At pH 7, tryptophan crosses a lipid bilayer at about...

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Key Concepts

Lipid Bilayer Permeability

Lipid bilayers consist of hydrophobic core regions that impede the passage of polar and charged species. Understanding the structural features of the bilayer is key to explaining why molecules with significant hydrophilic or ionic character struggle to traverse this barrier compared to more hydrophobic, uncharged molecules.

Effect of Molecular Charge and Ionization

The charge state of a molecule, which is influenced by the pH of its environment, plays a crucial role in its ability to permeate biological membranes. Molecules that are ionized, or exist as zwitterions at physiological pH, interact unfavorably with the nonpolar interior of lipid bilayers, resulting in significantly reduced rates of passive diffusion.

Hydrophobicity and Partition Coefficient

The partition coefficient is a measure of a molecule's preference for hydrophobic versus aqueous environments. Molecules with higher hydrophobicity more readily dissolve into the lipid membrane, facilitating faster permeability. Comparatively, molecules with lower hydrophobicity, or those that carry charges, have a lower partitioning into the lipid phase and hence traverse the membrane at a slower rate.

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