07 Cell Membranes (1)

Overview: 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 othersCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-1

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 itCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Membrane Models: Scientific InquiryMembranes have been chemically analyzed and found to be made of proteins and lipidsScientists studying the plasma membrane reasoned that it must be a phospholipid bilayer Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-2HydrophilicheadWATERHydrophobictailWATER

In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteinsLater studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regionsIn 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-3PhospholipidbilayerHydrophobic regionsof proteinHydrophilicregions of protein

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayerCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-4TECHNIQUEExtracellularlayerKnifeProteinsInside of extracellular layerRESULTSInside of cytoplasmic layerCytoplasmic layerPlasma membrane

The 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 membraneCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5Lateral movement(~107 times per second)Flip-flop(~ once per month)(a) Movement of phospholipids(b) Membrane fluidityFluidViscousUnsaturated hydrocarbontails with kinksSaturated hydro-carbon tails(c) Cholesterol within the animal cell membraneCholesterol

Fig. 7-5a(a) Movement of phospholipidsLateral movement(107 times per second)Flip-flop( once per month)

Fig. 7-6RESULTSMembrane proteinsMouse cellHuman cellHybrid cellMixed proteinsafter 1 hour

As 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 that those rich in saturated fatty acidsMembranes must be fluid to work properly; they are usually about as fluid as salad oilCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5b(b) Membrane fluidityFluidUnsaturated hydrocarbontails with kinksViscousSaturated hydro-carbon tails

The steroid cholesterol has different effects on membrane fluidity at different temperaturesAt warm temperatures (such as 37C), cholesterol restrains movement of phospholipidsAt cool temperatures, it maintains fluidity by preventing tight packingCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5cCholesterol(c) Cholesterol within the animal cell membrane

Membrane Proteins and Their FunctionsA membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayerProteins determine most of the membranes specific functions

Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-7Fibers ofextracellularmatrix (ECM)Glyco-proteinMicrofilamentsof cytoskeletonCholesterolPeripheralproteinsIntegralproteinCYTOPLASMIC SIDEOF MEMBRANEGlycolipidEXTRACELLULARSIDE OFMEMBRANECarbohydrate

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 helicesCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-8N-terminusC-terminus HelixCYTOPLASMICSIDEEXTRACELLULARSIDE

Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-9(a) TransportATP(b) Enzymatic activityEnzymes(c) Signal transductionSignal transductionSignaling moleculeReceptor(d) Cell-cell recognitionGlyco-protein(e) Intercellular joining(f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Fig. 7-9ac(a) Transport(b) Enzymatic activity(c) Signal transductionATPEnzymesSignal transductionSignaling moleculeReceptor

Fig. 7-9df(d) Cell-cell recognitionGlyco-protein(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 carbohydrates, on 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 individualCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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


Fig. 7-10ER1TransmembraneglycoproteinsSecretoryproteinGlycolipid2GolgiapparatusVesicle34SecretedproteinTransmembraneglycoproteinPlasma membrane:Cytoplasmic faceExtracellular faceMembrane glycolipid

Concept 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 cells molecular trafficCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 easilyCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 waterCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 movesCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 exhibit a net movement in one directionAt dynamic equilibrium, as many molecules cross one way as cross in the other directionAnimation: Membrane SelectivityAnimation: DiffusionCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-11Molecules of dyeMembrane (cross section)WATERNet diffusionNet diffusionEquilibrium(a) Diffusion of one soluteNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibrium(b) Diffusion of two solutes

Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to anotherNo work must be done to move substances down the concentration gradientThe diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happenCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Diffusion of two solutesFig. 7-11bNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibrium

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 concentrationCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Lowerconcentrationof solute (sugar)Fig. 7-12H2OHigher concentrationof sugarSelectivelypermeablemembraneSame concentrationof sugarOsmosis

Water Balance of Cells Without WallsTonicity is the ability of a 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 waterCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-13Hypotonic solution(a) Animal cell(b) Plant cellH2OLysedH2OTurgid (normal)H2OH2OH2OH2ONormalIsotonic solutionFlaccidH2OH2OShriveledPlasmolyzedHypertonic solution

Hypertonic or hypotonic environments create osmotic problems for organismsOsmoregulation, the control of 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 pumpVideo: ChlamydomonasVideo: Paramecium VacuoleCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-14Filling vacuole 50 m(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

Water 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 wiltCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Video: PlasmolysisVideo: Turgid ElodeaAnimation: OsmosisIn a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysisCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-15EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Solute Carrier protein (b) A carrier protein

Concept 7.4: Active transport uses energy to move solutes against their gradientsFacilitated diffusion is still passive because the solute moves down its concentration gradientSome transport proteins, however, can move solutes against their concentration gradientsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Need for Energy in Active TransportActive transport moves substances against their concentration gradientActive transport requires energy, usually in the form of ATPActive transport is performed by specific proteins embedded in the membranesAnimation: Active TransportCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Active transport allows cells to maintain concentration gradients that differ from their surroundingsThe sodium-potassium pump is one type of active transport systemCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-16-1EXTRACELLULARFLUID [Na+] high [K+] low Na+ Na+ Na+ [Na+] low[K+] high CYTOPLASM Cytoplasmic Na+ binds tothe sodium-potassium pump. 1

