Lipid Diversity and Its Implications for Membrane Organization

Biological membranes are complex, mesoscopic assemblies that possess functions, which are far more elaborate than a simple permeability barrier (or a passive matrix) in which proteins reside and carry out their associated functions. Instead, biomembranes are complex, highly functional dynamic machines that are central to a host of biological processes, including the transport of materials, cell defense, recognition, adhesion, and signaling. Although the exact composition of biomembranes varies among different types of organisms—and even within individual organisms—all cell membranes share a generic lipid bilayer architecture. Membrane lipid molecules contain a polar (hydrophilic) headgroup and typically, two long nonpolar (hydrophobic) hydrocarbon fatty acid chains (Figure 7.1). In a lipid bilayer, two sheets (leaflets) of lipids align to form a pseudoplanar structure with their polar headgroups exposed to water, and their hydrocarbon chains packed in an oil-like phase. For membrane models, bilayers can be prepared as vesicles (Iqbal et al. 2011), which are closed, spherical systems—ranging from nanometers to micrometers in diameter—or as supported bilayers (Katsaras 1997, 1998), which are planar and therefore, highly suitable for experimental techniques such as x-ray and neutron diffraction (both in-plane and out-of-plane), and different types of microscopies. Monolayers prepared at the air–water interface using a Langmuir–Blodgett trough are simplified models of bilayers, and are useful experimental platforms all by themselves (Abraham et al. 2008). 7