as tightly as saturated lipids and thus have lower melting points: less thermal energy is required to achieve the same level of fluidity as membranes made with lipids with saturated hydrocarbon chains. Incorporation of particular lipids, such as sphingomyelin, into synthetic lipid membranes is known to stiffen a membrane. Such membranes can be described as "a glass state, i.e., rigid but without crystalline order".