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.
One such enzyme is aminophospholipid transferase, an ATPase, which stimulates the movement of phosphatidylserine and phosphatidylethanolamine. Membrane topology can be varied including raised areas and invaginations. A large number of proteins are unique to or highly enriched in a specific organelle. Organelles differ qualitatively and quantitatively with regard to their lipid constituents, e.g., the inner mitochondrial membrane is highly enriched in cardiolipin, a phospholipid. Cilia and flagella 3.