Describe The Fluid Mosaic Model Of Membrane Structure.

Objective # 13Module 2C – Membranes and Cell Transport Allcells are surrounded by a plasmamembrane. Eukaryotic cells also containinternal membranes and membranemembranebound organelles. In this module, we will examine thestructure and function of cellmembranes. We will also look at howmaterials move within cells and acrosscell membranes.Describe the Fluid MosaicModel of membranestructure.12Objective 13Objective 13Further study has shown that cell membranesconsist of 4 major components:1) Lipid bilayer¾ mainly 2 layers of phospholipids; the nonnonpolarl tailsil pointi inwardid andd theh polarl hheadsdare on the surface¾ contains cholesterol in animal cells¾ is fluid, allowing proteins to move aroundwithin the bilayer In1972, Singer and Nicolsonproposed the Fluid Mosaic Model ofmembrane structure. Accordingg to their model, cellmembranes are composed of a lipidbilayer with globular proteinsembedded in the bilayer. Nonpolar Hydrophobic TaailsPolar Hydrophilic Heads3CH2N (CH3)3CH24Objective 13PhospholipidsOOPO–Phospholipid BilayerOHH2CCCH2OOC H3C OCH2CH2CH2CH2CH2CH2CH2CHCHCH2CH2CH2CH2CH2CH2CH2CH3 a. FormulaExtracellular fluidPolar hydrophilic headsNonpolarhydrophobic tailsc. IconPolar hydrophilic headsCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.b. Space-filling modelCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.5Intracellular fluid (cytosol)61

Objective 132) Transmembrane proteins or Integralmembrane proteins¾ globular proteins that run throughboth layers of the lipid bilayer¾ have hydrophobic regions thatanchor them in the hydrophobicinterior of the lipid bilayer andhydrophilic regions that protrudefrom the bilayer¾ float freely within the bilayerCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.78Objective 13Human cellMouse cellFusecellsAllow timefor mixing to occurIntermixedmembraneproteinsIntegral proteinsrun through bothlayers of the lipidbilayer.Hydrophobicregions of theprotein are shadedin blue andhydrophilic regionsin red.Experimentswith cellfusion showthat integralproteins canmove freelywithin thefluid lipidbilayer.3) Interior protein network¾ proteins associated with the surfaceof the membrane are calledperipheral membrane proteins¾ theh iinteriori surfacefoff theh plasmalmembrane is structurally supportedby a network of proteins calledspectrins and clathrins910Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Objective 13Extracellular matrix protein4) Cell surface markers¾ project from the external surface ofthe membrane¾ include glycolipids (lipids withcarbohydrateb h dgroups attached)h d) anddglycoproteins (proteins withcarbohydrate groups attached)¾ function as cell identity roteinsCholesterolPeripheralproteins11Actin filamentsof cytoskeletonIntermediate filamentsof cytoskeletonFluid Mosaic ModelCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.2

Objective # 14Functions of Plasma Membrane ProteinsOutsidecellList and describe the variousfunctions of membraneproteins.InsidecellTransporter13Cell surface identitymarkerEnzymeCell-to-cell adhesionCell surface receptorAttachment to thecytoskeletonCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Objective # 15Objective 15 Nocell exists as a closed system. Inorder to survive, materials must enterand leave the cell through the plasmamembrane. Because different processes take placein different parts of the cell, materialsmust be transported from one part ofthe cell to another.Explain the importance ofcell transport.p1516Objective # 16Objective 16Explain what passive transport is, anddescribe the following methods ofpassive transport across membranes:p diffusiona) Simpleb) Dialysisc) Osmosisd) Facilitated diffusion Passivetransport:¾ During passive transport, substancesmove according to their own naturaltendency without an inputp of energygfrom the cell. No ATP is required.¾ To understand how passive transportworks, we need to examine the kinetictheory of matter.17183

Objective 16theory of matter:matter:¾ All atoms and molecules are inconstant random motion. This energyof motion is called kinetic energyenergy.¾ The higher the temperature, the fasterthe atoms and molecules move.¾ We detect this motion as heat.¾ All motion theoretically stops atabsolute zero.Objective 16a Kinetic Diffusion:¾ Diffusionis the net movement of asubstance from an area where it has agconcentration to an area where ithigherhas a lower concentration i.e. down itsconcentration gradient.¾ Caused by the constant random motionof all atoms and molecules.1920Objective 16a Duringdiffusion, movementof individual atoms andmolecules is always randombut net movement of eachsubstance is down its ownconcentration gradient.2122Objective 16Objective 16b Inaddition to simple diffusiondiffusion,, thereare 3 special types of diffusion thatinvolve movement of materials acrossa semipermeable membrane:membrane:¾ dialysis¾ osmosis¾ facilitated diffusion Dialysisrefers to the diffusion ofsolutes across a semipermeablemembrane (i.e. a membrane wheresome substances can pass throughwhile others cannot). The ability of solutes to pass throughcell membranes depends mainly onsize and electrical charge.23244

