This figure summarizes the permeability properties of pure lipid bilayers. Biological membranes are composed mainly of lipids and proteins. The phospholipid part of the membrane, the lipid bilayer schematic shown in this figure, only allows the passage (i.e., permeation) of a limited number of molecules. Lipid-soluble (i.e., fat-soluble) molecules and some small polar molecules can readily permeate (i.e., pass through) the lipid bilayer. They are referred to as membrane-permeant molecules. Alternatively, it can be said that the membrane is permeable to these molecules. Examples include gas molecules such as oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂); steroid molecules; and fat-soluble vitamins (A, D, E, and K). Surprisingly, some small polar molecules are also 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). On the other hand, ions and large polar molecules cannot pass through the lipid bilayer. They are referred to as membrane-impermeant. Alternatively, it can be said that the membrane is impermeable (i.e., not permeable) to ions and large polar molecules. Therefore, ions and large polar molecules require the aid of membrane transport proteins in order to cross the membrane. Example of ions of physiological importance are sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), proton (H⁺), and chloride (Cl⁻). Examples of polar molecules include glucose, fructose, amino acids (glycine, glutamate, alanine, valine, cysteine, tyrosine, etc.), water-soluble vitamins, dipeptides, tripeptide, etc. Peptides and proteins most commonly utilize mechanisms such as endocytosis or exocytosis to cross biological membranes.

See also:
Lipid Bilayer Permeability (in Lecture Notes)
Impermeable (in Glossay)
Impermeant (in Glossay)
Permeability (in Glossay)
Permeable (in Glossay)
Permeant (in Glossay)