Differential stress in asymmetric membranes

Lipid membranes are called "asymmetric" if their two leaflets differ in at least one physical property, the most widely considered one being lipid composition. Many biological membranes are asymmetric, as has been known since the 1970s, but our ability to replicate asymmetry in artificial model membranes is a much more recent achievement, and it has led to surprising discoveries regarding their thermodynamic and mechanical properties. In this presentation I investigate the implications of a particular type of asymmetry, namely, a difference in lateral tension between the two leaflets, a situation I refer to as differential stress. Being a source of torque, it obviously couples to membrane shape and other drivers of bending, such as spontaneous curvature. But differential stress also affects lipid phase behavior, for instance by modifying the fluid-gel transition, and it is one of several factors that determine the equilibrium distribution of cholesterol between the leaflets. I will present a simple model for this cholesterol balance and, as an example, apply it to the human red blood cell.