Mechanism of passive permeation of ions and molecules through plant membranes

The Gibbs free energy of ion and molecule transfer ΔG(tr) from the aqueous phase to a hydrophobic part of a biomembrane can be calculated as a sum of all contributions ΔG(tr) = ΔG(el) + ΔG(hph) + ΔG(si), where ΔG(el) is electrostatic contribution, ΔG(hph) is the hydrophobic effect, and ΔG(si) is determined by specific interactions of the transferred particle (ion, dipole) with solvent molecules, such as hydrogen bond formation, donor-acceptor, and ion-dipole interactions. The electrostatic component of the Gibbs energy of ion transfer from medium w into the medium m was found from conventional Born expression corrected for the image energy in a thin membrane. The hydrophobic contribution to the Gibbs free energy of solute resolvation with surface area S can be calculated using the equation, ΔG_s = -N_ASγ, where γ is the surface tension in the cavity formed by the transferred particle in the media and N_A is the Avogadro's number. A significant point is that the free energy of the hydrophobic effect is opposite in sign to the electrostatic effect. As a result, the sum of electrostatic and hydrophobic components of the Gibbs free energy decreases with a solute size, so that ΔG(tr) > 0 only for small ions. The specific energy of ion/dipolar layer interaction depend on the dipolar membrane surface potential φ_s as ΔG(si) = -zFφ_s, where ze is the charge of ions and F is the Faraday constant. These calculations yielded the permeability of different ions and neutral molecules through plant membranes in good agreement with experimental data.