Na+ binding stimulatesphosphorylation by ATP. Fig. 7-16-2Na+ Na+ Na+ ATP P ADP 2

Fig. 7-16-3 Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside. Na+ P Na+ Na+ 3

Fig. 7-16-4 K+ binds on theextracellular side andtriggers release of thephosphate group. P P K+ K+ 4

Fig. 7-16-5 Loss of the phosphaterestores the proteins originalshape. K+ K+ 5

Fig. 7-16-6 K+ is released, and thecycle repeats. K+ K+ 6

2EXTRACELLULARFLUID [Na+] high [K+] low [Na+] low [K+] highNa+ Na+ Na+ Na+ Na+ Na+ CYTOPLASM ATP ADP P Na+ Na+ Na+ P 3K+ K+ 6K+ K+ 54K+ K+ P P 1Fig. 7-16-7

Fig. 7-17Passive transport Diffusion Facilitated diffusion Active transport ATP

How Ion Pumps Maintain Membrane PotentialMembrane potential is the voltage difference across a membraneVoltage is created by differences in the distribution of positive and negative ionsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:A chemical force (the ions concentration gradient)An electrical force (the effect of the membrane potential on the ions movement)


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 pumpCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-18EXTRACELLULARFLUID H+ H+ H+ H+ Proton pump + + + H+ H+ + + H+ ATPCYTOPLASM

Cotransport: Coupled Transport by a Membrane ProteinCotransport occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cellCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-19Proton pump ++++++ATPH+H+H+H+H+H+H+H+Diffusionof H+Sucrose-H+cotransporter Sucrose Sucrose

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 by transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesiclesBulk transport requires energy


ExocytosisIn exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contentsMany secretory cells use exocytosis to export their productsAnimation: ExocytosisCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 endocytosis:Phagocytosis (cellular eating)Pinocytosis (cellular drinking)Receptor-mediated endocytosis

Animation: Exocytosis and Endocytosis IntroductionCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In phagocytosis a cell engulfs a particle in a vacuoleThe vacuole fuses with a lysosome to digest the particle

Animation: PhagocytosisCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20aPHAGOCYTOSIS CYTOPLASM EXTRACELLULARFLUID Pseudopodium Food orother particle Foodvacuole Food vacuole Bacterium An amoeba engulfing a bacteriumvia phagocytosis (TEM) Pseudopodiumof amoeba 1 m

Fig. 7-20bPINOCYTOSIS Plasmamembrane Vesicle 0.5 m Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

In 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

Animation: Receptor-Mediated EndocytosisCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20cRECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein CoatedpitLigandCoatproteinPlasmamembrane0.25 m CoatedvesicleA coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)

Fig. 7-UN1Passive transport:Facilitated diffusion Channelprotein Carrierprotein

Fig. 7-UN2Active transport: ATP

Fig. 7-UN3Environment:0.01 M sucrose0.01 M glucose0.01 M fructose Cell 0.03 M sucrose0.02 M glucose

Fig. 7-UN4

You should now be able to:Define the following terms: amphipathic molecules, aquaporins, diffusionExplain how membrane fluidity is influenced by temperature and membrane compositionDistinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutionsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Explain how transport proteins facilitate diffusionExplain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumpsExplain how large molecules are transported across a cell membraneCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

**Figure 7.1 How do cell membrane proteins help regulate chemical traffic?*For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.

**Figure 7.2 Phospholipid bilayer (cross section)**Figure 7.3 The fluid mosaic model for membranes**Figure 7.4 Freeze-fracture**Figure 7.5 The fluidity of membranes*Figure 7.5a The fluidity of membranes*Figure 7.6 Do membrane proteins move?**Figure 7.5b The fluidity of membranes**Figure 7.5c The fluidity of membranes**Figure 7.7 The detailed structure of an animal cells plasma membrane, in a cutaway view**Figure 7.8 The structure of a transmembrane protein**Figure 7.9 Some functions of membrane proteins*Figure 7.9ac Some functions of membrane proteins*Figure 7.9df Some functions of membrane proteins***Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane******Figure 7.11 The diffusion of solutes across a membrane**Figure 7.11b The diffusion of solutes across a membrane**Figure 7.12 Osmosis**Figure 7.13 The water balance of living cells**Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation***For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.

*Figure 7.15 Two types of transport proteins that carry out facilitated diffusion*****For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files.*Figure 7.16, 1 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 2 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 3 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 4 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 5 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 6 The sodium-potassium pump: a specific case of active transport*Figure 7.16, 16 The sodium-potassium pump: a specific case of active transport*Figure 7.17 Review: passive and active transport****Figure 7.18 An electrogenic pump**Figure 7.19 Cotransport: active transport driven by a concentration gradient****For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.

*Figure 7.20 Endocytosis in animal cellsphagocytosis**Figure 7.20 Endocytosis in animal cellspinocytosis**Figure 7.20 Endocytosis in animal cellsreceptor-mediated endocytosis******

Documents

07 Cell Membranes (1)