TRANSIENTS FOR CARBON DIOXIDE 



441 



Brown : This is a specific case of a general statement, that if acid of any kind, 

 other than carbon dioxide, is produced or consumed, the pH will change. 



Gaffron: Precisely. 



Frenkel : You also have to consider that phosphorylation by itself will produce 

 chemical changes. 



Gaffron : This depends at what pH you have carbonates in the cell which de- 

 compose in order to give carbon dioxide. 



LIGHT 



WWxVDARkxWX ^^ 



20,000 



300 400 500 600 



1 TIME IN SECONDS 



light olt 



700 1900 2000 2400 2500 



t 

 light on 



Fig. 9. Light-dark changes in concentrations of PGA and RuDP (after Bassham, 



Shibata, and Steenberg). 



Bassham : I have some data on the specific carbon compound that relate very 

 closely to what Dr. Gaffron has been talking about. Unfortunatelj', I did not 

 foresee that they would be as important in this context as they are. Just some 

 curves on specific compounds that explain these results. Figure 9 shows two curves 

 which might explain these results. I won't explain the system except to say that 

 it is doing steady state photosynthesis with constant specific activity. The light 

 and dark periods are marked. The upper curve shows changes in the PGA level. 

 On turning off the light, it comes up as shown to a new high level. Turning the 

 light on again makes it go down first, then return to its initial level in the fight. 

 The lower curve is ribulose diphosphate. On turning the light off, its level falls 

 rapidly after a momentary small rise, and then it practically disappears. In the 

 light again, the level of ribulose diphosphate starts to rise but rather slowly. 



What does this mean? It simply means that in the light we have a steady-state 

 concentration of PGA. On turning off the light the PGA continues to be formed by 

 the carboxylation of the ribulose diphosphate. We get two molecules of PGA for 

 each ribulose used up. The initial slope of this curve is twice the rate of entry of 

 CO2 into the photosynthetic cycle, proving that you do get two molecules of 

 PGA per carbon dioxide going in. The increase in the citric acid cycle intermediates 



