Membranes-notes

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Unformatted text preview: M. S. Shell 2009 1/15 last modified 10/27/2010 Membranes Overview Membranes b very thin films of molecules that enclose cells, organelles, compartments Membrane composition Very different composition in prokaryotes and eukaryotes Typically composed of lipids and proteins, about 50%/50% by mass ~1 billion lipid molecules / cell Lipids provide basic structure, while proteins have specific functional roles Many different kinds of lipids: basic feature is that they are amphipathic , i.e., have both hydrophobic and a polar groups Typical phospholipid consists of head group consisting of a phosphate group plus a polar group glycerol hydrocarbon tail Phospholipids can vary in many respects; in particular, length of the tail (14-24 carbons) and number of unsaturated (double) bonds Most common type of phospholipid in cell membranes is phosphatidylcholine Cholesterols are also ampipathic and found in membranes Membrane structure The hydrophobic and polar parts of amphipathic molecules want to phase separate, but cannot because they are covalently bonded Instead, they can self-assemble to form various structures that minimize the exposure of the hydrophobic tails to water micelles bilayers M. S. Shell 2009 2/15 last modified 10/27/2010 vesicles Bilayers are about 5nm thick Structure is determined by the shape of the lipids and also their concentration Same principle as in soaps that use surfactants Liposomes are synthetic vesicles that are very important for drug delivery active therapeutic can be stored inside the vesicle for controlled release form spontaneously when phospholipids added to water diameters 25 nm 1 mm Thermodynamics of self assembly Self-assembly occurs at critical concentrations Consider the self assembly of n monomers M into an assembled micelle structure A: gG G Lets say we put some initial concentration of monomers in a solution, and we want to see what fraction are free versus in an assembled structure: G G g Substitute in the equilibrium expression: G G g G We could try to solve this expression for G , but analytical solutions cannot be found for arbitrary g . Instead, well rewrite it in a simpler fashion. Lets relate to the total concentration G at which half of the monomers as free: G for which G 1 2 G Substituting into the above equation: 1 2 g 2 M. S. Shell 2009 3/15 last modified 10/27/2010 Solving for g G , g G 2 2 Rewriting the above expression equilibrium expression using instead of g G : 1 2 What does this equation predict? Consider the limits for large values For , the second term in brackets is very small. Thus . In other words, almost all of the monomers are free and very few are associated...
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Membranes-notes - M. S. Shell 2009 1/15 last modified...

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