Determination of cell membrane permeability in concentrated cell ensembles

The method of volume averaging is used to analyze the process of diffusion in concentrated cell ensembles in which significant resistance to mass transfer is caused by the cellular membrane. A general closure scheme is given that allows for direct theoretical prediction of effective diffusivities for any cellular geometry. Numerical results are presented for the classical parallelepiped arrangement used to model cellular systems, and these results are used in conjunction with experimental studies of concentrated cell ensembles to determine membrane permeabilities for solute diffusion in several cellular systems. Membrane permeabilities are compared with predictions from other models of diffusion in cellular systems.

Nonequilibrium facilitated transport of carbon dioxide in bicarbonate and bovine albumin solutions

The nonlinear diffusion-reaction equations describing the nonequilibrium transport of CO2 through flat layers of complex but homogeneous aqueous media were solved by an approximate analytical method called the "Combined Damköhler Number" (CDN) technique. Unlike other approximate analytical solutions, the CDN technique is valid for the full range of Damköhler numbers, i.e., for any layer thickness. The present theoretical treatment uses as a basis the equilibrium approach of Stroeve, Hoofd, and Kreuzer which accounts for any species in the solution except for possible carbamate formation (binding of CO2 by the protein). The nonequilibrium model developed here for CO2 transport is the most general technique currently available in the literature. Theoretical results were compared to experimental data from the literature for diffusion in bicarbonate and albumin solutions and were generally in good agreement. Results obtained from numerical calculations were also compared and were found to be in excellent agreement with the CDN results.

Facilitated carbon dioxide transport in bovine albumin solutions

Steady-state CO2 diffusion in the presence of a CO2 gradient was measured in thin layers of bovine albumin solutions containing different amounts of buffer base, added as NaHCO3, and/or of NaCl. In the same solutions, electrical potentials due to the CO2 diffusion across the layers were measured. Addition of carbonic anhydrase induced a chemical reaction equilibrium to exist for the CO2 reaction system, and led to equilibrium values for facilitated CO2 transport due to a bicarbonate flux and to maximum values for the diffusion potential. The diffusion potentials are generated due to the large differences in the ionic mobilities of albumin and other ionic species such as bicarbonate. The diffusion potential markedly reduces the facilitated CO2 flux. The presence of sodium chloride had no significant effect on the CO2 transport rate. The total mass transfer rates of CO2 in albumin solutions were considerably lower than those found by Stroeve and Ziegler (23) in hemoglobin solutions at identical concentrations of buffer base.

Lateral diffusion of cholesterol in monolayers

The surface diffusion coefficient of cholesterol in cholesterol monolayers has been measured as a function of cholesterol surface concentration. Two different radiochemical methods, one integral and the other differential, were developed which gave comparable results. In the integral method two cholesterol monolayers, one of which is radioactive, are isolated on inert hydrophilic supports and then brought into contact. After some time the supports are separated and the radioactivity of the supports is measured. The differential method is an autoradiographic experiment. Two cholesterol monolayers, one of which is radioactive, are separated by means of a thin barrier. Upon removal of the barrier and at later times, an autoradiographic plate is brought to within a fraction of a mm from the aqueous surface and exposed. The plates are developed and analysed. The data show that the cholesterol surface diffusion coefficient in the dilute monolayers is approximately 10(-6)cm2/s and is nearly independent of surface concentration up to a concentration corresponding to an area of 40 A2/molecule. As the monolayer becomes compressed beyond this surface concentration, the diffusion coefficient decreases ubruptly with the deeply decreasing surface tension to about 10(-7) cm2/s, when a fully condensed surface layer of 38 A2/molecule is reached. This diffusion coefficient is of the same order of magnitude as the diffusion coefficients measured in lipid bilayers and in membranes.