Membramen

Membramen, the Dutch term for biological membranes, are the lipid-protein barriers that define the boundaries of cells and their organelles. They consist mainly of a phospholipid bilayer in which proteins are embedded or attached peripherally, with cholesterol modulating fluidity and glycoproteins and glycolipids decorating the extracellular surface.

The structure is described by the fluid mosaic model: a dynamic, two-layer sheet into which various proteins float, enabling selective permeability and functions beyond containment. The lipid bilayer forms a hydrophobic core that restricts polar molecules, while membrane proteins provide channels, transporters, receptors, and enzymes. In eukaryotes, mitochondria and chloroplasts contain specialized membranes with unique lipids and embedded protein complexes.

Functions include maintaining distinct internal environments, permitting selective transport, transmitting signals, and supporting energy conversion. Membranes regulate ion gradients and chemical gradients, essential for processes such as ATP synthesis, nutrient uptake, and waste removal. Transport occurs via passive diffusion, facilitated diffusion through channels and carriers, and active transport powered by pumps. Vesicular transport moves larger cargo, enabling endocytosis and exocytosis and intracellular trafficking.

Membrane composition and properties are modulated by temperature and sterols such as cholesterol, which influence fluidity and phase behavior. Microdomains, sometimes called lipid rafts, organize signaling and trafficking. Membranes also play roles in cell adhesion and recognition through surface carbohydrates.

In research and industry, artificial membranes and liposomes model membranes and enable drug delivery and biosensing.