Biophysical studies of heterogeneities in a model cell membrane

Abstract

A cellular membrane is also known as the plasma or cytoplasmic membrane, a thin layer surrounding every cell. It provides protection to a cell and functions as a semipermeable membrane to regulate the endo- and exocytosis processes. Further, it plays a significant role in cell signalling. It is composed of various inevitable components, such as lipids, cholesterol, proteins, ions, etc., that control many of the physiological processes in living organisms. The biological membranes are heterogeneous in structure, forming phase-separated domains of definite shape and composition to govern the permeability and selectivity against additives in a membrane. Even though these domains have been known to exist for the last few decades, their thermodynamic stability is still not understood fully. Therefore, this thesis investigates domain formation and various types of heterogeneities in model cellular membranes, which are composed of lipids, ionic liquids (ILs), and amino acids. The role of hybrid lipids has been studied in stabilizing and controlling the self-assembly of multiple lipids in the membrane. It is found that these lipids preferentially mix in a fluid phase. Further, the effects of IL molecules on two systems have been examined. One is composed of anionic lipids that mimic a bacterial membrane and the other mimics the mammalian cell membrane with a few mol% of added cholesterol. It is found that cholesterol protects the membrane against the adverse effects of ILs. The amino acids used in the study are found to alter the thermodynamic properties of a model membrane, which is complemented by its modified structure. Altogether, this thesis sheds light on various types of heterogeneities in a model membrane induced by a few biologically relevant additives and quantifies the corresponding changes in the physical properties of the membrane.