Objective 16c Osmosisrefers to the diffusion of thesolvent across a semipermeablemembrane. In livingg systems the solvent is alwayswater, so biologists generally defineosmosis as the diffusion of wateracross a semipermeable membrane:2526Objective 16cObjective 16c Theosmotic pressure of a solution isthe pressure needed to keep it inequilibrium with pure H20. If2 solutions have different [solutes]:¾ Theone with the higher [solutes], andlower [solvent], is hypertonic.hypertonicp. Thehigher the [solutes] in a solution,the higher its osmotic pressure.2 solutions have the same [solutes],they are called isotonic.isotonic.¾ Theone with the lower [solutes], andhigher [solvent], is hypotonic.hypotonic. If27282930Objective 16c Whatwill happen to a cell if it isplaced in a hypertonic solution? WWhatatwillw happenappe to a cellce if itt issplaced in a hypotonic solution?5

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Human Red Blood Cells – which cells arein a hypertonic environment?Shriveled cellsNormal cellsHypertonicsolutionIsotonicsolutionCells swell andeventually burstHypotonicsolutionCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.3132Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Objective 16cPlant cells – which cell is in a hypotonicenvironment?Cell body shrinksfrom cell wallHypertonicsolutionFlaccid cellIsotonicsolution Cellshave developed several ways tosurvive in a hypotonic environment:¾ Pump water out using a contractilevacuole.¾ Adjust [solutes] so it is isotonic relativeto the environment.¾ Develop a thick cell wall that canwithstand high turgor pressure.Normal turgid cellHypotonicsolutionCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.33Objective 16c Eventhough water molecules are polar,they can pass through the hydrophobicinterior of the lipid bilayer because theyare so small. However, the flow is fairlylimitedlimited. Recent studies have shown thatmovement of water molecules acrosscell membranes is facilitated by specialprotein channels called aquaporins.aquaporins.3534Pore proteins have a hydrophilic interiorchannel that allows ions and polar molecules,like water, to pass through the membranehydrophobiclipid bilayerpore protein36Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.6

Ions and large polar molecules cannot passthrough the hydrophobic interior of the lipidbilayer without the help of transport proteins:Objective 16d Facilitateddiffusion refers to thediffusion of solutes through asemipermeable membrane with the helpof special transport proteins.¾ NonNon--polar molecules and small polarmolecules can diffuse directly throughthe lipid bilayer of a membrane.¾ Ions and large polar molecules cannot,they need help from transport proteins.Outside- Hydrophobic interiorof the lipid bilayerInside37 Transport Protein38Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Objective 16dTwo types of transport proteins canhelp ions and large polar moleculesdiffuse through cell membranes:1) Channel proteins – have a hydrophiliciinteriori ffor ionsior polarl moleculesl l topass through. Some channel proteinscan be opened or closed in responseto a stimulus. These are called gatedchannels.channels 39404142Objective 16d2) Carrier proteins – physically bind tothe substance being transported onone side of membrane and release iton the other.7

Objective 16dObjective # 17 Facilitateddiffusion:¾ is specific – each channel or carriertransports certain ions or molecules only¾ iss passivepass ve – ddirectionect o ofo netet movementove e t issalways down the concentration gradient¾ saturates – once all transport proteins arein use, rate of diffusion cannot beincreased furtherExplain what active transport is, anddescribe the following methods ofactive transport across membranes:pumpspa) membrane pb) coupled transport4344Objective 17Objective 17a Activetransport:¾ a cell expends some of its own energy(from ATP) to move a substanceagainstgits natural tendency e.g.g uppaconcentration gradient.¾ Requires the use of carrier proteins(transport proteins that physically bindto the substance being transported). Wewill examine 2 types of activetransport: membrane pumps (protein(proteinmediated active transport) and coupledtransport (cotransport). With membrane pumps, a carrierprotein uses energy from ATP tomove a substance across a membrane,up its concentration gradient:4546Objective 17b Coupled transport occurs in 2 stages:¾ First, a carrier protein uses energyfrom ATP to move a substanceacross the membrane, up itsconcentration gradient. Thisgradient stores energy.47488

Objective 17b¾ Second, a coupled transport proteinallows the substance to move backdown its concentration gradient. Asthis happens, the stored energy isreleased and used to move a secondsubstance up its concentrationgradient:4950Objective # 18Objective 18Explain what bulk transport is, anddescribe the following methods ofbulk transport:p ga) Endocytosis includingg phagocytosis,pinocytosis, and receptorreceptor-mediatedendocytosisb) exocytosis Bulktransport allows small particles,or groups of molecules to enter orleave a cell without actually passingthrough the membrane. Wewill examine 2 types of bulktransport: endocytosis and exocytosis.5152Objective 18aObjective 18a Inendocytosisendocytosis,, part of the plasmamembrane envelops small particles orfluid, then seals on itself to form avesicle or vacuole which enters the cell:¾ Phagocytosis – the substance engulfedis a solid particle¾ Pinocytosis - the substance engulfed is aliquid53 Athird type of endocytosis is calledreceptor-mediated endocytosis.receptorendocytosis. In this process, the molecules to bepjjoin to specificpreceptorspontransportedthe membrane. This causes themembrane to indent , and the moleculesare engulfed in a coated vesicle whichenters the cell.549

Objective 18b Thereverse of endocytosis is calledexocytosis.exocytosis During this process, the membrane ofa vesicle fuses with the pplasmamembrane and its contents arereleased outside the cell:5556Table 5.2575810

Explain what active transport is, and describe the following methods of active transport across membranes: a)a) membrane membrane pumps 44 pp b)b) coupled transport coupled transport Objective 17 Active transport: ¾a cell expends some of its own energy (from ATP) to move a substance agains