Bài giảng Biology - Chapter 7: Membrane Structure and Function

Chapter 7Membrane Structure and FunctionOverview: Life at the EdgeThe plasma membraneIs the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective permeability It allows some substances to cross it more easily than othersFigure 7.1Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteinsPhospholipidsAre the most abundant lipid in the plasma membraneAre amphipathic, containing both hydrophobic and hydrophilic regionsThe fluid mosaic model of membrane structureStates that a membrane is a fluid structure with a “mosaic” of various proteins embedded in itMembrane Models: Scientific InquiryMembranes have been chemically analyzedAnd found to be composed of proteins and lipidsScientists studying the plasma membraneReasoned that it must be a phospholipid bilayer Figure 7.2HydrophilicheadHydrophobictailWATERWATERThe Davson-Danielli sandwich model of membrane structureStated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layersWas supported by electron microscope pictures of membranesIn 1972, Singer and NicolsonProposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayerFigure 7.3PhospholipidbilayerHydrophobic region of proteinHydrophobic region of proteinFreeze-fracture studies of the plasma membraneSupported the fluid mosaic model of membrane structureFigure 7.4A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.Extracellular layerCytoplasmic layerAPPLICATIONA cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior.TECHNIQUEExtracellularlayerProteinsCytoplasmic layerKnifePlasmamembraneThese SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. RESULTSThe Fluidity of MembranesPhospholipids in the plasma membraneCan move within the bilayerFigure 7.5 ALateral movement(~107 times per second) Flip-flop(~ once per month)(a) Movement of phospholipidsProteins in the plasma membraneCan drift within the bilayerEXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell.Membrane proteinsMouse cellHuman cellHybrid cellMixedproteinsafter1 hourRESULTSCONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.Figure 7.6+The type of hydrocarbon tails in phospholipidsAffects the fluidity of the plasma membraneFigure 7.5 BFluidViscousUnsaturated hydrocarbontails with kinksSaturated hydro-Carbon tails(b) Membrane fluidityThe steroid cholesterolHas different effects on membrane fluidity at different temperaturesFigure 7.5 (c) Cholesterol within the animal cell membraneCholesterolFigure 7.7GlycoproteinCarbohydrateMicrofilamentsof cytoskeletonCholesterolPeripheralproteinIntegralproteinCYTOPLASMIC SIDEOF MEMBRANEEXTRACELLULARSIDE OFMEMBRANEGlycolipidMembrane Proteins and Their FunctionsA membraneIs a collage of different proteins embedded in the fluid matrix of the lipid bilayerFibers ofextracellularmatrix (ECM)Integral proteinsPenetrate the hydrophobic core of the lipid bilayerAre often transmembrane proteins, completely spanning the membraneEXTRACELLULARSIDEFigure 7.8N-terminusC-terminusa HelixCYTOPLASMICSIDEPeripheral proteinsAre appendages loosely bound to the surface of the membraneAn overview of six major functions of membrane proteinsFigure 7.9 Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane.Enzymatic activity. A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.(a)(b)(c)ATPEnzymesSignalReceptorCell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions (see Figure 6.31).Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29).(d)(e)(f)Glyco-proteinFigure 7.9 The Role of Membrane Carbohydrates in Cell-Cell RecognitionCell-cell recognitionIs a cell’s ability to distinguish one type of neighboring cell from anotherMembrane carbohydratesInteract with the surface molecules of other cells, facilitating cell-cell recognitionSynthesis and Sidedness of MembranesMembranes have distinct inside and outside facesThis affects the movement of proteins synthesized in the endomembrane systemMembrane proteins and lipidsAre synthesized in the ER and Golgi apparatusERFigure 7.10TransmembraneglycoproteinsSecretoryproteinGlycolipidGolgiapparatusVesicleTransmembraneglycoproteinMembrane glycolipidPlasma membrane:Cytoplasmic faceExtracellular faceSecretedprotein4123Concept 7.2: Membrane structure results in selective permeabilityA cell must exchange materials with its surroundings, a process controlled by the plasma membraneThe Permeability of the Lipid BilayerHydrophobic moleculesAre lipid soluble and can pass through the membrane rapidlyPolar moleculesDo not cross the membrane rapidlyTransport ProteinsTransport proteinsAllow passage of hydrophilic substances across the membraneConcept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investmentDiffusionIs the tendency for molecules of any substance to spread out evenly into the available spaceFigure 7.11 ADiffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.Molecules of dyeMembrane (cross section)Net diffusionNet diffusionEquilibrium(a)Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to anotherFigure 7.11 BDiffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen-tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total soluteconcentration was initially greater onthe left side.