Change in orientation of a membrane lipid or protein from facing inward to facing outward, or vice versa.
While flip-flopping of membrane lipids is somewhat rare, flip-flopping of membrane proteins is all but non-existent. As a consequence, the orientation of membranes tends to stay fairly constant, with lipids that are found on one side staying on that side and with proteins retaining the same orientation as they had when they were inserted.
As a consequence of this relative lack of flip-flopping, membranes once formed can be hightly effective at carrying out their functioning unidirectionally. That is, with active transport, substances are pumped in only one direction. Similarly, cell attachment to other cells or extracellular matrix, signal reception, and cell-to-cell recognition tend to be mediated only on the outside of cells. In contrast but nevertheless consistent, membrane-associated cytoplasmic enzymes or membrane associations with cell cytoskeleton only occurs on the cytoplasmic side of membranes, that is, the orientation facing inward.
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