The study of biological systems offers opportunities for
interdisciplinary contributions from non-biological scientists. As a
physicist, I am particularly interested in how the underlying
thermodynamic behavior of lipid monolayers and bilayers determines
biological structure and function of lipid membranes.
Membranes carry out an important variety of functions at the interfaces of cells and organelles including providing simple barriers and localizing crucial proteins. Studying the physical properties of membranes begins with understanding the lipid and protein molecules that compose them. Lipids are the structural components of cell membranes; they are amphiphilic molecules simultaneously possessing both hydrophobic (tails) and hydrophilic (head group) groups.
In the presence of water lipids self-assemble into a variety of structures including lipid bilayers of cell membranes. In 1997 Simons and Ikonen proposed a new model of the cell membrane called the “raft hypothesis” in which small clusters (rafts) of certain lipids and proteins form within the cell membrane. This localization is implicated in many cellular processes such as membrane trafficking and signaling (Simons and Ikonen 1997). Separation of lipids into regions of different compositions is appealing to physicists because of its similarity to a miscibility phase transition (which occurs when a homogenous liquid phase separates into two distinct coexisting liquids). This phenomenon has been observed in mixtures of lipids deposited in monolayers at the air water interface (McConnell 1991).
More recently this phenomenon has been observed in lipid bilayer systems of giant unilamellar vesicles (GUVs) (Dietrich et al. 2001; Veatch and Keller 2002). Miscibility phase transitions in GUVs and monolayers are observed for the same compositions biologists often cite to be “raft forming.” This suggests phase separation may play a key role in membrane processes. While lipid monolayers and bilayers exhibit similar behavior, the two systems are strikingly different (Stottrup et al. 2004; Stottrup et al. 2005). Careful study of monolayer and bilayer behavior is crucial in appropriately applying lessons from model systems to biology.