Advanced Cell Biology Chapter 10 Notes
Advanced Cell Biology Chapter 10 Notes BCMB 311
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This 0 page Class Notes was uploaded by Izabella Nill Gomez on Monday February 8, 2016. The Class Notes belongs to BCMB 311 at University of Tennessee - Knoxville taught by Dr. Barry Bruce, Dr. J. Park in Spring 2016. Since its upload, it has received 27 views.
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Date Created: 02/08/16
Advanced Cell Bio Chapter 11 Notes A living cell is selfreproducing system of molecules held inside a containerpasma membraneprotein studded fatty lm so thin it can39t be seen directly by a light microscope Protects chemical components from outside Consists of 2ply sheet of lipids about 5nm thick into which proteins are inserted Properties unlike any sheet Membrane is penetrated by highly selective channelstransportersproteins that allow small molecules to be importedexported Other proteins act as receptorssensors for information about the environment changes Bacteria only have 1 membrane plasma eukarya also have internal membranes form various organelles like ER Golgi mitochondria All cell membranes are composed of lipids proteins and share common structure Lipids are arranged in two closely apposed sheetslipid bilayerserves as a permeability barrier for most H20 soluble molecules Structure of the cell membrane is determined by the way the membrane lipids behave in an aqueous environmentipid bilayer constitutes the fundamental structure of all cell membranes Most abundant lipids in the cell membrane are phospholipids which have hydrophilicphobic parts includes cholesterol glycerol lipids have sugars for a head Hydrophilic molecules dissolve readily in H20 because of chargeduncharged polar groups that can form electrostatic attractionsH bonds with H20 Hydrophobic molecules are insoluble because allalmost all atoms are unchargednonpolar Force adjacent H20 molecules to organize their structures around them highly orderedrequired free energycost minimized by phobic molecules clustering together Amphipathic molecules are subject to 2 con icting formshydrophiic head to H20 phobic tails reject and aggregate other phobic molecules Con ict resolved by forming bilayermost energetically favorable Same forces that drive bilayer formation help make it self sealingany tear will make the bilayer exposed to H20because energetically infavorable molecules will spontaneously rearrange to seal the free edge If the tear is small spontaneous rearrangement will exclude H20 and repair if it is a big sheet it may begin to fold in on itself and break up into separate closed vesicles Prohibition on free edges has consequencethe only way to avoid is by bonding and sealingforcing molecules to assemble into de ned closed compartments Fundamental to creation of a living cell result to property of amphipaths In the plane of the bilayer lipids can move and change spaces but not escape bilayer behaves as 2D uid Lipid bilayer is exibleabe to bindsets lower limit of 25 nm to size of vesicle membranes that can ll uidity and can be studies using synthetic bilayers easily provided by spontaneous aggregation of amphipathic lipids in H20 pure phospholipids will form spherical vesiclesiposomes Lipids rarely tumble from one monolayer to the otherquot ip op does not occur frequently without proteins to assistabout once per month for 1 lipid Lipids do continually exchange neighboring places through random thermal motions on the same monolayer Also ex tailsrotate around a long axis Fluidity of cell membrane is important for function how uid depends on phospholipid composition and nature of hydrocarbon tails closer and more regular packingmore viscous less uid 2 major properties affect how tightly the bilayer is packedength and number of 2x bonds they contain Shorter chain length decreases hydrocarbon interaction and makes membrane more uid Strength varies between 1224 C atoms 1820 common Most phospholipids have 1 tail with 1 or more 2x bonds between C39s and 2nCI tail with only 1x bonds Chain with 2x bonds does not have enough H atomsunsaturated With no 2x bondssaturated Each 2x bond creates a small kink which makes it more dif cult for tails to packthose with more unsaturated tails are more uid than those with lower ln bacteriayeast cellslengths and unsaturation of tails constantly adjusted to maintain constant uidityat higher temperatures the cell makes membrane lipids with more length less 2x bonds Similar trick is used to make margarine from vegetable oils fats generally unsaturated so hydrogenated to make single bonds In animal cells membrane uidity is modulated by inclusion of sterol cholesterol present in large amounts in the plasma about 20because it is short and stiff it lls the spaces between phospholipid kinks in unsaturated tailsstiffens bilayer to make less exible and less permeable Membrane uidity allows many membrane proteins to diffuse rapidly in the plane of the bilayer and interact with one anothercrucia in cell signaling Permits membrane lipids and proteins to diffuse from sites where inserted into the bilayer after synthesis to other regions of the cell Ensures membrane molecules are distributed evenly when the cell divides and to fuse membranes to mix molecules In eukarya new phospholipids are made by enzymes bound to the surface of the ER Using free fatty acids as substrates enzymes deposit newly made phospholipids exclusively on in the cystolic half of the bilayermembranes manage to grow evenly Transfer of lipids from one monolayer to the other rarely occurs spontaneously catalyzed by scramblases which remove randomly selected phospholipids from one half of the lipid to the othernew phospholipids are evenly distributed Some newly assembled membrane will remain in the ER rest for other compartments in the cell Random diffusion in vesicles to fuse with other membranes Most cell membranes are asymmetrical 2 halves of the bilayer have different sets of phospholipidsarises form Golgi Golgi has another family of phospholipid handling enzymes ippasesremove speci c phospholipids from a side of the bilayer facing the exterior space and ip them into the monolayer facing the cytosol Action