Fig. 2.

Model of ATP-dependent flippases and floppases. Flippases translocate lipids (typically phosphatidylserine and phosphatidylethanolamine) against a concentration gradient, towards the cytoplasmic face of the membrane. Floppases, exemplified by ABC transporters, translocate substrates (e.g., phosphatidylcholine, sphingolipids, cholesterol) in the opposite direction. Scramblases (not shown) are ATP-independent and calcium-dependent and transport lipids in both directions, along the concentration gradient, disrupting membrane asymmetry (Daleke, 2007; Sharom, 2011a).