CH7 - CH7 Cell Membranes The plasma membrane is the...

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1 CH7 – Cell Membranes • The plasma membrane is the boundary that separates the living cell from its surroundings • The plasma membrane exhibits selective permeability , allowing some substances to cross it more easily than others Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules , containing hydrophobic and hydrophilic regions • The FLUID MOSAIC MODEL states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER
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2 The Fluidity of Membranes • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, drift laterally • Rarely does a molecule flip-flop transversely across the membrane Fig. 7-5a (a) Movement of phospholipids Lateral movement ( 10 7 times per second) Flip-flop ( once per month) • As temperatures cool, membranes switch from a fluid state to a solid state • The temperature at which a membrane solidifies depends on the types of lipids • Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids • Membranes must be fluid to work properly; they are usually about as fluid as salad oil
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3 Fig. 7-5b (b) Membrane fluidity Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro- carbon tails • The steroid cholesterol has different effects on membrane fluidity at different temperatures • At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids • At cool temperatures, it maintains fluidity by preventing tight packing The affect of cholesterol on the Fluidity of Membranes Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane
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4 Membrane Proteins and Their Functions • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Illustrated in the FLUID MOSAIC MODEL • Proteins determine most of the membrane’s specific functions Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- protein Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Carbohydrate Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core • Integral proteins that span the membrane are called transmembrane proteins • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
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Fig. 7-8 N-terminus C-terminus α Helix CYTOPLASMIC SIDE EXTRACELLULAR SIDE Fig. 7-9ac (a) Transport (b) Enzymatic activity (c) Signal transduction ATP Enzymes Signal transduction Signaling molecule Receptor • Six major functions of membrane proteins: Fig. 7-9df (d) Cell-cell recognition Glyco- protein (e) Intercellular joining (f) Attachment to
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This note was uploaded on 07/08/2011 for the course BIOL 101 taught by Professor Smith during the Spring '11 term at Aachen University of Applied Sciences.

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CH7 - CH7 Cell Membranes The plasma membrane is the...

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