plasmamembranet

Plasmamembranet, also known as the plasma membrane or cell membrane, is the semi-permeable boundary that encloses the cytoplasm of a cell and separates it from the extracellular environment. In eukaryotic cells it forms a flexible, dynamic barrier that also participates in signal transduction and cell–cell interactions.

Structure and composition: It consists mainly of a phospholipid bilayer in which amphipathic phospholipid molecules arrange with hydrophobic tails inward and hydrophilic heads outward. Cholesterol sits within the bilayer, modulating fluidity and stability; proteins are embedded or attached to the membrane (integral/transmembrane and peripheral proteins). Carbohydrates are attached to lipids and proteins on the extracellular face, forming glycolipids and glycoproteins that contribute to cell recognition.

Model and asymmetry: The fluid mosaic model describes lateral mobility of lipids and proteins, allowing membrane remodeling, vesicle trafficking, and formation of microdomains. The two leaflets have different lipid compositions, creating membrane asymmetry.

Functions and transport: The plasma membrane acts as a selective barrier, controlling transport of ions, nutrients, and waste. Transport occurs via channels, carriers, and pumps (active transport, e.g., Na+/K+-ATPase), as well as through vesicular processes like endocytosis and exocytosis. It hosts receptors for external signals, supports enzymatic activity at the surface, and mediates cell adhesion and communication. The membrane maintains the electrochemical gradient across the cell, contributing to membrane potential.

Specialized features: Membrane microdomains (lipid rafts) organize signaling complexes. Surface extensions such as microvilli increase surface area; cilia and flagella in some cells extend from the membrane. Prokaryotic plasma membranes share core features but lack cholesterol and organized internal compartments.