The primary component of a cell membrane is phospholipids. These molecules consist of a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." When phospholipids are placed in an aqueous environment, they spontaneously arrange themselves into a bilayer, with the hydrophilic heads facing outward and the hydrophobic tails facing inward. This structure creates a semi-permeable barrier, allowing certain substances to pass while keeping others out. Besides phospholipids, cell membranes also contain proteins, cholesterol, and carbohydrates, contributing to various functions like signaling, transport, and structural integrity.