Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-hating) regions. The hydrophilic region of a phospholipid is the phosphate group, while the hydrophobic region is the fatty acid chains. When phospholipids are placed in water, they spontaneously form a bilayer with the hydrophilic regions facing outward and the hydrophobic regions facing inward. This bilayer structure is the basic building block of all cell membranes.