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3: Membrane Phases and Morphologies

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    One of the most important properties of a lipid bilayer is the relative mobility (fluidity) of the individual lipid molecules and how this mobility changes with temperature. This response is known as the phase behavior of the bilayer. The phase behavior of lipid bilayers is largely determined by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. The extent of this interaction is in turn governed by how long the lipid tails are and how well they can pack together.

    • 3.1: Membrane Phase Transitions
      Biological membranes are primarily composed of phospholipids—a diverse class of compounds composed of a hydrophilic head group covalently attached to a pair of hydrophobic fatty acids. This amphipathic structure leads phospholipid molecules to spontaneously form bilayers when placed in water, as the phospholipids are driven to orient their head groups towards water and shield their fatty acid tails from it via the hydrophobic effect.
    • 3.2: The Main Phase Transition
      The membranes that provide structure and definition to cellular compartments are composed of dynamic and heterogeneous lipids whose varying chemical properties allow for their function. The transition between the two main phases of a lipid bilayer, the liquid crystalline phase (Lα) and the gel phase (Lβ) determines how the membrane will behave in terms of fluidity at a particular temperature and the formation of biologically useful membrane microdomains.
    • 3.3: The Fluid Phase
      Membranes in functional condition are predominantly comprised of fluid phase lipid bilayers. Maintenance of membrane fluidity is crucial for integrity and functionality. However, phase coexistence is normal since the membrane is comprised of many different lipid molecules.
    • 3.4: The Gel Phase
      Biological plasma membranes express a plethora of unique lipids, sterols, and proteins and exhibit greater complexity than model lipid bilayer systems. At temperatures above the melting point the lipid bilayer exists in the liquid phase and at lower temperatures, the lipid bilayer enters the gel phase. In the gel phase, the lipid bilayer undergoes a fundamental molecular reorganization characterized by an increase in order, a loss of lateral mobility and a change in the broad topology of the mem
    • 3.5: The Ripple Phase
      This phase is characterized by corrugations of the membrane surface with well-defined periodicity with an axis parallel to the mean bilayer plane [1]. The molecular origin of ripple-phase formation is traditionally been associated with the lipid headgroup region and hence lipids can be classified into ripple-forming and non-ripple forming lipids based on their headgroups.
    • 3.6: Rafts
      Lipid “rafts” are membrane microdomains composed of higher order lipids that have tight interaction with each other relative to the “sea” of liquid phase lipids known as bulk lipids. These rafts have been estimated to be 10-200 nm in size. Small rafts can merge to form large rafts through protein-protein and protein-lipid interactions. The rafts are important because certain membrane proteins preferentially localize to these domains and biological processes such as signal transduction occur ther
    • 3.7: Lipid Phase Coexistence
      Lipid phase coexistence occurs when a membrane is composed of a heterogeneous mix of lipid physical states organized into lateral domains that can range from gel (solid phase) to liquid disordered and to liquid ordered phases.

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