Chapter 7


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  • Figure 7.2 Phospholipid bilayer (cross section)
  • Figure 7.3 The fluid mosaic model for membranes
  • Figure 7.5b The fluidity of membranes
  • Figure 7.5c The fluidity of membranes
  • Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
  • Figure 7.8 The structure of a transmembrane protein
  • Figure 7.9 Some functions of membrane proteins
  • Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane
  • Figure 7.17 Review: passive and active transport
  • Figure 7.11 The diffusion of solutes across a membrane
  • 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
  • Figure 7.12 Osmosis
  • Figure 7.13 The water balance of living cells
  • Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport
  • Figure 7.18 An electrogenic pump
  • Figure 7.20 Endocytosis in animal cells
  • Chapter 7

    1. 1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPowerPoint®Lecture Presentations forBiologyEighth EditionNeil Campbell and Jane ReeceLectures by Chris Romero, updated by Erin Barley with contributions from Joan SharpChapter 7Membrane Structure andFunction
    2. 2. Plasma Membrane• the boundary that separates the living cell from itssurroundings• An amphipathic molecules– contains hydrophobic and hydrophilic regions• Exhibits selective permeability– allows some substances to cross a membrane more easily thanothers• Cellular membranes are fluid mosaics of lipids and proteinsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    3. 3. Fig. 7-2HydrophilicheadWATERHydrophobictailWATERAmphipathic MoleculesPlasma membrane structure
    4. 4. Fig. 7-3PhospholipidbilayerHydrophobic regionsof proteinHydrophilicregions of proteinMembranes are fluid mosaics of lipids and proteins
    5. 5. Cell membrane movementLateral movement(~107times per second)Flip-flop(~ once per month)
    6. 6. Fig. 7-5bFluidUnsaturated hydrocarbonsViscousSaturated hydrocarbon tailsMembrane fluidity• As temperatures cool, membranes switch from a fluid state to a solidstate• Membranes rich in unsaturated fatty acids are more fluid that thoserich in saturated fatty acids• Membranes must be fluid to work properly; they are usually about asfluid as salad oil****
    7. 7. Fig. 7-5cCholesterolCholesterol’s affect on membrane fluidity• The steroid cholesterol has different effects on membrane fluidity atdifferent temperatures• At warm temperatures cholesterol restrains movement ofphospholipids• At cool temperatures it maintains fluidity by preventing tight packing
    8. 8. Membrane Proteins and Their Functions• A membrane is a collage of different proteins embedded inthe fluid matrix of the lipid bilayer• Proteins determine most of the membrane’s specificfunctions• Peripheral proteins are bound to the surface of themembrane• Integral proteins penetrate the hydrophobic core– Integral proteins that span the membrane are calledtransmembrane proteins– The hydrophobic regions of an integral protein consist of nonpolaramino acids, often coiled into alpha helicesCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    9. 9. Fig. 7-7Fibers ofextracellularmatrix (ECM)Glyco-proteinMicrofilamentsof cytoskeletonCholesterolPeripheralproteinsIntegralproteinCYTOPLASMIC SIDEOF MEMBRANEGlycolipidEXTRACELLULARSIDE OFMEMBRANECarbohydrate
    10. 10. Fig. 7-8N-terminusC-terminusα HelixCYTOPLASMICSIDEEXTRACELLULARSIDETransmembrane proteins consist of α helicesA transmembrane protein is an example of an integral protein
    11. 11. • Six major functions of membrane proteins:– Transport– Enzymatic activity– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to the cytoskeleton andextracellular matrix (ECM)Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsKNOW THESE!!!
    12. 12. Fig. 7-9(a) TransportATP(b) Enzymatic activityEnzymes(c) Signal transductionSignal transductionSignaling moleculeReceptor(d) Cell-cell recognitionGlyco-protein(e) Intercellular joining (f) Attachment tothe cytoskeletonand extracellularmatrix (ECM)
    13. 13. Synthesis and Sidedness of Membranes• Membranes have distinct inside and outsidefaces• Membranes are built by the ER and Golgi– The asymmetrical distribution of proteins,lipids, and associated carbohydrates in theplasma membrane is determined when themembrane is builtCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    14. 14. Fig. 7-10ER1TransmembraneglycoproteinsSecretoryproteinGlycolipid2GolgiapparatusVesicle34SecretedproteinTransmembraneglycoproteinMembrane glycolipidSynthesis of a membraneComponents via theendomembrane system1. ER producestransmembraneglycoprotiens and secretoryproteins2. The golgi apparatuspackages transmembraneglycoproteins, secretoryproteins and membraneglycolipids3. Vessicles containingmembrane componentsfuse with the membrane torelease those componentsfrom the cell
    15. 15. The Permeability of the Lipid Bilayer• A cell must exchange materials with its surroundings, aprocess controlled by the plasma membrane• Plasma membranes are selectively permeable, regulatingthe cell’s molecular traffic• Hydrophobic (nonpolar) molecules can dissolve in the lipidbilayer and pass through the membrane rapidly• Polar molecules, such as sugars, do not cross themembrane easilyCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    16. 16. Transport Proteins• A transport protein is specific for the substance it moves• Transport proteins allow passage of hydrophilicsubstances across the membrane– Channel proteins have a hydrophilic channel that certainmolecules or ions can use as a tunnel– Aquaporins facilitate the passage of water– Carrier proteins bind to molecules and change shape toshuttle them across the membrane• Active transport requires energy while passive transportdoes notCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    17. 17. Fig. 7-17Passive transportDiffusion Facilitated diffusionActive transportATPMolecules flow with theconcentration gradient andtherefore do not requireenergyMolecules flow againstthe concentrationgradient whichrequires energy
