Key Points

• Biologic membranes are composed of a phospholipid bilayer containing peripheral and integral proteins.

• Diffusion of nonelectrolytes takes place from a higher concentration to a lower concentration. The rate of diffusion across a membrane is a linear function of the concentration difference.

• The permeability coefficient for diffusion of a solute through the lipid matrix of the membrane increases with increasing solubility.

• Water-soluble solutes cross membranes by restricted diffusion though water-filled protein channels. The permeability coefficient for diffusion of uncharged solutes through channels decreases with increasing molecular radius.

The permeability coefficients of ions depend on radius and charge.

• The rate of diffusion of charged solutes depends on both the concentration gradient and the electrical potential difference across the membrane.

• Diffusion potentials arise to preserve electro-neutrality in the face of differences in ion mobility.

• The orientation of a diffusion potential is such that it accelerates the movement of less mobile ions and retards the movement of more mobile ions.

• The Nernst potential is the electrical potential difference at which there is no ionic flow in

Essential Medical Physiology, Third Edition

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