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Section 5

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Biology 2382B
Sashko Damjanovski

SECTION 5 Biomembranes • Membranes define what is a cell.Allow specialized cellular functions to occur in a localized manner. • Basic components: lipids, sterols and proteins • Due to amphipathicity, phospholipids spontaneously form lipid bilayers in aqueous solution where properties of the fatty acids confer properties onto the bilayers Phospholipid bilayer – polar groups face outward to shield hydrophobic fatty acyl tails from water. The hydrophobic effect and van der Waals interactions between the fatty acyl tails drive the assembly of the bilayer. FattyAcid • Long hydrocarbon chain attached to a polar carboxyl head group • Amphipathic • Melting point increases with increasing chain length o The more carbons, the higher the melting point o Melting point decreases with unsaturation, so the more double bonds, the lower the melting point • Often Cx:y, where x = number of carbon molecules and y = number of double bonds • No double bonds = saturated • One double bond = unsaturated • More than one double bond = polyunsaturated Effect of Double Bond on Structure • Tm increases with chain length, decreases with unsaturation. • Saturated chains can tightly packed – numerous van der Waal’s forces • cis double bonds common, trans rare • Over 100 fatty acids known • Only a few fatty acids are common in membranes • Melting point increases with increasing chain length, but melting point decreases with increasing unsaturation Three Classes of Membrane Lipids • Phosphoglycerides o Most common, 2 esterfied fatty acid tails and polar head group o Less common, 2 fatty acid tails but only one esterfied (plasmalogen) • Sphingolipids o Sphingosine o Amino alcohol with long hydrocarbon chain and fatty acyl chain attached via amide bound o Can be glycolipid (sugar head group) • Sterols o Four ring hydrocarbon o Most common example is cholesterol o Sterols are amphipathic. Can intercalate between phospholipids, into the bilayer, where they orient themselves with their polar hydroxyl groups facing the same direction as the polar head groups of the fatty acids • Lipid composition can affect thickness and curvature Properties of Biomembranes 1. Fluid 2. Closed compartments 3. Semi-permeable 4. Asymmetric Membrane Fluidity • Membranes are dynamic macromolecular assemblies • They are two dimensional fluids: o Rapid lateral diffusion (within the leaflet, laterally) o Slow, rare transverse (flip-flop) movement (between leaflets) • Fluidity is temperature - and composition-dependent o Phospholipids o cis double bonds o Steroids o Proteins Fluid – Measuring Lipid/Protein Movement • Up to 50% immobile • Diffusion 10X slower in plasma membranes than pure bilayers • Distances moved also restricted • Heat does alter membrane fluidity • Fluorescence graph is
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