Biological membranes are thin structures that surround and protect cells and their internal compartments. They are primarily composed of a double layer of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This arrangement creates a barrier that regulates the movement of substances in and out of the cell, maintaining homeostasis. In addition to phospholipids, biological membranes contain proteins, cholesterol, and carbohydrates. These components play essential roles in various functions, such as cell signaling, transport, and communication. The fluid mosaic model describes the dynamic nature of these membranes, highlighting their flexibility and the diverse functions of their embedded molecules.