3 membranes_07_aris

3 membranes_07_aris - Medical Cell and Tissue Biology BMS...

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Medical Cell and Tissue Biology, BMS 6110C John P. Aris, Ph.D. Rm B1-8, 392-1873, [email protected] Chapters 2 and 3 Cell Membranes
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Cell Compartments Cells and compartments (e.g., organelles) are membrane-bound
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Cell Membranes Membranes consist of lipids, proteins, carbohydrates Carbohydrates are present only as modifications Asymmetric distribution of almost all components Glycosyl modifications on exterior (ectoplasmic) side
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Membrane Composition is Regulated Percentage by Weight Protein Lipid Carbohydrate Plasma membrane Erythrocyte 49 43 8 Intestinal Epithelium 45 49 6 Myelin (mostly PM) 18 79 3 Mito Inner Membrane 76 24 0 Erythrocyte PM - transport (e.g., ions); mechanical strength Epithelium PM - transport (e.g., solutes); cell-cell contacts Myelin - acts as “insulator” around axons of neurons Mitochondrial IM - H + transport and ATP synthesis
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Lipid Bilayer Membrane lipids - amphiphilic (hydrophilic + hydrophobic) Phospholipids - head group + fatty acid chains Cholesterol - amphiphilic sterol lipid Hydrophobic core - impermeable to charged, polar molecules Bilayers spontaneously close to form sealed compartment
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Lateral diffusion Flip-flop Flexion Exchange Rotation Phase transition “Solid” (gel-like) Fluid Lipid Bilayer Properties Lipids move in plane of bilayer, which is like a viscous fluid Flip-flop and exchange out of bilayer are very rare events Synthetic membranes - exist in "fluid" or "solid" state Biological membranes - must be “fluid” to function in cells
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Phospholipids Backbone is 3-carbon glycerol molecule (phosphoglyceride) Hydrophilic head group - choline (phosphatidylcholine) Hydrophobic tails - fatty acids (may be unsaturated)
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Head groups - charged or polar (hydrophilic) Hydroxyl (OH) groups are linked to phosphate Phosphatidylinositol (PI) is a glycophospholipid with 2 functions: adds to bilayer structure; is a precursor for signaling molecules Glycolipids - no phosphate in molecule, found in exoplasmic leaflet of plasma membrane Common Head Groups
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Cholesterol is a sterol lipid that doesn’t form bilayers by itself but intercalates between phospholipids. Cholesterol
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Lipid Composition of Cell Membranes Chol PC PE PS PI PG CL SM Plasma Membrane 30 18 11 9 4 0 0 14 Golgi Complex 8 40 15 4 6 0 0 10 Smooth ER 10 50 21 0 7 0 2 12 Rough ER 6 55 16 3 8 0 0 3 Nuclear Envelope 10 55 20 3 7 0 0 3 Lysosome 14 25 13 0 7 0 5 24 Mito Inner 3 45 24 1 6 2 18 3 Mito Outer 5 45 23 2 13 3 4 5 Percentage by weight from hepatocytes. Chol, cholesterol; PC,
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3 membranes_07_aris - Medical Cell and Tissue Biology BMS...

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