Sinh học - Chapter 7: Membrane structure and function

Membrane Structure and FunctionChapter 7Overview: Life at the EdgeThe plasma membrane is the boundary that separates the living cell from its surroundingsThe plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others© 2011 Pearson Education, Inc.Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteinsPhospholipids are the most abundant lipid in the plasma membranePhospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regionsThe fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it© 2011 Pearson Education, Inc.Figure 7.2HydrophilicheadHydrophobictailWATERWATERFigure 7.3PhospholipidbilayerHydrophobic regionsof proteinHydrophilicregions of proteinThe Fluidity of MembranesPhospholipids in the plasma membrane can move within the bilayerMost of the lipids, and some proteins, drift laterallyRarely does a molecule flip-flop transversely across the membrane© 2011 Pearson Education, Inc.Figure 7.5Glyco-proteinCarbohydrateGlycolipidMicrofilamentsof cytoskeletonEXTRACELLULARSIDE OFMEMBRANECYTOPLASMIC SIDEOF MEMBRANEIntegralproteinPeripheralproteinsCholesterolFibers of extra-cellular matrix (ECM)Figure 7.6Lateral movement occurs107 times per second.Flip-flopping across the membraneis rare ( once per month).Figure 7.7Membrane proteinsMouse cellHuman cellHybrid cellMixed proteinsafter 1 hourRESULTSAs temperatures cool, membranes switch from a fluid state to a solid stateThe temperature at which a membrane solidifies depends on the types of lipidsMembranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acidsMembranes must be fluid to work properly; they are usually about as fluid as salad oil© 2011 Pearson Education, Inc.The steroid cholesterol has different effects on membrane fluidity at different temperaturesAt warm temperatures (such as 37°C), cholesterol restrains movement of phospholipidsAt cool temperatures, it maintains fluidity by preventing tight packing© 2011 Pearson Education, Inc.Figure 7.8FluidUnsaturated hydrocarbontailsViscousSaturated hydrocarbon tails(a) Unsaturated versus saturated hydrocarbon tails(b) Cholesterol within the animal cell membraneCholesterolEvolution of Differences in Membrane Lipid CompositionVariations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditionsAbility to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary© 2011 Pearson Education, Inc.Membrane Proteins and Their FunctionsA membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayerProteins determine most of the membrane’s specific functions© 2011 Pearson Education, Inc.Peripheral proteins are bound to the surface of the membraneIntegral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteinsThe hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices© 2011 Pearson Education, Inc.Figure 7.9N-terminus helixC-terminusEXTRACELLULARSIDECYTOPLASMICSIDESix major functions of membrane proteinsTransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)© 2011 Pearson Education, Inc.Figure 7.10aEnzymesSignaling moleculeReceptorSignal transductionATP(a) Transport(b) Enzymatic activity(c) Signal transductionFigure 7.10bGlyco-protein(d) Cell-cell recognition(e) Intercellular joining(f) Attachment to the cytoskeleton and extracellular matrix (ECM)The Role of Membrane Carbohydrates in Cell-Cell RecognitionCells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membraneMembrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual© 2011 Pearson Education, Inc.Synthesis and Sidedness of MembranesMembranes have distinct inside and outside facesThe asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus© 2011 Pearson Education, Inc.Figure 7.12TransmembraneglycoproteinsERER lumenGlycolipidPlasma membrane:Cytoplasmic faceExtracellular faceSecretoryproteinGolgiapparatusVesicleTransmembraneglycoproteinSecretedproteinMembraneglycolipidConcept 7.2: Membrane structure results in selective permeabilityA cell must exchange materials with its surroundings, a process controlled by the plasma membranePlasma membranes are selectively permeable, regulating the cell’s molecular traffic© 2011 Pearson Education, Inc.The Permeability of the Lipid BilayerHydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlyPolar molecules, such as sugars, do not cross the membrane easily© 2011 Pearson Education, Inc.Transport ProteinsTransport proteins allow passage of hydrophilic substances across the membraneSome transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnelChannel proteins called aquaporins facilitate the passage of water© 2011 Pearson Education, Inc.Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membraneA transport protein is specific for the substance it moves© 2011 Pearson Education, Inc.Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investmentDiffusion is the tendency for molecules to spread out evenly into the available spaceAlthough each molecule moves randomly, diffusion of a population of molecules may be directionalAt dynamic equilibrium, as many molecules cross the membrane in one direction as in the other© 2011 Pearson Education, Inc.Figure 7.13aMolecules of dyeMembrane (cross section)WATER(a) Diffusion of one soluteNet diffusionNet diffusionEquilibriumFigure 7.13b(b) Diffusion of two solutesNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibriumSubstances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreasesNo work must be done to move substances down the concentration gradientThe diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen© 2011 Pearson Education, Inc.Effects of Osmosis on Water BalanceOsmosis is the diffusion of water across a selectively