Measurement of Hydrodynamic Boundary Conditions and Viscosity of Lipid Membranes

Cell membranes have a difficult role - they must be able to separate and protect the interior of the cell from its environment, but they also must be able to selectively allow desired materials in and waste out, while remaining flexible and being able to grow. This range of functions is accomplished with a complicated and dynamic mixture of lipids and proteins. While the proteins are responsible for specific tasks, it is the lipids that determine the structure of the membrane and their fluidity that allows the embedded proteins to arrange themselves. Understanding lipid fluid properties is therefore important in order to have a full picture of cellular dynamics. We examine the interaction between a lipid bilayer and the surrounding fluid in order to measure the boundary conditions. With an extremely precise measurement of the diffusion of giant unilamellar vesicles we are able to use the Stokes-Einstein relation to determine that despite its fluidity, a lipid membrane has boundary conditions that match those of a solid. Next we assess the accuracy of a technique used to measure the viscosity of a membrane using elliptical beads by performing it simultaneously with an established technique using the motion of phase separated domains. After showing its reliability we apply it to determine if there is any viscosity dependence on lipid chain length, which would be impossible using the phase separated domain method. We find that there is a small increase in viscosity with chain length. We also examine the effect a recently discovered protein, $\beta$ cell expansion factor A, or BefA, has on membrane morphology. We find that it causes membrane budding, resulting in clusters of small vesicles adhered to the outside of a larger one. This dissertation contains previously published and unpublished material