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  The cell membrane is a thin, flexible barrier that surrounds all living cells. It is also known as the plasma membrane or cytoplasmic membrane. The primary function of the cell membrane is to regulate the movement of molecules in and out of the cell, thereby maintaining the cell's internal environment and enabling it to carry out its various functions. The cell membrane is composed of a phospholipid bilayer, which is made up of two layers of phospholipid molecules. Phospholipids are molecules that have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. When these molecules are arranged in a bilayer, the hydrophilic heads face outward, towards the aqueous environment both inside and outside the cell, while the hydrophobic tails face inward, towards each other. The phospholipid bilayer provides a barrier that is selectively permeable, meaning that it allows certain molecules to pass through while blocking others. This is achieved through the presence of vario...