Solupinnan

Solupinnan, known in English as the cell surface or plasma membrane, is the flexible boundary that encloses a cell and separates its internal milieu from the external environment. It is composed mainly of a phospholipid bilayer with hydrophilic heads facing the aqueous surroundings and hydrophobic tails forming an interior barrier. The membrane is interspersed with proteins, glycoproteins, and glycolipids that contribute to structure and function.

Proteins within the membrane include channels, carriers, and pumps that regulate the selective transport of ions and molecules, as well as receptors that detect signals from the outside. The layer often carries carbohydrates on its extracellular face, creating a glycocalyx that supports cell recognition, protection, and adhesion. Cholesterol or other sterols modulate membrane fluidity and stability, varying in different organisms and tissues.

Functions of the solupinnan include maintaining homeostasis through selective permeability, enabling signaling pathways, mediating cell–cell and cell–matrix adhesion, and facilitating endocytosis and exocytosis. The membrane is fluid and dynamic, allowing proteins and lipids to diffuse and reorganize into microdomains, sometimes called lipid rafts, that concentrate specific activities.

Compared with prokaryotes, eukaryotic cells possess additional outer structures such as a cell wall (in plants, fungi, and some algae) adjacent to the plasma membrane. In bacteria, the cytoplasmic membrane underlies the cell wall and hosts essential processes. The solupinnan is a frequent target for pharmaceuticals and is central to many cellular processes and diseases.