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Lipid Bilayer Permeability
All living cells must be able to exchange materials (nutrients and waste products) with their external environments in order to remain alive. Because the phospholipid bilayer is responsible for forming membranes and hence compartments, it is important to understand how various molecules can pass through this lipid bilayer. Simply stated, biological membranes are semipermeable lipid bilayers. Permeability refers to the ease with which molecules cross biological membranes. Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic core), only lipid-soluble molecules and some small molecules are able to freely pass through the lipid bilayer. Ions and large polar molecules cannot pass through the lipid bilayer. But more specifically, whether a molecule can pass through the membrane depends on its size and its electrical nature. The membrane is highly permeable to non-polar (fat-soluble) molecules. The permeability of the membrane to polar (water soluble) molecules is very low, and the permeability is particularly low to large polar molecules. The permeability to charged molecular species (ions) is very low. Therefore, the passage of most molecules and ions is aided by the presence of specific membrane transport proteins.
The proper way to state these features is to say that the membrane is highly permeable to lipid-soluble molecules, or that the membrane is not permeable to ions. It may also be said that membrane permeability is high for lipid-soluble molecules, and that membrane permeability is low for ions and polar molecules. Another way of stating this is that lipid-soluble molecules are highly permeant, or that ions are impermeant (i.e., not permeant).
Permeation through a pure lipid bilayer.
Figure 1. Permeation through a pure lipid bilayer.
Only a limited number of molecules can cross biological membranes without the aid of transport proteins. Membrane impermeant molecules and ions require the aid of membrane transport proteins in order to cross the membrane. See text for details. O2, oxygen; CO2, carbon dioxide, N2, nitrogen, H2O, water; Na+, sodium ion; K+, potassium ion; H+, proton; Ca2+, calcium ion; Cl-, chloride ion.
Figure 1 summarizes the permeability properties of pure lipid bilayers. Lipid-soluble molecules can readily pass through a lipid bilayer. Examples include gas molecules such as oxygen (O2) and carbon dioxide (CO2), steroid molecules, and fat-soluble vitamins (A, D, E, and K). Surprisingly, some small polar molecules are capable of permeating the lipid bilayer without the aid of a membrane transport protein. Examples include water (H2O), glycerol (C3H5(OH)3), urea (CH4N2O), and ethanol (C2H6O). Membrane permeability to large polar molecules is very low. Ions are not membrane-permeant.

Posted: Monday, January 17, 2011
Last updated: Saturday, November 17, 2012