TRANSIENT CHANGES IN GAS EXCHANGE 107 



If the quantum requirement for oxyfren |)rodurtion was calculated 

 from the maxima in light and darkness, values of 3 or 4 quanta per 

 oxygen were commonly obtainable. The evidence indicated, however, 

 that these maxima were probably not representatives of synthesis of 

 carbohydrate from carbon dioxide and water, but were more likely the 

 result of changes in the size of pools of metabolic intermediates. 



Calculations based on steady-rate gas exchange led in general to 

 maximum efficiencies which represented no less than about 8 or 10 

 quanta per molecule of oxygen. 



Discussion 



Emerson : I shoiilil like to ask Professor Rrown whether the mass spectrometer 

 has confirmed the maxima for oxygen? 



Brown : I have never seen the O2 peak in the light. Although, when Whittingham 

 and I were doing this work, we looked at the CO2 much more than at the oxygen, 

 we never observed it for oxygen. 



Emerson : I believe that while it may soon be superseded by better and more 

 convincing methods, the evidence from manometric measurements is indicative 

 that for a few moments in light there may be a peak in oxygen production. 



Brown : I have two questions. First, can you specify conditions under which you 

 can reproduce the oxygen burst? I have had the impression that j'ou can write the 

 prescription for the CO2 burst, but not nearly so easily for the oxygen burst. Is 

 this correct? 



Emerson : You are right. We have less experience with the ox3^gen burst. I don't 

 think, though, that it would be difficult for us to write the prescription for it as 

 precisely as we have for the carbon dioxide burst if we gave a little more attention 

 to it. 



Brown: My other question is, what is the half-time for equilibration for a 

 single gas under conditions of shaking, and so on, in these experiments j^ou showed 



us 



«? 



Emerson : If we don't put cells in the vessel, just phosphate buffer and gas, our 

 best measurements indicate that the half-time is around 15 seconds. With the cell 

 suspension, however, the half-time appears to be 55 seconds or so. Part of this 

 may be a matter of induction, although I am inclined to doubt it. I think for the 

 the most part this 55-second half-time represents the sum total of the physical 

 lag of the system. 



Brown: Have you tried it with dead cells or with poisoned cells, so that it is 

 strictly a physical measurement you make? 



Emerson : Yes, it has been done. The lag remains nearer 55 than 15 seconds. 



Gaffron : You mean this minute is not the total time but the half-time? 



Emerson : Yes. It is a surprisingly long half-time. 



Gaffron : I think you have reported that there is a bvnst of oxygen but also a 

 quick uptake of CO2. So we have a burst of photosynthesis. Considering the half- 



