Biomembranes

Biomembranes are thin, flexible barriers that enclose cells and organelles, defining compartments and regulating the exchange of matter and information with the environment. They are primarily composed of a lipid bilayer formed by amphipathic phospholipids, with cholesterol and other lipids modulating fluidity and thickness. Membrane proteins, including integral transmembrane proteins and peripheral proteins, perform transport, signaling, and enzymatic roles. Carbohydrates are often attached to lipids and proteins on the extracellular face, forming a glycocalyx involved in recognition.

Most biomembranes follow the fluid mosaic model: lipids diffuse laterally in the bilayer, proteins diffuse within the membrane, and the bilayer has orientation with asymmetric lipid distribution between the inner and outer leaflets.

Biomembranes include the plasma membrane and internal membranes such as the nuclear envelope, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, lysosomes, and vesicles. They create distinct environments for processes like oxidative phosphorylation, photosynthesis, protein synthesis, and transport.

Functions include selective permeability, maintaining ion and pH gradients, and enabling energy conversion and signaling. Transport occurs via passive diffusion, facilitated diffusion through channels and carriers, active transport by pumps, and vesicular processes such as endocytosis and exocytosis. Receptors detect signals; adhesion molecules mediate cell contacts; membranes participate in membrane fusion and fission during trafficking.

Membrane synthesis and maintenance rely on coordinated lipid and protein production in the endoplasmic reticulum and Golgi, followed by vesicular trafficking to correct destinations. The properties of membranes—fluidity, thickness, and curvature—are tuned by lipid composition and cholesterol, accommodating dynamic processes such as vesicle budding and organelle biogenesis.