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Cell Membranes
BIOLOGYA Global Approach
Campbell Reece Urry Cain Wasserman Minorsky Jackson 2015 Pearson Education LtdTENTH EDITION
Global Edition
Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick8
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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
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Figure 8.1
2015 Pearson Education Ltd3Figure 8.1 How do cell membrane proteins help regulate chemical traffic?
Figure 8.1a
2015 Pearson Education Ltd4Figure 8.1a How do cell membrane proteins help regulate chemical traffic? (part 1: ion channel)
Video: Structure of the Cell Membrane
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Video: Water Movement Through an Aquaporin
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Concept 8.1: 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 regionsA phospholipid bilayer can exist as a stable boundary between two aqueous compartments
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Figure 8.2
Hydrophilic headHydrophobic tailWATERWATER
2015 Pearson Education Ltd8Figure 8.2 Phospholipid bilayer (cross section)
The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in itProteins are not randomly distributed in the membrane
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Figure 8.3Glyco-protein
CarbohydrateGlycolipidEXTRACELLULARSIDE OFMEMBRANEMicrofilamentsof cytoskeletonFibers of extra-cellular matrix (ECM)CholesterolPeripheralproteinsIntegralproteinCYTOPLASMICSIDE OFMEMBRANE
2015 Pearson Education Ltd10Figure 8.3 Updated model of an animal cells plasma membrane (cutaway view)
Figure 8.3aGlyco-protein
CarbohydrateMicrofilamentsof cytoskeletonFibers of extra-cellular matrix (ECM)Cholesterol
2015 Pearson Education Ltd11Figure 8.3a Updated model of an animal cells plasma membrane (cutaway view) (part 1)
Figure 8.3b
GlycolipidEXTRACELLULARSIDE OFMEMBRANEIntegralproteinCYTOPLASMICSIDE OFMEMBRANEPeripheralproteins
2015 Pearson Education Ltd12Figure 8.3b Updated model of an animal cells plasma membrane (cutaway view) (part 2)
The Fluidity of MembranesPhospholipids in the plasma membrane can move within the bilayerMost of the lipids, and some proteins, drift laterallyRarely, a lipid may flip-flop transversely acrossthe membrane
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Figure 8.4-1Membrane proteinsMouse cellHuman cell
2015 Pearson Education Ltd14Figure 8.4-1 Inquiry: Do membrane proteins move? (step 1)
Figure 8.4-2
Membrane proteinsMouse cellHuman cellHybrid cell
2015 Pearson Education Ltd15Figure 8.4-2 Inquiry: Do membrane proteins move? (step 2)
Figure 8.4-3
Membrane proteinsMouse cellHuman cellHybrid cellMixed proteinsafter 1 hour
2015 Pearson Education Ltd16Figure 8.4-3 Inquiry: Do membrane proteins move? (step 3)
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 than those rich in saturated fatty acidsMembranes must be fluid to work properly; they are usually about as fluid as salad oil
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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 packing
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Figure 8.5FluidViscousUnsaturated tailsprevent packing.Saturated tails packtogether.CholesterolCholesterol reducesmembrane fluidity atmoderate temperatures,but at low temperatureshinders solidification.(a) Unsaturated versus saturated hydrocarbon tails(b) Cholesterol within the animal cell membrane
2015 Pearson Education Ltd19Figure 8.5 Factors that affect membrane fluidity
Evolution 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
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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 membranes specific functions
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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 nonpolaramino acids, often coiled into alpha helices
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Figure 8.6
N-terminusC-terminus helixEXTRACELLULARSIDECYTOPLASMICSIDE
2015 Pearson Education Ltd23Figure 8.6 The structure of a transmembrane protein
Six major functions of membrane proteinsTransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)
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Figure 8.7
(a) Transport(b) Enzymatic activity(c) Signaltransduction(d) Cell-cell recognition(e)Intercellular joining(f)Attachment tothe cytoskeletonand extracellularmatrix (ECM)EnzymesATPSignalingmoleculeReceptorSignal transductionGlyco-protein
2015 Pearson Education Ltd25Figure 8.7 Some functions of membrane proteins
Figure 8.7a
(a) TransportEnzymesATPReceptorSignal transduction(b)Enzymaticactivity(c) SignaltransductionSignalingmolecule
2015 Pearson Education Ltd26Figure 8.7a Some functions of membrane proteins (part 1: transport, enzymatic activity, and signal transduction)
Figure 8.7b
(d) Cell-cellrecognition(e) Intercellularjoining(f)Attachment tothe cytoskeletonand extracellularmatrix (ECM)Glyco-protein
2015 Pearson Education Ltd27Figure 8.7b Some functions of membrane proteins (part 2: cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and ECM)
HIV must bind to the immune cell surface protein CD4 and a co-receptor CCR5 in order to infecta cell HIV cannot enter the cells of resistant individuals that lack CCR5
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Figure 8.8
HIVReceptor(CD4)Co-receptor(CCR5)Receptor (CD4)but no CCR5Plasmamembrane(a)(b)
2015 Pearson Education Ltd29Figure 8.8 The genetic basis for HIV resistance
The Role of Membrane Carbohydrates in Cell-Cell RecognitionCells recognize each other by binding to 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
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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
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TransmembraneglycoproteinsFigure 8.9SecretoryproteinGolgiapparatusVesicleAttachedcarbohydrateER lumenGlycolipidTransmembraneglycoproteinPlasma membrane: Cytoplasmic face Extracellular faceMembraneglycolipidSecretedprotein
2015 Pearson Education Ltd32Figure 8.9 Synthesis of membrane components and their orientation in the membrane
Concept 8.2: Membrane structure results in selective permeabilityA cell must exchange materials with its surroundings, a process controlled by theplasma membranePlasma membranes are selectively permeable, regulating the cells molecular traffic
