membrana

Membrana, from Latin membrana meaning a thin film or parchment, is a thin, pliable layer that forms a boundary separating two environments. In biology, membranes define the borders of cells and organelles and regulate the movement of substances between compartments. The most familiar example is the plasma membrane of the cell, which, along with internal membranes such as the nuclear envelope and the membranes of mitochondria and endoplasmic reticulum, forms a lipid bilayer.

Biological membranes are primarily composed of phospholipids arranged in a bilayer, with embedded proteins and lipids such as cholesterol that modulate fluidity. The hydrophobic core of the bilayer acts as a barrier to most polar molecules, while proteins provide selective transport, signaling, and enzymatic functions. The membrane is often described by the fluid mosaic model, recognizing lateral mobility of components in a two-dimensional fluid.

Functions include maintaining cell integrity, controlling the import and export of ions and nutrients, enabling cell communication through receptors, and organizing cellular processes by compartmentalization. Transport occurs via diffusion, facilitated diffusion, active transport, and vesicular mechanisms such as endocytosis and exocytosis. Carbohydrates on the extracellular face, forming the glycocalyx, contribute to cell recognition and protection.

In addition to biological membranes, the term membrana is used for synthetic or industrial membranes—thin polymer films designed to separate substances in filtration, desalination, and chemical processing. Understanding membranes is central to cells, physiology, and materials science.