Membranes

Membranes are thin barriers that enclose cells and organelles, separating compartments and regulating exchange with the surroundings. The core structure is a phospholipid bilayer in which amphipathic lipids form a hydrophobic interior and hydrophilic surfaces face water. Proteins embedded in or attached to the bilayer perform transport, signaling, and enzymatic functions. Cholesterol helps modulate membrane fluidity and stability. The fluid mosaic model describes membranes as dynamic, with lipids and proteins able to move laterally and unevenly distributed, yielding membrane asymmetry between inner and outer leaflets.

Membranes control substance movement through passive and active mechanisms. Small nonpolar molecules diffuse readily; ions and most polar molecules require channels, carriers, or pumps. Active transport uses energy, typically from ATP, to move substances against gradients. Membranes also host receptors and signaling complexes, enabling cells to respond to hormones, nutrients, and stress. They participate in energy metabolism, for example the inner mitochondrial membrane is crucial for oxidative phosphorylation, and chloroplast thylakoid membranes carry photosynthetic components.

Biological membranes include the plasma membrane and the membranes of organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

Synthetic or artificial membranes are designed for filtration, separation, and medical applications, using polymers, ceramics, or other materials.

Techniques such as patch-clamp recording, fluorescence microscopy, and cryo-electron microscopy advance understanding of membrane structure and transport dynamics.