LS1a F08 Lecture Notes 10-16-3

LS1a F08 Lecture Notes 10-16-3 - 1 A lipid bilayer has...

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2 A lipid bilayer has edges in which the hydrophobic lipid tails are exposed to water. Because the contact of hydrophobic molecules with water is energetically unfavorable, synthetic bilayers organize themselves to eliminate free edges. The free edges can be eliminated by forming a sealed compartment, which places the polar head groups in contact with water and buries the hydrophobic fatty acid tails inside the compartment and away from water. This spontaneous formation of sealed compartments is an amazing property that is fundamental to living cells; it arises because of the amphipathic properties of phospholipids, their shape, and the fact that these molecules exist in an aqueous environment. The hydrophobic effect is the major driving force for bilayer formation and the formation of sealed compartments - it is the configuration of lowest free energy which minimizes the amount of hydrophobic surface area in contact with water.
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3 Although we draw them as static structures, biological membranes are very dynamic because individual phospholids in bilayers are highly mobile. The two halves of the bilayer are referred to as leaflets . Individual phospholipids can rotate about their long axis, their fatty acid tails can flex , and they can diffuse laterally in the plane of the bilayer. Lateral diffusion is so fast that a phospholipid will move through the membrane from from one end of a bacterial cell to the other (~2 um) in 1 second. By contrast, exchange of phospholipids between the two leaflets - called flip-flop - is rare; on average it occurs less than once per month for a single phospholipid.
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4 Animation of dynamics of lipid and protein movement in cell membranes
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5 Cell membranes are two dimensional fluids or liquid crystals in which hydrophobic components in the membrane - lipids and membrane proteins - are constrained within the plane of the membrane, but are free to diffuse laterally. How do we know that membranes are so dynamic ? Proteins or membranes can be labeled with a fluorescent molecule (the fluorescent molecule is covalently attached to the protein or lipid, or a fluorescent antibody is used that interacts non- covalently) so that they can be visualized by fluorescence microscopy. Fluorescent molecules absorb light of a certain wavelength and then emit light of a different wavelength; the emitted light can then be measured with a microscope and sensitive camera attached to it (fluorescence microscope). A technique called photobleaching is commonly used to measure lipid and protein mobility in membranes. Photobleaching is light-induced inactivation or fading of a fluorescent molecule, resulting from chemical damage and covalent modification. In this experiment the molecule whose mobility you want to measure in the membrane is labeled with a fluorescent dye. A very intense laser is used to produce light and inactivate (bleach) the fluorescence in a small area of the membrane (this is the decrease in fluorescence and the production of the dark square in
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LS1a F08 Lecture Notes 10-16-3 - 1 A lipid bilayer has...

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