The outer membrane is unlike any other membrane in nature. Its lipid bilayer structure is typical of other membranes, but the outside leaflet is made up of lipopolysaccharides rather than phospholipids. For this reason, the outer membrane is also called the lipopolysaccharide layer or LPS. The outer membrane is joined to peptidoglycan by means of lipoprotein molecules. ■ lipoproteins, p. 35
Like the cytoplasmic membrane, which in Gram-negative bacteria is sometimes called the inner membrane, the outer membrane serves as a barrier to the passage of most molecules. However, specialized proteins, porins, span the outer membrane. These proteins have a small channel that runs through them, allowing passage of certain small molecules and ions either into or out of the periplasm. Some porins are specific for certain molecules; others allow many different molecules to pass. The size of the porin channel partially determines the size of the molecule that can pass through it. In addition to the general secretory pathway already discussed, Gram-negative bacteria have additional secretion systems to translocate proteins to the outside of the outer membrane.
The outer membrane functions as a protective barrier and excludes many toxic compounds. These include certain antimicrobial medications. This is one reason why Gram-negative bacteria are generally less sensitive to many such medications.
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