Membrani

Membrani, or membranes, are lipid bilayer structures that enclose cells and organelles, forming selective barriers that regulate exchange with the environment and internal compartments. They define the boundaries of life and organize the chemical reactions that sustain it.

They are primarily composed of phospholipids arranged in a bilayer with hydrophobic tails hidden from water and hydrophilic heads facing outward. Cholesterol modulates membrane fluidity in animal cells, especially at varying temperatures. Proteins can be embedded (integral) or attached to the surface (peripheral) and carry out transport, signaling, and enzymatic roles. Carbohydrate moieties on lipids and proteins contribute to the glycocalyx, which aids in cell recognition and interaction.

The plasma membrane is the best-known example, but every organelle in a eukaryotic cell has its own membrane system, including those of mitochondria, chloroplasts, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and the nuclear envelope. Membranes support compartmentalization, enabling specialized environments for diverse processes. The fluid mosaic model describes membranes as dynamic, with lipids and proteins able to move laterally within the bilayer.

Membranes regulate transport through diffusion, channels, carriers, and pumps, and facilitate active transport that requires energy. Water movement is governed by osmosis. Large molecules and particles cross membranes via vesicular processes such as endocytosis and exocytosis. Membranes also participate in signaling, cell recognition, and adhesion, and are continually remodeled during growth and development. Membrane biogenesis occurs mainly in the endoplasmic reticulum and Golgi apparatus, where lipids and proteins are targeted to specific destinations. Synthetic membranes and liposomes model their properties for research and applications.