Due to its polar nature, the head of a phospholipid is attracted to water (it is hydrophilic). The nonpolar head is not attracted to water and is said to be hydrophobic. When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other. This forms a membrane with hydrophillic heads on both sides. This membrane is partially permeable and very flexible.
- See also: biochemistry, lipid, lipid bilayer