Freeze fracture studies of the plasma membrane

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Freeze-fracture studies of the plasma membrane supported the fluid mosaic modelv.Freeze-fracture is a specialized preparation technique that splits a membrane along the middleof the phospholipid bilayerc.The Fluidity of Membranesi.Phospholipids in the plasma membrane can move within the bilayerii.Most of the lipids, and some proteins, drift laterallyiii.Rarely does a molecule flip-flop transversely across the membraneiv.As temperatures cool, membranes switch from a fluid state to a solid statev.The temperature at which a membrane solidifies depends on the types of lipidsvi.Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acidsvii.Membranes must be fluid to work properly; they are usually about as fluid as salad oilviii.The Role of Cholesterol1.The steroid cholesterol has different effects on membrane fluidity at differenttemperatures2.At warm temperatures (such as 37°C), cholesterol restrains movement ofphospholipids3.At cool temperatures, it maintains fluidity by preventing tight packingd.The Functions of Membrane Proteins
Biological Science ITuesdays and Thursdays 9:30-10:45, FLH 0255Wednesdays 5:15-6:15, KIN 1024BSC 2010Dr. Dennis28i.A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayerii.Proteins determine most of the membrane’s specific functionsiii.Peripheral proteins are bound to the surface of the membraneiv.Integral proteins penetrate the hydrophobic core1.Integral proteins that span the membrane are called transmembrane proteins2.The hydrophobic regions of an integral protein consist of one or more stretches ofnonpolar amino acids, often coiled into alpha helicesv.Six major functions of membrane proteins:1.Transport2.Enzymatic activity3.Signal transduction4.Cell-cell recognition5.Intercellular joining6.Attachment to the cytoskeleton and extracellular matrix (ECM)vi.Cell-Cell Recognition1.Cells recognize each other by binding to surface molecules, often carbohydrates, on theplasma membrane2.Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) ormore commonly to proteins (forming glycoproteins)3.Carbohydrates on the external side of the plasma membrane vary among species,individuals, and even cell types in an individuale.Synthesis and Sidedness of Membranesi.Membranes have distinct inside and outside facesii.The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasmamembrane is determined when the membrane is built by the ER and Golgi apparatusf.Membrane Structure Results in Selective Permeabilityi.Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer andpass through the membrane rapidlyii.Polar molecules, such as sugars, do not cross the membrane easilyg.Transport Proteinsi.Transport proteins allow passage of hydrophilic substances across the membraneii.Some transport proteins, called channel proteins, have a hydrophilic channel that certain

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Term
Spring
Professor
Pf
Tags
Biology, DNA, Chemical bond, Dr Dennis

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