Water Permeation Measured by Solvent Isotope Effects

Abstract

In the following sections methods to measure the passive water permeation across biological membranes will be discussed together with some results obtained. The systems considered always consist of vesicles surrounded by lipid bilayers or single cells surrounded by a plasma-membrane. Thus the membrane separates the aqueous intracellular from the extracellular compartments. If τ is the mean lifetime (= 1/k_ex with k_ex the exchange relaxation rate) of water-molecules in the intracellular compartment the permeability coefficient, P_d, is given by P_d=V/A·1/τ, where V/A is the volume to surface ratio of the vesicles or cells. With the exception of rather specialized cases (e.g. epithelial cells (Parisi et al. 1985)) the barrier for the transfer of water molecules across lipid bilayers or plasma membranes is rather low and, in particular, water channels are usually absent.