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. 2002 Apr;119(4):341–354. doi: 10.1085/jgp.20028565

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Copyright © 2002, The Rockefeller University Press

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Figure 5. — Diagram of the energy profile of Ca²⁺ and Na⁺ permeation through CNG ion channels of rods and cones. The rate model hypothesizes there exist two binding sites (energy wells G2 and G3), separated from each other and the cytoplasmic and extracellular surfaces by energy barriers (G12, G23, and G34). Each of the energy minima and maxima are located at defined positions along the electrical thickness of the membrane (D12, D2, D23, D3, D34) between 0 (extracellular membrane surface) and 1 (cytoplasmic). Amplitudes of wells and peaks are computed in units of RT. The absolute values of peaks and wells are somewhat arbitrary since they depend on the frequency factor value selected (see text and ). However, the relative thermodynamic differences between ions in a given channel, or between channels for a given ion are informative since these differences will be the same regardless of the frequency factor value.