PhospholipidsPhospholipids belong to the lipid family of biological polymers. A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group and a polar molecule. The phosphate group and polar head region of the molecule is hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the non-polar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the water.
Phospholipids are a major component of cell membranes which enclose the cytoplasm and other contents of a cell. Phospholipids form a lipid bilayer in which their hydrophillic head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane to enter or exit the cell.