membranien

Membranes are flexible barriers that enclose cells and organelles. They are primarily composed of a phospholipid bilayer with embedded proteins, cholesterol or other sterols in animals, and carbohydrate groups on the extracellular face. The bilayer forms a hydrophobic core that restricts many substances, while proteins and small nonpolar molecules enable selective transport and communication.

Membranes are described by the fluid mosaic model, in which lipids and proteins diffuse within the layer, giving the structure both fluidity and functionality. The distribution of lipids and proteins is asymmetric, which affects signaling, transport, and membrane potential.

Key functions include defining boundaries, mediating selective transport and homeostasis, supporting energy transduction in organelles (for example, mitochondrial inner membranes and chloroplast membranes), and enabling compartmentalized metabolism. They host receptors, channels, transporters, enzymes, and adhesion molecules that participate in signaling and cell recognition.

Major types include the plasma membrane and internal membranes such as the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, peroxisomes, and in plants, chloroplasts. Bacterial and archaeal membranes also share the same fundamental architecture but differ in lipid composition; bacteria typically lack sterols, while archaea use ether-linked lipids and may have monolayer membranes in some species.

Transport mechanisms encompass passive diffusion, facilitated diffusion via channels and carriers, active transport against gradients, and vesicular transport (endocytosis and exocytosis). Membrane proteins are classified as integral (transmembrane) or peripheral and perform transport, signaling, and enzymatic roles.

Membranes are central to bioenergetics, maintaining resting potentials and gradients, and they are widely exploited in biotechnology, for example in liposome drug delivery and synthetic biology research.