$\bullet$ The plasma membrane is composed mainly of phospholipids. They are arranged in two layers, called a phospholipid bilayer. Each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic).
$\bullet$ The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.