Outermembrane

Outer membrane refers to the outermost lipid bilayer found in certain cells and organelles. In biology, the term is most commonly used for the outer membrane of Gram-negative bacteria and for the outer membranes of mitochondria and chloroplasts in eukaryotic cells. It functions as a boundary and a selective barrier, while also participating in signaling and transport.

In Gram-negative bacteria, the outer membrane is asymmetric, with lipopolysaccharide (LPS) in the outer leaflet and phospholipids in the inner leaflet. It contains a diverse set of outer membrane proteins (OMPs), including porins that form channels for small hydrophilic molecules. Larger substances cross the membrane through specific transporters. The outer membrane is linked to the peptidoglycan-containing cell wall via lipoproteins such as Lpp, reinforcing the envelope and helping maintain shape. The periplasm lies between the outer and inner membranes and houses enzymes and chaperones. Functionally, the outer membrane acts as a protective barrier against harmful compounds, contributes to antibiotic resistance by limiting permeability, and presents LPS as a potent endotoxin that stimulates immune responses.

In eukaryotic organelles, the outer mitochondrial membrane and the outer membrane of chloroplasts envelope their respective organelles. These membranes are rich in different phospholipids and contain porins, such as voltage-dependent anion channels (VDACs) in mitochondria, that permit diffusion of small solutes. Protein import in mitochondria is mediated in part by the translocase complexes that span the outer membrane. The outer membranes are permeable to small metabolites yet generally restrict larger proteins, and in animals can participate in signaling pathways that regulate apoptosis.

The outer membrane is a remnant of the endosymbiotic origin of these organelles and differs in composition from the inner membrane, reflecting distinct functional requirements across different organisms.