Animals exploit the phospholipid asymmetry of their plasma membrane to distinguish between live cells and dead ones. When animal cells undergo a form of programmed cell death called apoptosis, phosphatidylserine—a phospholipid that is normally confined to the cytosolic monolayer of the plasma membrane—rapidly translocates to the extracellular, outer monolayer. The presence of phosphatidylserine on the cell surface serves as a signal that helps direct the rapid removal of the dead cell.

For a cell to engineer this process for apoptosis to be activated, a protein called scramblase begins transferring phospholipids from one monolayer of the plasma membrane to the other monolayer.

When phosphatidylserine, which is normally confined to the cytosolic monolayer, is translocated to the outer monolayer, apoptosis is activated.

For this phospholipid to remain on the outer layer, the protein flippase that would normally transfer phosphatidylserine from the extracellular monolayer to the cytosolic monolayer is inactivated.

With the joined work of these two proteins, phosphatidylserine rapidly accumulates at the cell surface. This signals the proteolytic caspase cascade that will lead to the death of the cell.