The cytoplasmic membrane, also known as the cell membrane or plasma membrane, can best be described as a selectively permeable barrier that separates the interior of a cell from its external environment. The membrane is composed of a phospholipid bilayer, which consists of two layers of phospholipid molecules arranged tail-to-tail. The hydrophobic tails face inward, while the hydrophilic heads face outward. This arrangement creates a barrier that prevents most substances from freely crossing the membrane, while allowing certain molecules to pass through via various transport mechanisms such as diffusion, facilitated diffusion, and active transport. The membrane also contains proteins that play various roles in maintaining the structure of the membrane, transporting molecules across the membrane, and communicating with the external environment.