The cell membrane is a selectively permeable barrier that controls the passage of substances into and out of the cell. Its structure is key to its functions. • The phospholipid bilayer forms the basic structure. Phospholipids have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. They arrange themselves with the tails facing inward, creating a barrier to water-soluble substances. • Proteins are embedded within or attached to the bilayer. These include integral proteins that span the membrane and peripheral proteins that are on the surface. Proteins function as channels for specific ions and molecules, carriers that transport substances, receptors for cell signaling, enzymes, and anchors for the cytoskeleton. • Cholesterol is found in animal cell membranes. It helps to regulate membrane fluidity, making it less fluid at high temperatures and more fluid at low temperatures. • Carbohydrates are often attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the outer surface of the membrane. These play roles in cell recognition, cell adhesion, and as receptors.