The cell membrane, also called the plasma membrane or biomembrane, is a thin, quasi-fluid structure found both around the whole cell (extracellularly) and around most organelles inside a eukaryotic cell (intracellularly), where it separates each organelle from the cytosol. It is about 75 Angstrom thick and appears trilamellate (three-layered) under the electron microscope, built mainly from a bilayer of phospholipids -- each molecule having one hydrophilic head and two hydrophobic tails (amphipathic), arranged so the tails face inward, away from the surrounding water -- along with proteins and carbohydrates in ratios that vary between cell types. The most accepted description of its structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972: proteins are embedded in the phospholipid bilayer like icebergs in a sea of lipid and are free to move within it, a mobility that is measured as the membrane's fluidity and comes mainly from the lipid component. Integral (intrinsic) proteins sit at different depths in the bilayer, some spanning it completely as transmembrane proteins that form channels, for instance for water; peripheral (extrinsic) proteins sit only on one surface or the other. This model replaced an earlier, rigid 'sandwich' model (Danielli and Davson), which pictured static protein layers coating the lipid core and so could not explain the mobility membrane proteins actually show. The membrane's main function is to control transport across it -- it is selectively permeable. In passive transport, molecules move down their concentration gradient without using energy (simple diffusion of neutral solutes; osmosis for water); in active transport, ions or molecules move against their gradient and require ATP, such as the Na+/K+ pump. Polar molecules cannot cross the non-polar lipid bilayer directly and need carrier proteins. Membrane glycoproteins, such as those normally present on the red blood cell surface, are what the ABO blood-typing system actually detects, so they play a direct role in determining blood group.