Liposome with water trapped in its core, cut-away illustration. This artificially-constructed, spherical vesicle possesses a selectively-permeable wall that closely resembles the membrane of a living cell. The membrane consists of a dual layer of phospholipids. Each phospholipid molecule consists of a hydrophilic (water-soluble) phosphate group head (round) and a hydrophobic (water-insoluble) fatty acid tail. Liposomes can be used to carry drugs or genes to target cells. This is particularly useful for highly toxic cancer drugs as it reduces unpleasant side effects.