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The cell membrane, also known as the plasma membrane, is a thin, flexible barrier that surrounds all cells in living organisms. It plays a crucial role in regulating the movement of materials into and out of the cell, providing protection for the cell's internal structures, and maintaining the cell's shape and integrity. The cell membrane is composed of two layers of phospholipids, which have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail, arranged in a way that creates a barrier to the movement of polar molecules while allowing nonpolar molecules to pass through. Additionally, the cell membrane contains various proteins that serve as channels or pumps for specific molecules, providing selective permeability and active transport capabilities. In the context of cellular biology, the cell membrane is a key component in understanding how cells interact with their environment and maintain homeostasis. It serves as the boundary between the intracellular and extracellular environments, and its properties and functions are tightly regulated to ensure proper cell function. The study of cell membranes falls under the broader field of cellular biology, which investigates the structure, function, and processes of cells, and is a crucial area of research in life science, which encompasses all aspects of living organisms.