The primary component of cell membranes is the phospholipid bilayer. Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, which arrange themselves into two layers. This structure creates a semi-permeable barrier that regulates the movement of substances into and out of the cell. Besides phospholipids, proteins, cholesterol, and carbohydrates are also integral to the membrane's structure and function, contributing to various roles, including cell signaling and communication.