The most important function of phospholipids is to form the phospholipid bilayer of the plasma membrane. In this bilayer, phospholipids are arranged so that their hydrophobic heads are pointing outwards and their hydrophilic tails are pointing inwards. This arrangement allows plasma membranes to be selectively permeable to solutes such as proteins, ions and water.
Phospholipids are composed of two fatty acid chains and one glycerol compound. The fatty acid chains act as the tails of the phospholipid while glycerol acts as a head. The glycerol molecule also attaches to a phosphate group. The phosphate group causes the glycerol head to become hydrophilic. The ability for phospholipids to be both hydrophobic and hydrophilic causes the compounds to be amphipathic.
In biological system, phospholipids allow cell membranes to be fluid. Their unique characteristics allow the cell membrane to take different shapes and expand and shrink when necessary.
Phospholipids can also be used as signal transducers between cells. They can be split to produce products that function as second messengers in cellular systems. As a second messenger, phospholipids can signal for leukocytes to migrate to a site of infection, and they can also inhibit neurons. Phospholipids can also be found in plants and can help produce certain plant hormones.