Membrane

Membrane refers to a thin, flexible barrier that encloses a cell or organelle and separates its internal environment from the outside. In biology, membranes are primarily composed of a phospholipid bilayer with embedded proteins, and sometimes cholesterol or other sterols, which confer fluidity and stability. The prevailing model, the fluid mosaic model, describes membranes as dynamic, protein-containing fluids where lipids diffuse and proteins move laterally.

Components and organization: The bilayer forms a hydrophobic core, with hydrophilic heads facing outward. Integral (transmembrane) proteins span the bilayer; peripheral proteins associate with surfaces. Carbohydrates may be attached to lipids or proteins on the extracellular face, forming glycoproteins and glycolipids involved in cell recognition.

Functions: membranes act as selective barriers that regulate transport, signaling, and communication. They enable passive diffusion of small uncharged molecules, and mediate selective transport through channels, carriers, and pumps. Active transport uses energy to move substances against gradients. Membranes participate in vesicular trafficking via endocytosis and exocytosis, and maintain electrochemical gradients essential for processes such as nerve signaling and ATP production. Organelles such as mitochondria and chloroplasts have specialized membranes, including the inner mitochondrial membrane and thylakoid membranes.

Variations: plasma membranes differ among cell types; organelle membranes vary in composition. Prokaryotes have cytoplasmic membranes, while Gram-negative bacteria possess an outer membrane. In eukaryotes, membrane lipids include phospholipids, cholesterol, and sphingolipids; membrane proteins provide receptors and enzymes. Synthetic and polymeric membranes are used in filtration, dialysis, and separation technologies.

History: the fluid mosaic model proposed by Singer and Nicolson in 1972 replaced earlier models of static membranes.