(b)Net diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibriumEffects of Osmosis on Water BalanceOsmosisIs the movement of water across a semipermeable membraneIs affected by the concentration gradient of dissolved substancesFigure 7.12Lowerconcentrationof solute (sugar)Higherconcentrationof sugarSame concentrationof sugarSelectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules canMore free watermolecules (higherconcentration)Water moleculescluster around sugar moleculesFewer free watermolecules (lowerconcentration)Water moves from an area of higher free water concentration to an area of lower free water concentrationOsmosisWater Balance of Cells Without WallsTonicityIs the ability of a solution to cause a cell to gain or lose waterHas a great impact on cells without wallsIf a solution is isotonicThe concentration of solutes is the same as it is inside the cellThere will be no net movement of waterIf a solution is hypertonicThe concentration of solutes is greater than it is inside the cellThe cell will lose waterIf a solution is hypotonicThe concentration of solutes is less than it is inside the cellThe cell will gain waterWater balance in cells without wallsFigure 7.13 Hypotonic solutionIsotonic solutionHypertonic solutionAnimal cell. Ananimal cell fares bestin an isotonic environ-ment unless it hasspecial adaptations tooffset the osmoticuptake or loss ofwater.(a)H2OH2OH2OH2OLysedNormalShriveledAnimals and other organisms without rigid cell walls living in hypertonic or hypotonic environmentsMust have special adaptations for osmoregulationH2OFigure 7.13 Hypotonic solutionIsotonic solutionHypertonic solutionAnimal cell. Ananimal cell fares bestin an isotonic environ-ment unless it hasspecial adaptations tooffset the osmoticuptake or loss ofwater.(a)H2OH2OH2OLysedNormalShriveledWater Balance of Cells with WallsCell wallsHelp maintain water balanceIf a plant cell is turgidIt is in a hypotonic environmentIt is very firm, a healthy state in most plantsIf a plant cell is flaccidIt is in an isotonic or hypertonic environmentWater balance in cells with wallsPlant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.(b)H2OH2OH2OH2OTurgid (normal)FlaccidPlasmolyzedFigure 7.13 Facilitated Diffusion: Passive Transport Aided by ProteinsIn facilitated diffusionTransport proteins speed the movement of molecules across the plasma membraneChannel proteinsProvide corridors that allow a specific molecule or ion to cross the membraneFigure 7.15 EXTRACELLULARFLUIDChannel proteinSoluteCYTOPLASMA channel protein (purple) has a channel through which water molecules or a specific solute can pass.(a)Carrier proteinsUndergo a subtle change in shape that translocates the solute-binding site across the membraneFigure 7.15 Carrier proteinSoluteA carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.(b)Concept 7.4: Active transport uses energy to move solutes against their gradientsThe Need for Energy in Active TransportActive transportMoves substances against their concentration gradientRequires energy, usually in the form of ATPThe sodium-potassium pumpIs one type of active transport systemFigure 7.16PP iEXTRACELLULARFLUID Na+ binding stimulatesphosphorylation by ATP.2Na+ Cytoplasmic Na+ binds tothe sodium-potassium pump.1 K+ is released and Na+sites are receptive again; the cycle repeats.3 Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.4 Extracellular K+ binds to the protein, triggering release of the Phosphate group.6 Loss of the phosphaterestores the protein’s original conformation.5CYTOPLASM[Na+] low[K+] highNa+Na+Na+Na+Na+PATPNa+Na+Na+PADPK+K+K+K+K+K+[Na+] high[K+] lowReview: Passive and active transport comparedFigure 7.17Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.ATPMaintenance of Membrane Potential by Ion PumpsMembrane potentialIs the voltage difference across a membraneAn electrochemical gradientIs caused by the concentration electrical gradient of ions across a membraneAn electrogenic pumpIs a transport protein that generates the voltage across a membraneFigure 7.18EXTRACELLULARFLUID+H+H+H+H+H+H+Proton pumpATPCYTOPLASM++++–––––+Cotransport: Coupled Transport by a Membrane ProteinCotransportOccurs when active transport of a specific solute indirectly drives the active transport of another solute Cotransport: active transport driven by a concentration gradientFigure 7.19Proton pumpSucrose-H+cotransporterDiffusionof H+SucroseATPH+H+H+H+H+H+H+++++++––––––Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosisLarge proteinsCross the membrane by different mechanismsExocytosisIn exocytosisTransport vesicles migrate to the plasma membrane, fuse with it, and release their contentsEndocytosisIn endocytosisThe cell takes in macromolecules by forming new vesicles from the plasma membraneEXTRACELLULARFLUIDPseudopodiumCYTOPLASM“Food” or other particleFoodvacuole1 µmPseudopodiumof amoebaBacteriumFood vacuoleAn amoeba engulfing a bacterium viaphagocytosis (TEM).PINOCYTOSISPinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).0.5 µmIn pinocytosis, the cell “gulps” droplets of extracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.PlasmamembraneVesicleIn phagocytosis, a cellengulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Three types of endocytosisFigure 7.20PHAGOCYTOSIS0.25 µmRECEPTOR-MEDIATED ENDOCYTOSISReceptorLigandCoat proteinCoatedpitCoatedvesicleA coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).PlasmamembraneCoatproteinReceptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.