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# Membranes Properties and Morphologies

• Diffusion in Membranes
The amphipathic nature of the lipid bilayer, whose tails are hydrophobic and associate with each other and whose head groups are hydrophilic and interact with the aqueous environment, are critical to its structure. The composition of the lipid bilayer is also important for the diffusion both across and within the membrane. This membrane diffusion is important for a variety of functions, some of which include regulating the fluidity of the membrane, and the uptake of metabolites into the cell.
• Membrane asymmetry
Membrane asymmetry happens when a membrane no longer resembles uniformity in terms of lipid or protein distribution and relative leaflet curvature. The degree of asymmetry covers a wide range since, theoretically, asymmetry is any deviation of the 50:50 proportion when comparing two surfaces. Asymmetry can occur on both sides of a biological membrane or on just one (transverse asymmetry). Phase separation is one of the simplest examples of membrane asymmetry.
• Membrane Compressibility
Compressibility is a measure of the relative volume change of a substance in response to stress. Lipid membrane compressibility has been extensively studied and multiple models developed. Researchers have employed a combination of classic biochemical methods and computational analyses for the purpose of characterizing the elastic nature of mono- and bilayers.
• Membrane Curvature
There are significant local differences in membrane curvature even within one single cell: plasma membranes, organelle membranes of Golgi, endosome and ER. Conformation of these semi-permeable phospholipid bilayers are actively modulated by an interplay and interaction between lipids and proteins. It is essential for living organisms to regulate membrane curvatures since processes like endocytosis, exocytosis and tubulation depend on membrane dynamics to sustain life.
• Membrane Fluctuations
The fluid mosaic model for the cell membrane views the entire membrane as a dynamic system constantly in flux. Since then, the dynamics of cell membranes were studied extensively and have shown that the membrane is in even more flux than the fluid mosaic model had claimed. The membrane is inhomogeneous, with some parts changing their position and composition more rapidly than others.
• Membrane Permeability
All cells are contained by a cell membrane (biomembrane) selectively open to some chemicals and ions but acts as a barrier to undesired components. Here the focus would be on biological membranes in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integral, and/or peripheral proteins responsible for communication and transportation of chemicals and ions.
• Membrane Scattering
Membrane scattering encompasses a huge variety of methods for characterizing lipid membranes. For example, a few popular techniques include X-ray or neutron reflectivity and diffraction, Brewster angle microscopy, ellipsometry, X-ray interferometry, infrared reflection-adsorption spectroscopy, and vibrational sum frequency generation spectroscopy. These methods can be used with a variety of different model membranes types (described briefly below).
• Non-Membrane Lipid Assemblies
Amphipathic compounds (detergents) are a unique set of molecules with the ability of manipulation (distortion or formation) of the hydrophobic-hydrophilic interactions in biological samples. Detergents in an aqueous solution self-associate to colloid particles. The formation of the colloidal aggregates by detergents are termed micelles. In research micelle-forming detergents provide an amphipathic environment that can mimic lipid bilayers.
• Protein-lipid interactions
lipids also have integral roles such as storing energy or being a major component of a membrane. Despite their individual importance, interaction of lipids and proteins  can provide functions that would not be possible individually. The greatest number of these interactions are seen in membranes, which are composed of a wide variety of lipids and proteins.
• Surface Tension and Line Tension
Membrane surface tension, which consist of in-plane membrane tension and membrane-cytoskeleton adhesion, is the cohesive force that keep cell membranes intact. In order to deform the cell membrane, you need to overcome both the in-plane membrane tension and membrane-cytoskeleton adhesion. Line tension is the interfacial energy at the edge of membrane domain or at the lipid phase separation in cell membrane.
• Vesicles
The most basic definition of a vesicle is a compartment composed of many phospholipids with some form of head group. In a biological context, vesicles are typically formed by cells to uptake, excrete, or otherwise transport materials between membranous compartments in the cell. A synthetic vesicle, called a liposome, can be created by mixing phospholipid molecules in an aqueous environment.