Phospholipid

A phospholipid is a lipid whose head contains a positively charged (polar) group that is linked to its two (nonpolar) tails by a negatively charged phosphate group. Both tails consist of a fatty acid, each 14-24 carbon groups long. Phospholipids are used as building blocks of biological membranes.

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