Soft Matter Physics of Lipid Membrane-Based Assemblies

Mention lipids to a biologist and you likely conjure images of biomembranes that form the boundaries between living cells and their exterior and between organelles within the cell. These membranes are complex and heterogeneous complex fluids, typically composed of phospholipid bilayer aggregates that contain in addition proteins and other guest molecules. As borders of living cells, these bilayers must be sturdy and selective on the one hand, yet flexible enough to adapt to the surrounding environmental conditions on the other. This dual nature is achieved, in part, by the amphipathic structure of lipids composed of hydrophobic tails and hydrophilic headgroups. The fluid character of bilayers allows them to respond to the presence of interacting macromolecules through membrane deformations or variations in local lipid composition. In turn, these macromolecules may respond by changing their stable conformation, or by altered associations and organization. The cross talk between lipids and guests such as proteins is increasingly recognized as an important part of the physiological role of membranes in the cell.