Proton transport electrostatic activation energy

Near the surface, AG is dominated by the Coulomb energy profile and, therefore, it is approximately equal to the difference of the electrostatic potentials at the proton positions before and after the transfer. This difference depends strongly on the distance of the proton from the surface. Values of AG were found in the range of 0.5 eV. This value decreases, however, to the activation energy of proton transport in bulk water when the proton-surface separation exceeds 3 A (the thickness of one monolayer of water). Moreover, the electrostatic activation energy is a function of the separation between surface charges, which lies in the range of 7 to 15 A. 

Several energy sources are possible for active transport, including electrostatic or pH gradients of the proton motive force (PMF), secondary gradients derived from the PMF by other active transport systems, and by the hydrolysis of ATP. The development of these ion gradients enables the cell to store potential energy in the form of these gradients. 

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