Cellgränsen

Cellgränsen, eller cell boundary, describes the limiting border that defines a cell and separates its interior from the external environment. In most cells this boundary is the plasma membrane, a phospholipid bilayer about 5–10 nanometers thick. The bilayer consists of amphipathic phospholipids with hydrophilic heads facing water and hydrophobic tails inward, creating a semi-permeable barrier. The membrane contains cholesterol and a variety of proteins, including channels, transporters, receptors, and enzymes, as well as glycoproteins and glycolipids on the outer surface that form the glycocalyx.

Functions include maintaining structural integrity, regulating the movement of ions and molecules, generating and maintaining electrochemical gradients, and enabling signaling and communication with other cells. The membrane supports endocytosis and exocytosis, which move materials into and out of the cell, and it participates in vesicular trafficking within the cell.

Transport occurs via simple diffusion for small nonpolar molecules, through channels for ions and polar molecules, or by carrier proteins and pumps for larger or charged substances. Active transport consumes energy, usually ATP, to move substances against gradients. The membrane potential is created by ion pumps and selective permeability and is essential for nerve and muscle function in animals.

Some cells have additional boundary structures. Plant, fungal, and some bacterial cells possess a rigid cell wall outside the plasma membrane, which provides mechanical support while still allowing selective transport. In tissues, cell–cell junctions further define boundaries between cells.

The term cell boundary is closely linked to the fluid mosaic model, which describes membranes as dynamic and heterogeneous assemblies of lipids and proteins.