The main chemical component of cell membranes is phospholipids. These molecules consist of a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. Phospholipids arrange themselves into a bilyaer, forming the basic structure of the membrane, which serves as a barrier to separate the internal environment of the cell from the external surroundings. This arrangement allows for selective permeability, enabling the cell to regulate the entry and exit of substances.