![]() ![]() Using this same concept, Danielli and Davson proposed that cellular membranes were lipid bilayers, as proposed by Gorter and Grendel, that interacted with flattened or beta-sheet proteins via the hydrophilic head groups of membrane phospholipids. Edidin has discussed the historical concepts that cell membranes are composed of phospholipid bilayers plus some membrane proteins. In 1925, Gorter and Grendel used Langmuir’s methods to assess the notion that that red blood cells were surrounded by two layers of membrane lipids, which was consistent with Fricke’s estimate from cell membrane capacitance experiments that cell membranes were approximately 4 nm thick. This concept was implied by the experiments of Langmuir, who used oil layers on aqueous surfaces and measured surface tensions. In contrast, the hydrophilic portions of their structures interact with the aqueous environment and other hydrophilic molecules. ![]() Ī basic concept in the organization of cellular membranes is that they are made up of amphipathic molecular components that associate into macro-structures that exclude water interactions on their hydrophobic surfaces. Cells are also compartmentalized into organelles by various complex intracellular membranes that are also responsible for the biosynthesis of various molecules, energy production, replication, transportation, reutilization, destruction, secretion and other activities that are essential in cell and tissue organization and maintenance. In addition, cells release signals and molecules to adjacent cells, tissues and distant organs, including lipid vesicles and globules containing other molecules (proteins, DNA etc.), and in doing so, they can condition host micro- and macro-environments. Thus, cell membranes are important filters that provide a cellular barrier and continuity, while selectively transmitting signals, nutrients and substances from outside to inside cells and then to various cellular organelles. Cell membrane interactions with extracellular molecules determine how individual cells process nutrients, initiate cellular signaling and respond to and maintain normal cellular physiology. In this Special Issue, the use of membrane phospholipids to modify cellular membranes in order to modulate clinically relevant host properties is considered.Ĭell or plasma membranes are the first cellular barriers encountered by extracellular ions, molecules, lipid vesicles and globules, viruses and other cells. They can also be externalized in a reverse process and released as extracellular vesicles and exosomes. Various lipid globules, droplets, vesicles and other membranes can fuse to incorporate new lipids or expel damaged lipids from membranes, or they can be internalized in endosomes that eventually fuse with other internal vesicles and membranes. However, the fluid regions of membranes are very important in lipid transport and exchange. The presence of specialized membrane domains significantly reduced the extent of the fluid lipid matrix, so membranes have become more mosaic with some fluid areas over time. In addition, lipid–lipid and lipid–protein membrane domains, essential for cellular signaling, were proposed and eventually discovered. Subsequently, the structures associated with membranes were considered, including peripheral membrane proteins, and cytoskeletal and extracellular matrix components that restricted lateral mobility. This simplified version of cell membrane structure was never proposed as the ultimate biomembrane description, but it provided a basic nanometer scale framework for membrane organization. Integral membrane proteins can transform into globular structures that are intercalated to various degrees into a heterogeneous lipid bilayer matrix. The Fluid–Mosaic Membrane Model accounted for these and other properties, such as membrane asymmetry, variable lateral mobilities of membrane components and their associations with dynamic complexes. These thermodynamically untenable structures did not allow lipid lateral movements independent of membrane proteins. Early cell membrane models placed most proteins external to lipid bilayers in trimolecular structures or as modular lipoprotein units. ![]()
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