The cell membrane is a semi-fluid barrier that encloses the cytoplasm of a cell, regulating the passage of substances into and out of the cell. Structurally, it is primarily composed of a phospholipid bilayer, which determines the fundamental boundary of the cell. Embedded within this bilayer are various proteins and other associated molecules that provide crucial functional capacity. The membrane exhibits selective permeability, meaning it controls which materials, such as ions, nutrients, and waste products, are permitted to cross, thus maintaining the necessary internal chemical balance and structural integrity required for the cell's survival and metabolic activities.