19.3 Passive Transbilayer Diffusion

Passive Transbilayer Diffusion describes the movement of solutes directly across a lipid bilayer without the assistance of any embedded transport protein, driven purely by the solute's own physical properties and the concentration gradient it experiences. It covers the step-by-step physical process a solute undergoes to cross the hydrophobic bilayer core, which solute categories are able to cross readily, and the quantitative measures used to describe the rate at which this unassisted crossing occurs.

Basic Directional Movement

Direct Bilayer Permeation

Direct bilayer permeation is the general phenomenon of a solute crossing the lipid bilayer without protein assistance, representing the overarching process that the more specific steps and solute categories in this branch elaborate on.

Passive Solute Entry

Passive solute entry is direct bilayer permeation occurring in the direction from the external medium into the compartment lumen, driven by a driving force favoring inward movement.

Passive Solute Exit

Passive solute exit is direct bilayer permeation occurring in the direction from the compartment lumen into the external medium, driven by a driving force favoring outward movement.

Solute Categories Capable of Passive Diffusion

Hydrophobic Molecule Diffusion

Hydrophobic molecule diffusion describes the passive crossing behavior of molecules that favor the bilayer's hydrophobic interior, generally permeating readily due to their chemical compatibility with the membrane core.

Small Neutral Molecule Diffusion

Small neutral molecule diffusion describes the passive crossing behavior of small, uncharged molecules, which tend to cross the bilayer relatively easily due to their limited size and lack of a charge barrier.

Gas Transbilayer Diffusion

Gas transbilayer diffusion describes the passive crossing behavior of dissolved gas molecules, which typically permeate the bilayer with particular ease given their small size and generally nonpolar character.

Weak Acid Membrane Permeation

Weak acid membrane permeation describes the passive crossing behavior of weak acid molecules, whose permeation rate depends on the proportion of the molecule present in its uncharged, protonated form under given pH conditions.

Weak Base Membrane Permeation

Weak base membrane permeation describes the passive crossing behavior of weak base molecules, whose permeation rate similarly depends on the proportion of the molecule present in its uncharged form under given pH conditions.

Charged Solute Passive Exclusion

Charged solute passive exclusion describes the strong resistance charged, ionic solutes present to direct bilayer permeation, since the energetic cost of moving a charge through the hydrophobic core is generally prohibitive without protein assistance.

The Physical Steps of Crossing

Solute Partitioning into Membrane

Solute partitioning into membrane is the initial step of passive diffusion in which a solute transitions from the aqueous phase into the outer region of the bilayer, governed by the solute's relative chemical compatibility with water versus the membrane interior.

Solute Diffusion through Hydrophobic Core

Solute diffusion through hydrophobic core is the intermediate step in which a partitioned solute moves through the bilayer's hydrophobic interior from one side toward the other, driven by random thermal motion combined with any relevant concentration gradient.

Solute Release from Opposite Leaflet

Solute release from opposite leaflet is the final step of passive diffusion in which a solute exits the bilayer interior into the aqueous phase on the far side, completing its crossing from one side of the boundary to the other.

Quantifying Passive Diffusion

Passive Permeability Coefficient

Passive permeability coefficient is a quantitative measure of how readily a specific solute crosses a bilayer through passive diffusion, incorporating the combined effects of partitioning and diffusion through the hydrophobic core into a single rate-related value.

This relationship expresses the net solute flux across the membrane as the product of the permeability coefficient and the concentration difference between the external medium and the lumen, providing the quantitative link between a solute's inherent permeability and its actual observed rate of passive movement.

Passive Diffusion Rate

Passive diffusion rate is the actual observed speed at which a given solute crosses the bilayer under specific conditions, determined by the combination of its permeability coefficient and the prevailing concentration difference.

Passive Diffusion Direction Reversal

Passive diffusion direction reversal describes a change in the net direction of a solute's passive movement, occurring whenever the underlying concentration or electrochemical gradient driving diffusion changes sign.

Inherent Limitation

Bilayer-Mediated Selectivity Limit

Bilayer-mediated selectivity limit describes the constraint that passive diffusion alone provides only a crude form of selectivity, based primarily on solute size and hydrophobicity, and cannot achieve the fine-grained, chemically specific selectivity that protein-mediated transport mechanisms are capable of providing.