Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Lipids and Membranes
Membranes are asymmetric, meaning they have non-equivalent sides

Most membranes are asymmetric, meaning they have unequal faces, which is in full agreement with the Fluid Mosaic Model. This Asymmetry manifests itself, firstly, in the different polar lipid compositions of the inner and outer surfaces of Plasma Membranes in bacterial and animal Cells. For instance, the inner lipid monolayer of the human erythrocyte membrane predominantly contains phosphatidylethanolamine and phosphatidylserine, whereas the outer layer contains phosphatidylcholine and sphingomyelin. Secondly, certain membrane transport systems operate in only one direction. For example, Erythrocyte membranes feature a transport system (“pump”) that extrudes Na⁺ ions from The Cell into the extracellular environment while pumping K⁺ ions inward, utilizing energy released from ATP Hydrolysis. This pump, known as the Na⁺, K⁺-transporting ATPase, never transports Na⁺ and K⁺ ions in the opposite directions. Thirdly, the outer surface of plasma membranes bears a very high number of oligosaccharide groups representing the HEAD groups of Glycolipids and the oligosaccharide side chains of Glycoproteins, whereas the inner surface is virtually devoid of oligosaccharide groups.

The asymmetry of Introduction/36.html">Biological Membranes is maintained primarily because the translocation of individual phospholipid molecules from one monolayer of The Lipid Bilayer to the other is severely restricted (Fig. 12-19). This barrier to transversal movement is due to the high energy required to force the polar, charged head groups of phospholipid molecules through the hydrophobic hydrocarbon core of the membrane. Consequently, a polar lipid molecule can diffuse freely within its own side of the bilayer, but its ability to "flip-flop" to the opposite surface is severely restricted (Fig. 12-19).

Class="center">

Fig. 12-19. Asymmetric Distribution of Lipids across the two faces of The cell membrane lipid bilayer. Polar lipid molecules can diffuse freely across the plane of either membrane surface; however, the "flip-flop" of lipid molecules from one side of the membrane to the other, as depicted in the scheme, occurs only on rare occasions.