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Stephen Fish, Ph.D. Marshall University J. C. E. School of Medicine Fish@Marshall.edu Note to instructors: I use these PowerPoint slides in cell biology lectures that I give to first year medical students. Copy the slides, or just the illustrations into your own teaching media. We all know that teaching science often requires compromises and simplification for specific student populations, or the requirements of a specific course. Please feel free to offer suggestions for improvements, corrections, or additional illustrations. I would be pleased to hear from anyone who finds my work useful, and am always willing to make it better. Also, the images have been compressed to screen resolution to keep PowerPoint file size down, and I can provide them at any resolution. Stephen E. Fish, Ph.D. Membrane Lipids Generic cell 95% of cellular membrane is in intracellular organelles Membranes are made of lipids and proteins Three membrane lipid types Phospholipids Cholesterol Glycolipids Phospholipids • Phosphatidylcholine • Phosphatidylserine • Phosphatidylethanolamine • Sphingomyelin (serine replaces glycerol backbone) • Phosphatidylinositol (one of the less common types) Hydrophilic (soluble) Hydrophobic (not soluble = lipophilic) Cholesterol • Up to 40% of mammalian cell membranes • Small polar head makes it weakly amphipathic Hydroxyl group is a very small polar (hydrophilic) head Rigid steroid hydrocarbon ring structure Flexible Nonpolar hydrocarbon (hydrophobic) chain Glycolipids • Pattern of sugar residues is variable • Always in outer leaflet of cell membrane, & inner leaflet of organelles Hydrophilic (soluble) Hydrophobic (not soluble = lipophilic) The hydrophobic effect & amphipathic lipids Polar (hydrophilic) head Nonpolar (hydrophobic) tail Air Water Amphipathic lipids under water form membranes automatically More lipid molecules form a sphere • Membrane disk (seen on edge) curves to minimize hydrophobic core exposure at edges • Enough lipids automatically forms a liposome • Similarly, punctured membranes seal themselves Why does it do that? • Compare the amount of exposed edge of curved & flat membrane for each size • Curved membrane exposes less of the hydrophobic interior at the edges Membrane self assembly is useful to science Motion of lipids in membranes • Molecular movement from heat – Rotate in place – Hydrophobic tails flex – Flip flop across membrane • Phospholipids rare • Cholesterol common – Lateral movement • Phase transition- cool down & motion stops Membrane phase transition • Lipid lateral, rotational, & flexional movement from heat spreads apart & maintains fluidity • Cooling reduces movement & lipids stick together (congeals like bacon grease) – Stiffens, is easily torn & can’t reseal – Interferes with membrane protein function Liquid Solidified when warm when cooled Types of lipids affect phase transition • Unsaturated phospholipids with bend in tail – Spread lipids out & makes more fluid – Lowers temperature of phase transition • Cholesterol with bend & rigid steroid rings – Lowers temperature of phase transition – Increases range over which transition occurs – Net affect- membrane is more fluid but stiffer (tougher) Lipid asymmetry in leaflets of the cell membrane Extracellular leaflet has more Phosphatidylcholine Sphingomyelin, & Glycolipids (opposite for organelles) Equal for Cholesterol Intracellular leaflet has more Phosphatidylserine, Phosphatidylethanolamine, & Phosphatidylinositol Some lipids in the outer leaflet of cellular membrane aggregate to form rafts • Rafts have lower phase transition temperature & are stiffer • Function discussed later with the membrane proteins associated with the rafts Lipid rafts have higher concentrations of - • Sphingolipid • Cholesterol • Glycolipids Although a minor membrane component (~2%), glycolipids have some special functions • Gangliosides in neurons – Oligosaccharides with negatively charged sialic acid residues – Attract positive ions, e.g. Ca++ – Affects electrical properties & signaling Glycolipids help form insulation for nervous system electrical activity • Negatively charged gangliosides in glia membrane • Repel negative ions & attract positive ions • Myelin insulation greatly increases the speed of action potentials Membrane proteins are next and Sherman says Lipids & proteins, OH BOY!
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