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Lecture 6 (Sept 7) - Biological S nce 110A I ntroduction to...

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Lecture6: Membrane structure& membraneproteins Plasma and organellemembranes Structureand properties of phospholipids Properties of themembrane lipid bilayer Structureand properties of sphingolipids Roles of cholesterol in themembrane Asymmetric distribution of lipids in membranes Classes and structureof membraneproteins Biological Sciences 110A: Introduction to Biology Kendal Broadie Reading in Chapter 4 (120-136) Karp
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Plasma membrane of red blood cell All cells aresurrounded by a hydrophobic plasma membranecomposed primarily of lipids Membranes act as “ semi-permeable barriers ” between the cytoplasmand extracellular environment Membrane receptors senseenvironmental signals and transduce information into thecell interior Membrane transport machinery allows cell to concentrate substances (sugars, salts (ions), etc.) Themembrane scaffolds biochemical reactions , providing a defined spacefor ordered molecular associations and interactions SeeFig. 4.1 Karp
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Eukaryotecells arecompartmentalized into many membrane- bound organelles Organelles dividenumerous aqueous compartments within thecell Organelles protect thegenomeand spatially separate transcription, translation and protein modification Organelles sequester “dangerous” metabolic reactions involving enzymatic catalysts that could damagethe cell Organelles provideacidic (low pH) chambers used in degradation Organelles providehigh [H + ] and [Ca 2+ ] reserves: energy production & signaling SeeFig. 4.2 Karp
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Primary component of membranes arephospholipids composed of: also called phosphoglycerides owing to glycerol backbone 16-20 carbons SeeFig. 4.3 Karp
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Thepolar head group is variablegiving riseto a complex mixtureof phospholipids
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Lipids arenot soluble in water, spontaneously form “bilayers” Can be dispersed in water as: Micelles: aggregates of 50-100 of lipid molecules with polar heads facing out lipids act as “emulsifying agents:” hydrophobic compounds aresequestered in micelleinterior, allowing them to bedispersed in water as an emulsion. This is being developed as a drug delivery system Bilayer membranes: polar heads facewater and nonpolar hydrocarbon tails buried in interior. Hydrophilic surfaces, hydrophobic interior H 2 O Lipid molecule Micelle Bilayer membrane ~3 nm SeeFig. 4.3 Karp
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Membrane sheets spontaneously close to form “liposomes” Open sheets of membraneareenergetically unfavorable…becausethehydrophobic lipid tails areexposed to water at theedges.
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