Abstract: Nature uses lipid membranes as a primary mechanism for compartmentalizing the living world. They enclose cells, and in eucaryotes form the compartments of many well-known organelles. It has been known for more than half a century that many of these biomembranes—and certainly the plasma membrane—are asymmetric: their lipid content on the two leaflets forming the bilayer is different, often dramatically so. Given time, lipids will spontaneously hop between leaflets via random thermal fluctuations, hence asymmetry must be maintained by proteins that actively transport lipids against chemical potential gradients, telling us that cells indeed care about this deeply.
While phospholipid composition is the best studied manifestation of asymmetry, any observable that can be defined at the level of a single leaflet could potentially be asymmetric—in fact, it very likely is given that compositional asymmetry is already broken. In this talk I will discuss two examples that are highly consequential for many biomembrane properties and processes, but whose asymmetry remains very difficult to establish directly: lateral membrane tension and cholesterol content. I will show that these two are intimately related, and how they may require us to rethink many old ideas: from mechanical stability to phase transitions and lipid rafts.