Outer membrane
The outer membrane of gram-negative bacteria occurs mainly in proteo and cyanobacteria . It is part of a membrane system consisting of two double membranes. The inner membrane is always formed by the actual cell membrane , while the outer membrane acts as an additional boundary to the outside. Both membranes enclose a cell compartment known as the periplasmic space .
The two layers of the outer membrane are constructed differently. The composition of the inner layer is similar to that of a unit membrane. It mainly consists of phospholipids in which lipoproteins are embedded. This lipoprotein complex maintains a connection to the cell wall in the periplasmic space. The composition of the outer layer, on the other hand, differs significantly from that of a biological unitary membrane: phospholipids are largely replaced by polysaccharides , which form complex lipopolysaccharides (LPS) with lipids . Together with numerous proteins, they form the essential components of the outer layer. In contrast to the inner membrane, the outer membrane is considered to be "not energized", meaning that no energy in the form of ATP and no proton gradient is available for biochemical processes . The exact composition of the membrane is usually highly variable and plays an important role in the adaptability of the bacteria to changed environmental conditions ( e.g. changed osmolarity ) and in the pathogenicity of individual strains. The lipopolysaccharides act as strong antigens , pyrogens and endotoxins .
The outer membrane primarily serves as a diffusion barrier for large molecules. Like a "filter", it prevents biopolymers from entering the periplasmic space, but above all it prevents proteins and enzymes from escaping from the periplasm into the environment. In the membrane there are numerous porins and other integral proteins that are necessary for transport processes or the reception of signals and environmental stimuli. Barrel-shaped β-barrel structures are characteristic of integral proteins of the outer membrane . Parts of the anchorage of the flagella and flagella (the so-called L-ring) are also located in the outer membrane.
