The cytomembrane system manages access into and out of a cell, recognizes other cells of the body, provides communication with other cells, and supplies structural support. Also known as the plasma or cell membrane system, its function and components vary based on the organism's domain and the cell's complexity.
Lipids are the primary structural elements of the cytomembrane system and include phospholipids, cholesterol and glycolipids. Phospholipids arrange themselves in two rows, with their head towards either the cell fluid or the external fluid and tail towards the opposite row. This lipid bilayer allows certain substances through while blocking others. Cholesterol inserts itself between the phospholipids, preventing them from packing tightly together and becoming rigid. A flexible membrane is a feature in bacterial prokaryotes, archaea and eukaryotes, except for plants, in which the cholesterol spacers are missing.
Glycolipids form a bridge between the lipid bilayer and carbohydrate chains on the membrane surface. Proteins act as controllers in the cytomembrane system and include structural proteins, receptor proteins, transport proteins and glycoproteins. Structural proteins embed themselves in the lipid bilayer, with their ends out of the membrane. Receptor proteins exist in eukaryotes and use signaling molecules, such as neurotransmitters and hormones, to promote communication.
Transporter proteins include those that change to an alpha-helical shape to carry blocked substances through the membrane. Glycoproteins embed themselves in the membrane and form a bridge to carbohydrate chains. They are active in both cellular communication and transport. The cytomembrane system acts as a security guard, a taxi service and a communications utility for the cell.