The membrane structure of cells is primarily composed of a phospholipid bilayer, which consists of two layers of phospholipids. Each phospholipid has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. This arrangement allows the membrane to be flexible and semi-permeable, controlling the movement of substances in and out of the cell. Embedded within this bilayer are various proteins, which serve multiple functions, including transport, signaling, and structural support. These proteins can be integral, spanning the membrane, or peripheral, located on the surface. Together, the phospholipids and proteins create a dynamic and functional structure essential for cellular processes.