Lecithin is (organic chemistry) the principal phospholipid in animals; it is particularly abundant in egg yolks, and is extracted commercially from soy. It is also a major constituent of cell membranes, and is commonly used as a food additive (as an emulsifier). While cephalin is (biochemistry) a phospholipid found particularly in the cells of nervous tissue; it is also the primary phospholipid in bacteria.