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# 2: Membranes - Aggregated Lipids

Biological membranes consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. The bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins.

• 2.1: 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.
• 2.2: 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.
• 2.3: 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.
• 2.4: 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.
• 2.5: 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.
• 2.6: 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.
• 2.7: 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.