The thin, flexible membrane surrounding all cells is called the cell membrane, also known as the plasma membrane. It serves as a crucial barrier between the cell's interior and its surrounding environment. The cell membrane is composed primarily of a phospholipid bilayer, which consists of two layers of phospholipids arranged with their hydrophilic (water-loving) heads facing outward and their hydrophobic (water-hating) tails facing inward. This arrangement creates a selectively permeable barrier that regulates the movement of molecules in and out of the cell.

The cell membrane also contains various proteins embedded within the phospholipid bilayer. These proteins perform a wide range of functions, such as transporting molecules across the membrane, acting as receptors for external signals, and providing structural support. Additionally, the cell membrane contains cholesterol molecules, which help to maintain the membrane's fluidity and stability.

The cell membrane is a dynamic structure that is constantly in flux, with phospholipids and proteins constantly moving and interacting with each other. This fluidity is crucial for the proper functioning of the cell membrane and allows it to adapt to changes in the environment.