    18. 18. Passive Transport• Transport of molecules in/out of cell does notrequire energy• Examples– Diffusion– Facilitated diffusion– Osmosis• Animal cells (without cell walls)• Plant cells (with cell walls)
    19. 19. Diffusion• the tendency for molecules to spread out evenlyinto the available space• At dynamic equilibrium, as many molecules crossone way as cross in the other direction• Substances diffuse down their concentrationgradient, the difference in concentration of asubstance from one area to another– No energy required– Sustances move to less concentrated areaCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    20. 20. Fig. 7-11(a) Diffusion ofone soluteMolecules of dye Membrane (cross section)Net diffusion Net diffusion EquilibriumNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibrium(b) Diffusion oftwo solutes
    21. 21. Facilitated Diffusion• Passive transport aided by proteins• Transport proteins speed the passive movement ofmolecules across the plasma membrane• Channel proteins provide corridors that allow a specificmolecule or ion to cross the membrane– Ex: Aquaporins– Ex: Ion channels• Carrier proteins undergo a subtle change in shape thattranslocates the solute across the membrane• No energy requiredCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    22. 22. Fig. 7-15EXTRACELLULARFLUIDChannel protein(a) A channel proteinSoluteCYTOPLASMSoluteCarrier protein(b) A carrier protein
    23. 23. Osmosis• Osmosis is the diffusion of water across a selectivelypermeable membrane• Water diffuses across a membrane from the region oflower solute concentration to the region of higher soluteconcentration• Water diffuses across a membrane to balance theconcentrations of different solutes– Low concentration solute  high concentration solute• No energy requiredCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    24. 24. Lowerconcentrationof solute (sugar)Fig. 7-12H2OHigherconcentrationof sugarSelectivelypermeablemembraneSame concentrationof sugarOsmosisWater moves from lowconcentration of solute tohigh concentration of solute
    25. 25. Osmosis in Animal Cells• Tonicity is the ability of a solution to cause a cell togain or lose water (always compared to inside the cell)– In an isotonic solution• Solute concentration is the same as that inside the cell• Outside cell = inside cell• No net water movement across the plasma membrane– In a hypertonic solution• Solute concentration is greater than that inside the cell• Outside cell > inside cell• Cell loses water– In a hypotonic solution• Solute concentration is less than that inside the cell• Outside cell < inside the cell• Cell gains water
    26. 26. Osmosis in Plant Cells• Cell walls help maintain water balance• In a hypotonic solution– Plant cell swells until the wall opposes uptake (water travels to the higherconcentration of solute)– The cell is now turgid (firm)• In an isotonic solution– No net movement of water into the cell– Cell becomes flaccid (limp), and the plant may wilt• In a hypertonic solution– Plant cells lose water (water travels to higher concentration of solute)– The membrane pulls away from the wall, a usually lethal effect calledplasmolysisCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    27. 27. Fig. 7-13Hypotonic solution(a) Animalcell(b) PlantcellH2OLysedH2OTurgid (normal)H2OH2OH2OH2ONormalIsotonic solutionFlaccidH2OH2OShriveledPlasmolyzedHypertonic solution***Water moves to higher concentration of solute***Need to know which condition is normal for plant and animal cells
    28. 28. Active Transport• Active transport– moves substances against their concentration gradient– requires energy (ATP)– allows cells to maintain concentration gradients thatdiffer from their surroundings• Examples– Sodium-potassium pump (animals)– Proton pump (plants)Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    29. 29. K+K+6K+K+5 4K+K+PP2[Na+] high[K+] low[Na+] low[K+] highNa+Na+Na+Na+Na+Na+ATPADPPNa+Na+Na+P31Fig. 7-16-7The Sodium-Potassium Pump: 3 Na+ out and 2 K+ inATP hydrolysis
    30. 30. Fig. 7-18EXTRACELLULARFLUIDH+H+H+H+Proton pump+++H+H+++H+––––ATPCYTOPLASM–The Proton Pump pumps H+ against their concentration gradient
    31. 31. Bulk transport across the plasma membrane• Small molecules and water enter/leave the cell through thelipid bilayer or by transport proteins• Large molecules, such as polysaccharides and proteins,cross the membrane in bulk via vesicles• Bulk transport requires energy• Exocytosis vs EndocytosisCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    32. 32. Exocytosis• In exocytosis,transport vesiclesmigrate to themembrane, fusewith it, andrelease theircontents• Many secretorycells useexocytosis toexport theirproductsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    33. 33. Endocytosis• In endocytosis, the cell takes in macromoleculesby forming vesicles from the plasma membrane• Endocytosis is a reversal of exocytosis, involvingdifferent proteins• There are three types of endocytosis:– Phagocytosis (“cellular eating”)– Pinocytosis (“cellular drinking”)– Receptor-mediated endocytosisCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
    34. 34. Fig. 7-20PHAGOCYTOSISa cell engulfs a particlein a vacuoleEXTRACELLULARFLUIDCYTOPLASMPseudopodium“Food”orother particleFoodvacuolePINOCYTOSISextracellular fluid is“gulped” into tinyvesicles1 µmPseudopodiumof amoebaBacteriumFood vacuoleAn amoeba engulfing a bacteriumvia phagocytosis (TEM)PlasmamembraneVesicle0.5 µmPinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)RECEPTOR-MEDIATEDENDOCYTOSISbinding of ligands to receptorstriggers vesicle formationReceptorCoat proteinCoatedvesicleCoatedpitLigandTypes of Endocytosis