Life depends on plasma membranes. These thin films envelop all cells, separating the cell's interior, or cytoplasm, from the surrounding extracellular environment. But this edge of life is not solid or static. It regulates a steady flow of materials into and out of a cell and performs many crucial functions. Let's begin with the structure of the plasma membrane, whose key components are phospholipids and proteins. Phospholipids form a bilayer in which their nonpolar, hydrophobic tails are oriented toward the interior of the membrane, and their hydrophilic phosphate heads face the watery environment on either side of the membrane. Suspended in and attached to this bilayer of phospholipids are various proteins. Biologists use a fluid mosaic model to describe a membrane. The kinky tails of many phospholipids, along with cholesterol, found in animal cell membranes, keep the molecules from packing tightly. Thus, the membrane remains fluid, and its components can drift about like party-goers elbowing their way through a crowded room. What about the mosaic part of the model? Just as a mosaic is formed by differently colored and shaped tiles, a membrane is a mosaic of proteins with different shapes and functions.