The cell membrane, also known as the plasma membrane, is a thin, flexible barrier that surrounds the cell. It is composed of a phospholipid bilayer with embedded proteins and carbohydrates, and it separates the cell from its external environment.
The phospholipid bilayer is composed of two layers of phospholipid molecules. Each phospholipid molecule has a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The hydrophilic heads face the external environment and the cytoplasm, while the hydrophobic tails face each other in the interior of the membrane. This arrangement creates a barrier that restricts the movement of certain molecules into and out of the cell.
The membrane also contains embedded proteins that have various functions, including transport of molecules across the membrane, cell signaling, and cell adhesion. Carbohydrates are also present on the outer surface of the membrane and are involved in cell recognition and communication.
The cell membrane is selectively permeable, meaning that it allows certain molecules to pass through while restricting the passage of others. Small and nonpolar molecules such as oxygen and carbon dioxide can diffuse across the membrane, while larger and polar molecules require specific transport proteins to cross the membrane.
The cell membrane is essential to the survival of the cell, as it regulates the movement of molecules in and out of the cell, maintains cell shape and structure, and provides protection from the external environment. It also plays a crucial role in cell signaling and communication with other cells.