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KeywordsSmall Molecule Outer Membrane Bioorganic Chemistry Outer Layer Cellular Function
The outer membrane is a phospholipid- and polysaccharide-containing membrane that lies external to the peptidoglycan layer of gram-negative bacteria. This layer is effectively a second lipid bilayer, but in addition to phospholipids and proteins found in the cytoplasmic membrane, it contains polysaccharides. It is a rather complex structure only found in gram-negative bacteria. The lipid and polysaccharides are linked to form a lipopolysaccharide complex. For this reason, the outer membrane is often called the lipopolysaccharide layer (LPS). In the outer membrane, LPS associates with several proteins to form the outer leaflet of the membrane. Lipoproteins are also found in the inner leaflet of the outer membrane and their function is to anchor the membrane to the peptidoglycan. In the outer part of the membrane lipopolysaccharides replace phospholipids. Thus, although the outer membrane can be considered a lipid bilayer, its structure and function is quite distinct from that of the cytoplasmic membrane. Although the major function of the outer membrane of gram-negative bacteria is structural, its components can produce toxicity to animals. The outer membrane has special types of proteins called porins, which are channels responsible for the permeability of small molecules. The periplasm is the space delimited by the outer layer and the cytoplasmic membrane of the gram-negative bacteria in which different important cellular functions are performed.