of ippases initiates and maintains asymmetric arrangement of phospholipids characteristic of animal membranespreserved as membranes but from one organelle and fuse with another Glycolipids are located mainly in plasma and only in noncystolic half of the bilayer Sugar groups face the exterior of the cell where they form continuous coat of carb that surroundsprotects animal cells Sugar groups are acquired in the Golgi where the enzymes that engineer this modi cation are con ned No ippases transfer glycolipids to the other half of the bilayer cystolic side In cystol phospholipids have a special role in relaying signals from the cell surface to the interiorusuay in cystolic half Most membrane functions are carried out by membrane proteins In animals about 50 of plasma mass remainder being lipid and small number of carbs glycolipids and glycoproteins 50x more lipid molecules than protein because of size Some transport particular nutrients metabolizes ions across bilayerothers anchor membrane to macromolecules on either sideothers are receptors that detect signals in the cell39s environment and relay to the cell interior or work as enzymes to catalyze speci c reactions at the membrane Each type of membrane contains a different set of proteins re ecting specialized functions of the particular membrane Transmembrane proteins are amphipathic hydrophobic regions in the interior of the membrane with tails of phospholipids philic regions exposed to aqueous environment on either side of the membrane Other membrane proteins are located almost entirely in the cytosol associated with the cystolic half of the bilayer by amphipathic alpha helix exposed to the surface Some outside the bilayer one on one side or either attached to membrane by one or more lipid bilayer groups Can be indirectly bound to one side or the other by other proteinsintegra membrane proteins are lipidlinked can only be removed by detergents associate with the monolayer or transmembrane Remaining are peripheral can be released from the membrane by gentle attraction procedures that interfere with the proteinprotein interaction but leave the bilayer intact All membrane proteins have unique con rmation in the bilayer unique to function Peptide bonds that join successive amino acids in the protein are normally polar making the backbone hydrophilic because the interior of the bilayer has no H20 H bonds form with each other maximized if in an alpha helix formationphobic side chains are exposed to the outside philic to the inside In many transmembrane proteins polypeptide chains cross the membrane only oncemany are receptors for extracellular signals others are channels forming pores across a bilayermust form a series of alpha helices multipasscontain phobicphilic side chainsphobes on one side philics on the other Although alpha is te most common conformation beta sheets rolled into a cylinder can form in a barrel shape Interior is hydrophilic ex porin protein To study the membrane protein must be separated from all othersinvolves solubilizing the membrane with agents that destroy the lipid bilayer by disrupting associationswidey used are detergentssmal amphipathic lipidlike molecules that have only one phobic tailcone shaped aggregate to form into micelles when mixed in solution phobic ends interact with membranespanning phobic regions of the transmembrane proteins and phobic tails of phosphatesphiic regions bring proteins in detergent at the same time solubilizes Standard method to learn the 3D structure of protein is crystallography but requires ordered arraysmembrane proteins are harder to organize because of puri cation process and heterogeneous in size Bacteriorhodopsin is a small protein found in large amounts of archaean Halobacterium halobumlives in salt marshes Bacteriorhodopsin acts as a membrane transport protein that pumps H protons out of the cell Pumping requires energy and Bacteriorhodopsin gets it from sunlight has 1 light absorbing nonproteinretinalgives purple color Retianl is covalently attached to one of Bacteriorhodopsin39s 7 alpha heliceschanges shape when absorbing light causing small conformational changes causes transfer of H proton Retinal is regenerated by taking up the H proton from the cytosol returning the proton to the original conformation and repeating Cell used protein gradient to store energy to convert to ATP Cell membranes are fragile require proteins for support attached to membrane via transmembrane proteins For plants yeast bacteriacel39s shape and mechanical properties conferred by the cell wallmeshwork of proteins sugars other macromolecules that encase the plasma membrane Membrane in animals is stabilized by a meshwork of brous proteins attached to the underside of the membrane cell cortexcortex of human red blood cells simpleregular in structure attenedcontain olmeric protein spectrin as main component of the cortexong thin exible red about 100 nm Maintains cell39s biconcave shape Spectrin is connected to the membrane through intracellular attachment proteins that link spectrin to speci c transmembrane proteins These without spectrin are very fragile organisms tend to be anemic Cortex in other cells are rich in actin and myosin more complexother cells need to selectively take up materials from environment change shape actively moveneed cortex Also restrain diffusion of proteins within the plasma membrane Because the membrane is a 2D uid many proteins like lipids can move freely within the plane of the lipid bilayeratera diffusion demonstrated by the fusion of human and mouse cell After fusion initially proteins stay separated but after diffusedmix Cells have ways of con ning particular proteins to localized areas within the bilayer membrane creating functionally specialized regionsmembrane domains on cellorganelle surface Most proteins in the plasma are covered by sugarsshort chainsgycoproteins Proteoglycans contain one or more polysaccharide chains All carbs are located outside of the plasma where it forms a layer of glycocalyxprotects the cell from mechanical damageadsorb water and give cell slimy surfaceimportant role in cell recognitionlectins bind to special oligosaccharide chains
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