The underlying structure that supports all living cells is known as the cell membrane or plasma membrane. This semi-permeable barrier surrounds the cell, regulating the movement of substances in and out, thus maintaining cellular integrity and homeostasis. The membrane is composed primarily of a phospholipid bilayer, with embedded proteins that facilitate various functions, including signaling, transport, and communication. The cell membrane's structure allows it to play a critical role in cellular processes, contributing to cell communication and metabolism.