||The cell membrane, or plasmalemma, forms the boundary between the cell and its environment. Other mebranes within the cell bound the organelles, and thus segregate different biochemical activities. They limit diffusion and actively control the passage of ions and small molecules between functionally distinct compartments. Although the various membranes of the cell differ slightly in thickness and protein content, they all have the same basic molecular organization. In electron micrographs, they have a trilaminar appearance, consisting of two dense lines, 2.5-3.0 nm in thickness, separated by a less dense intermediate zone. They consist of a bimolecular layer of mixed phospholipids, with the hydrophylic heads of the lipid molecules at the outer and inner surfaces, and their hydrophobic chains projecting toward the middle of the bilayer. These lines correspond to the hydrophilic heads of the lipid bilayer.