When phospholipid vesicles bound to a planar membrane are osmotically swollen, they develop a hydrostatic pressure (Δp) and fuse with the membrane. We have calculated the steady-state Δp, from the equations of irreversible thermodynamics governing water and solute flows, for two general methods of osmotic swelling. In the first method, vesicles are swollen by adding a solute to the vesicle-containing compartment to make it hyperosmotic. Δp is determined by the vesicle membrane’s permeabilities to solute and water. If the vesicle membrane is devoid of open channels, then Δp is zero. When the vesicle membrane contains open channels, then Δp peaks at a channel density unique to the solute permeability properties of both the channel and the membrane. The solute enters the vesicle through the channels but leaks out through the region of vesicle-planar membrane contact. Δp is largest for channels having high permeabilities to the solute and for solutes with low membrane permeabilities in the contact region. The model predicts the following order of solutes producing pressures of decreasing magnitude: KCI > urea > formamide ≥ ethylene glycol. Differences between osmoticants quantitatively depend on the solute permeability of the channel and the density of channels in the vesicle membrane. The order of effectiveness is the same as that experimentally observed for solutes promoting fusion. Therefore, Δp drives fusion. When channels with small permeabilities are used, coupling between solute and water flows within the channel has a significant effect on Δp. In the second method, an impermeant solute bathing the vesicles is isosmotically replaced by a solute which permeates the channels in the vesicle membrane. Δp resulting from this method is much less sensitive to the permeabilities of the channel and membrane to the solute. Δp approaches the theoretical limit set by the concentration of the impermeant solute.
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