Some membrane proteins are modified. Thickness of lipid bilayer can vary. In addition, some membrane lipid can be in a nonbilayer configuration, such as some of the lipid directly associated with an integral membrane protein. Membrane proteins can form dimers or larger aggregates. An example of unique protein localization in a membrane is the location of viral envelope proteins in a portion of a membrane which does not contain any host cell membrane proteins.
Fluidity is an important characteristic of biological membranes.
In addition to the fluidity of the lipid components, described above, some membrane proteins can also migrate and rotate in the plane of the bilayer. Thus biological membranes are dynamic structures. Phospholipids, glycolipids and proteins do not or slowly migrate from one leaflet of the membrane to the other. This movement is termed flip-flop. Enzymes can stimulate the movement of phospholipids across membranes.