The determination of membrane transport parameters with the cell pressure probe: theory suggests that unstirred layers have significant impact

A simulation model was written to compute the time-kinetics of turgor pressure, P, change in Chara corallina during cell pressure probe experiments. The model allowed for the contribution of a membrane plus zero, one, or two unstirred layers of any desired thickness. The hypothesis that a cell with an unstirred layer is a composite membrane that will follow the same kind of kinetics with or without unstirred layers was tested. Typical ‘osmotic pulse’ experiments yield biphasic curves with minimum or maximum pressures, P_min(max), at time t_min(max) and a solute exponential decay with halftime . These observed data were then used to compute composite membrane properties, namely the parameters L_p = the hydraulic conductance, σ = reflection coefficient and P_s = solute permeability using theoretical equations. Using the simulation model, it was possible to fit an experimental data set to the same values of P_min(max), t_min(max) and incorporating different, likely values of unstirred layer thickness, where each thickness requires a unique set of plasmalemma membrane values of L_p, σ and P_s. We conclude that it is not possible to compute plasmalemma membrane properties from cell pressure probe experiments without independent knowledge of the unstirred layer thickness.