A cell membrane consists of a lipid bilayer, which is a double layer of phospholipids. Each phospholipid has a hydrophilic phosphate head and a hydrophobic lipid tail. These phospholipids are not chemically bound to one another, but rather maintain their structure due to the fact that the lipid tails do not like water. They’re hydrophobic. If they all buddy up, then they can all stay dry. (See the figure below.) What is the function of the phospholipid bilayer?