permeable membraneWater diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides© 2011 Pearson Education, Inc.Figure 7.14Lowerconcentrationof solute (sugar)Higher concentrationof soluteSugarmoleculeH2OSame concentrationof soluteSelectivelypermeablemembraneOsmosisWater Balance of Cells Without WallsTonicity is the ability of a surrounding solution to cause a cell to gain or lose waterIsotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membraneHypertonic solution: Solute concentration is greater than that inside the cell; cell loses waterHypotonic solution: Solute concentration is less than that inside the cell; cell gains water© 2011 Pearson Education, Inc.Figure 7.15HypotonicsolutionOsmosisIsotonicsolutionHypertonicsolution(a) Animal cell(b) Plant cellH2OH2OH2OH2OH2OH2OH2OH2OCell wallLysedNormalShriveledTurgid (normal)FlaccidPlasmolyzedHypertonic or hypotonic environments create osmotic problems for organismsOsmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environmentsThe protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump© 2011 Pearson Education, Inc.Figure 7.16Contractile vacuole50 mWater Balance of Cells with WallsCell walls help maintain water balanceA plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt© 2011 Pearson Education, Inc.In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis© 2011 Pearson Education, Inc.Facilitated Diffusion: Passive Transport Aided by ProteinsIn facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membraneChannel proteins provide corridors that allow a specific molecule or ion to cross the membraneChannel proteins includeAquaporins, for facilitated diffusion of waterIon channels that open or close in response to a stimulus (gated channels)© 2011 Pearson Education, Inc.Figure 7.17EXTRACELLULARFLUIDCYTOPLASMChannel proteinSoluteSoluteCarrier protein(a) A channel protein(b) A carrier proteinCarrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane© 2011 Pearson Education, Inc.Concept 7.4: Active transport uses energy to move solutes against their gradientsFacilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energySome transport proteins, however, can move solutes against their concentration gradients© 2011 Pearson Education, Inc.The Need for Energy in Active TransportActive transport moves substances against their concentration gradientsActive transport requires energy, usually in the form of ATPActive transport is performed by specific proteins embedded in the membranes© 2011 Pearson Education, Inc.Active transport allows cells to maintain concentration gradients that differ from their surroundingsThe sodium-potassium pump is one type of active transport system© 2011 Pearson Education, Inc.Figure 7.18-6EXTRACELLULARFLUID[Na] high[K] low[Na] low[K] highCYTOPLASMNaNaNa123456NaNaNaNaNaNaKKKKKKPPPP iATPADPFigure 7.19Passive transportActive transportDiffusionFacilitated diffusionATPHow Ion Pumps Maintain Membrane PotentialMembrane potential is the voltage difference across a membraneVoltage is created by differences in the distribution of positive and negative ions across a membrane© 2011 Pearson Education, Inc.Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membraneA chemical force (the ion’s concentration gradient)An electrical force (the effect of the membrane potential on the ion’s movement)© 2011 Pearson Education, Inc.An electrogenic pump is a transport protein that generates voltage across a membraneThe sodium-potassium pump is the major electrogenic pump of animal cellsThe main electrogenic pump of plants, fungi, and bacteria is a proton pumpElectrogenic pumps help store energy that can be used for cellular work© 2011 Pearson Education, Inc.Figure 7.20CYTOPLASMATPEXTRACELLULARFLUIDProton pumpHHHHHHCotransport: Coupled Transport by a Membrane ProteinCotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell© 2011 Pearson Education, Inc.Figure 7.21ATPHHHHHHHHProton pumpSucrose-HcotransporterSucroseSucroseDiffusion of HConcept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosisSmall molecules and water enter or leave the cell through the lipid bilayer or via transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesiclesBulk transport requires energy© 2011 Pearson Education, Inc.ExocytosisIn exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contentsMany secretory cells use exocytosis to export their products© 2011 Pearson Education, Inc.EndocytosisIn endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membraneEndocytosis is a reversal of exocytosis, involving different proteinsThere are three types of endocytosisPhagocytosis (“cellular eating”)Pinocytosis (“cellular drinking”)Receptor-mediated endocytosis© 2011 Pearson Education, Inc.In phagocytosis a cell engulfs a particle in a vacuole. The vacuole fuses with a lysosome to digest the particleIn pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesiclesIn receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formationA ligand is any molecule that binds specifically to a receptor site of another molecule© 2011 Pearson Education, Inc.Figure 7.22aPseudopodiumSolutes“Food”or otherparticleFoodvacuoleCYTOPLASMEXTRACELLULARFLUIDPseudopodiumof amoebaBacteriumFood vacuoleAn amoeba engulfing a bacteriumvia phagocytosis (TEM).Phagocytosis1 mFigure 7.22bPinocytosis vesicles formingin a cell lining a small bloodvessel (TEM).Plasma membraneVesicle0.5 mPinocytosisFigure 7.22cTop: A coated pit. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosis(TEMs).Receptor0.25 mReceptor-Mediated EndocytosisLigandCoat proteinsCoatedpitCoatedvesicleCoatproteinsPlasma membraneFigure 7.UN04