The Necessity of an Electric Potential Difference and its Use for Photophosphorylation in Short Flash Groups.

The ATP-synthesis and the electric phenomena on the thylakoid membrane have been studied under excitation of photosynthesis with short flash groups. The light induced electric potential difference has been measured by means of field indicating absorption changes around 523 nm. It is shown that upon excitation with short flash groups a certain electric potential difference required before ATP can be synthesized. Moreover it is confirmed that the ATP-synthesis accompanied by an additional flux of protons down their electrochemical potential gradient. Correlation between the number of protons which have flowed across the ATPase coupled pathway, and the number of ATP-molecules which have been formed, yields a ratio H⁺/ATP = 3. Since this ratio includes only those protons, which have interacted with the ATPase complex, it represents the lower limit for H⁺/ATP-ratios determined by any other method. The necessity of a certain electric potential difference and the fact that 3 H⁺ have to be translocated down their electrochemical potential difference before one molecule of ATP can be synthesized is consistent with the chemiosmotic postulate, that the ADP ~ P bond energy derived from the electrochemical potential difference of the proton across the coupling membrane. With respect to the alternative hypothesis for phosphorylation, the results impose grave restriction on a phosphorylation driven only by the free energy of a chemical intermediate, if this does exist at all. [ABSTRACT FROM AUTHOR]