Ulkikalvo

Ulkikalvo, or outer membrane, is the outermost lipid bilayer that surrounds certain cells or organelles. In biology, the term most often refers to the outer membrane of Gram-negative bacteria, which lies outside the peptidoglycan layer. It also occurs as the outer envelope of mitochondria and chloroplasts in eukaryotic cells, reflecting their endosymbiotic origins.

Structure and composition: The outer membrane is typically a phospholipid bilayer studded with proteins. In Gram-negative bacteria, the outer leaflet often contains lipopolysaccharide (LPS) and lipoproteins that contribute to its barrier properties. The membrane hosts channels such as porins and various transport receptors to control diffusion of small molecules. In mitochondria and chloroplasts, the outer membrane contains channel proteins like porins (VDAC in mitochondria) and a set of proteins involved in import and assembly.

Functions: The ulkikalvo provides chemical and physical protection while regulating exchange with the environment or cytosol. It acts as a selective barrier, permitting nutrients and other small molecules to pass through porins while restricting larger or harmful substances. In Gram-negative bacteria, LPS contributes to the immune response in hosts and can act as an endotoxin. The outer membrane also influences antibiotic susceptibility by limiting drug entry and participating in efflux mechanisms.

Biogenesis and evolution: In bacteria, outer membrane components are assembled by systems such as the Bam complex for beta-barrel proteins and the Lpt pathway for LPS transport to the surface. In mitochondria and chloroplasts, protein import and assembly involve organelle-specific pathways (for example TOM and SAM) and reflect their endosymbiotic origin. Ulkikalvo is a central concept in microbiology and cell biology, with relevance to health, disease, and antibiotic resistance.