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A phospholipid is a lipid; this is the primary component of animal cell membranes.

Phospholipids consist of a water-soluble head (a positively charged (polar) group), linked to two water-insoluble nonpolar tails (by a negatively charged phosphate group). Both tails consist of a fatty acid, each 14-24 carbon groups long.

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