Contribution of Membrane Potentials to Energy Conversion in Photosynthetic Membranes

Abstract

The conversion of the energy of light into the chemical energy of ATP occurs at the photosynthetic membrane with the transmembrane electrochemical gradient of protons as intermediate. The electric portion, i.e. the light‐induced change of the electric potential difference (photopotential), quickly attains a maximum of 200 mV after the onset of illumination and decreases toward the steady state for the benefit of the H+‐concentration gradient. The resulting stationary photopotential is determined by two different methods, on the one hand directly through the electrochromic absorption change of membrane pigments, on the other hand by calculating the change of the surface potential at the inner side of the membrane from the surface charge densities. The charge density in the dark state is derived from the kinetics of an electron transfer reaction in the membrane (reduction of oxidized chlorophyll‐aI) sensitive to the pH of the inner surface. Its dependence on pH is characterized by an isoelectric point of 4.8. The light‐induced change of the charges is determined from the amount of protons additionally bound to the inner membrane surface. For the calculation of surface potentials from the charge densities the conventional Gouy‐Chapman theory is modified by assuming a transverse spreading of surface charges over a layer of 20 A thickness. — The results of both the methods are similar. The stationary transmembrane photopotential is dependent on the concentration of electrolyte and exhibits a maximal value of 60–70 mV at 6 mM KCl. Under physiological conditions the contribution of the photopotential is below 20 mV and can be neglected compared to the light‐induced difference in proton concentration with respect to energetic and kinetic considerations.

Regulation of the photosynthetic electron transport during dark-light transitions by activation of the ferredoxin-NADP+-oxidoreductase in higher plants

“…The second phase reflects an higher activity of photosystem II compared to photosystem I, which is inverted during the third phase. Several possibilities of explanation exist as far as this inversion is concerned: --changes in the membranal energy distribution by the phosphorylat]on of the light-harvesting complex [2]; --the decrease of the cytochrome f reduction rate by the formation of the proton gradient [11];…”

Kinetic modelling of the proton translocating CF0CF1‐ATP synthase from spinach

“…Reversal of this proton flow takes place if in the presence of excess ATP the hydrolysis of ATP is forced to occur. During steady-state conditions and with an ionic strength above 50 mM the proton-motive force controlling the ATP synthase consists of a transmembrane pH difference only, without the contribution of an electrical potential difference [9].…”

Storage of light-driven transthylakoid proton motive force as an electric field (Δψ) under steady-state conditions in intact cells of Chlamydomonasreinhardtii

“…While in past years the notion that Dw comprised a significant fraction of photosynthetic pmf in higher plants was controversial (Vredenberg and Tonk 1975;Vredenberg and Bulychev 1976;Bulychev 1984;van Kooten et al 1986), observations by others (Huber et al 1980;Siggel 1981;Tiemann and Witt 1982;Tikhonov et al 1981;Tikhonov and Timoshin 1985) as well as more recent arguments (Kramer et al 1999) favored this position. In previous work, we have demonstrated that thylakoids from higher plants can store pmf in the form of Dw for extended periods provided that the activities of membrane-permeable counterions is relatively low and/or the proton buffering capacity of the lumen is relatively high (Cruz et al 2001).…”