Asymmetrische lipidendubbellaag

The inner and outer layers of the plasma membrane have different lipid compositions, creating asymmetry in the membrane.

Due to the membrane asymmetry, the inside and outside membrane faces can each function distinctly.

Membrane asymmetry was first identified and has been extensively studied in erythrocytes.

In this cell type, the outer layer contains choline-phospholipids — sphingomyelin and phosphatidylcholine — which help in transmembrane signaling. The inner layer contains amino-phospholipids — phosphatidylserine and phosphatidylethanolamine — which act as binding sites for cytosolic enzymes and proteins.

Phosphatidylinositol is present at a relatively low concentration compared to other phospholipids and is primarily found in the inner layer.

Unlike the other membrane lipids, cholesterol is typically evenly distributed between the two layers.

Membrane asymmetry affects the fluidity and curvature of the inner and the outer layers, allowing the membrane to undergo bending, fission, and fusion.

The small head group of phosphatidylethanolamine enables it to fit into the inner layer of the curved membrane. In contrast, the large head groups of phosphatidylcholine and sphingomyelin fit into the outer curved membrane.