Phospholipids are prevalent in the cells of bacteria and eukaryotes. They are molecules made of a phosphate head and a lipid tail. The head is considered water-loving or hydrophilic, whereas the tail is hydrophobic, or repellent to water. Phospholipids are therefore called amphiphilic. Because of this dual nature of phospholipids, many types arrange themselves into two layers in a watery environment. This is called a phospholipid bilayer. Phospholipid synthesis occurs primarily in the endoplasmic reticulum. Other areas of biosynthesis include the Golgi apparatus and mitochondria. Phospholipids function in various ways inside cells.