process, but the time lags following changes from dark to light, as already 

 described, are such that they would seem to make difficult the determi- 

 nation- of significant quantum-requirement measurements in periods of 

 1 min dark and 1 min light, such as have been used in these experiments 

 in which a 1-quantum process has been reported. As yet, no 1-quantum 

 results have been reported by Warburg or Burk in alkaline solutions, 

 nor have there been checks to date by other laboratories in acid solutions. 

 They are, of course, thermodynamically impossible as a continuing 

 process. 



Evans (1951) has determined both oxygen and carbon dioxide with 

 methods that are specific for the two gases. He found a quantum require- 

 ment of 9-14 photons per molecule, as calculated for oxygen and inde- 

 pendently and simultaneously for carbon dioxide. The experimental 



