The cell membrane acts as the barrier between inside and outside of the cell. It is composed of an asymmetric phospholipid bilayer with embedded proteins and carbohydrates and is approximately 7 nm thick. The phospholipids in the membrane can move through lateral diffusion, flexion, or rotation, a process called the fluid mosaic model. The asymmetric bilayer serves an important function for the cell by regulating molecular diffusion across the membrane based on charge and size. The two general categories of proteins present in the cell membrane are referred to as integral or paramembrane proteins. Integral proteins form hydrophobic bonds with lipids and other integral proteins. These proteins penetrate partially or through lipid bilayer and may be glycoproteins (oligosaccharides attached to N-terminal end of protein). Paramembrane proteins are less hydrophobic than integral proteins. They may associate with lipid polar headgroups or integral proteins via H-bonds or ionic interactions.
Some proteins exist as receptors located within the membrane, with functions ranging from activating downstream intracellular signaling when bound by ligand or acting as channels to allow ions to flow into or out of the cell. Other functions of the membrane are to subdivide the cytoplasm within the cell and increase surface area of the cell. Within the membrane are structures named lipid rafts. These areas are enriched in cholesterol, sphingolipid, and proteins. Lipid rafts are important to the cell for signal transduction across the membrane.