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The Permeability of the Lipid BilayerHydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlyHydrophilic molecules including ions and polar molecules do not cross the membrane easily
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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
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Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membraneA transport protein is specific for the substanceit moves
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Concept 8.3: 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
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Figure 8.10
Molecules of dyeMembrane (cross section)WATER(a) Diffusion of one solute(b) Diffusion of two solutesNet diffusionNet diffusionNet diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibriumEquilibrium
2015 Pearson Education Ltd38Figure 8.10 The diffusion of solutes across a synthetic membrane
Animation: Diffusion
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Animation: Membrane Selectivity
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Substances 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
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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
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Figure 8.11
Lower concentrationof solute (sugar)Higher concentrationof soluteMore similarconcentrations of soluteSugarmoleculeH2OSelectivelypermeablemembraneOsmosis
2015 Pearson Education Ltd43Figure 8.11 Osmosis
Figure 8.11a
SelectivelypermeablemembraneOsmosisWatermolecules canpass throughpores, but sugarmoleculescannot.Water moleculescluster aroundsugar molecules.This side hasmore solutemolecules,fewer freewater molecules.This side hasfewer solutemolecules,more freewater molecules.
2015 Pearson Education Ltd44Figure 8.11a Osmosis (part 1: detail of membrane)
Animation: Osmosis
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Water Balance of Cells Without Cell 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
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Figure 8.12
HypotonicIsotonicHypertonicH2OLysedNormalShriveledPlasmamembraneCellwallTurgid (normal)FlaccidPlasmolyzed(a) Animal cell(b) Plant cellPlasmamembraneH2OH2OH2OH2OH2OH2OH2O
2015 Pearson Education Ltd47Figure 8.12 The water balance of living cells
Video: Plasmolysis
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Video: Turgid Elodea
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Hypertonic 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
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Figure 8.13
Contractile vacuole50 m
2015 Pearson Education Ltd51Figure 8.13 The contractile vacuole of Paramecium caudatum
Video: Chlamydomonas
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Video: Paramecium Vacuole
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Water Balance of Cells with Cell 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)
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In a hypertonic environment, plant cells lose water The membrane pulls away from the cell wall causing the plant to wilt, a usually lethal effect called plasmolysis
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Facilitated Diffusion: Passive Transport Aided by ProteinsIn facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membraneTransport proteins include channel proteins and carrier proteins
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Channel proteins provide corridors that allow a specific molecule or ion to cross the membraneAquaporins facilitate the diffusion of waterIon channels facilitate the diffusion of ions Some ion channels, called gated channels, open or close in response to a stimulus
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Figure 8.14
(a) A channel protein(b) A carrier proteinCarrier proteinChannel proteinSoluteSoluteEXTRACELLULARFLUIDCYTOPLASM
2015 Pearson Education Ltd58Figure 8.14 Two types of transport proteins that carry out facilitated diffusion
Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
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Concept 8.4: 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
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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
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Active transport allows cells to maintain concentration gradients that differ from their surroundingsThe sodium-potassium pump is one type of active transport system
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Figure 8.15
EXTRACELLULARFLUIDCYTOPLASM
1
2
5
6
4
3[Na+] low[K+] high[Na+] high[K+] lowNa+K+K+K+K+K+K+Na+Na+Na+Na+Na+Na+Na+Na+ATPADPPPPiP
2015 Pearson Education Ltd63Figure 8.15 The sodium-potassium pump: a specific case of active transport
Figure 8.15aCYTOPLASM
2[Na+] low[K+] high[Na+] high[K+] lowNa+Na+Na+Na+Na+Na+ATPADPPEXTRACELLULARFLUID
1 Cytoplasmic Na+binds to the sodium-potassium pump. Theaffinity for Na+ is highwhen the proteinhas this shape. Na+ bindingstimulatesphosphorylationby ATP.
2015 Pearson Education Ltd64Figure 8.15a The sodium-potassium pump: a specific case of active transport (part 1: Na+ binding)
Figure 8.15b
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3 Phosphorylationleads to a change inprotein shape, reducingits affinity for Na+, whichis released outside. The new shape has ahigh affinity for K+, whichbinds on the extracellularside and triggers releaseof the phosphate group.Na+Na+PNa+K+K+PPi
2015 Pearson Education Ltd65Figure 8.15b The sodium-potassium pump: a specific case of active transport (part 2: K+ binding)
Figure 8.15c
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K+ is released;affinity for Na+ ishigh again, and thecycle repeats. Loss of thephosphate grouprestores the proteinsoriginal shape, whichhas a lower affinityfor K+.5K+K+K+K+
2015 Pearson Education Ltd66Figure 8.15c The sodium-potassium pump: a specific case of active transport (part 3: K+ release)
Animation: Active Transport
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Video: Na+/K+ ATPase Cycle
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Figure 8.16
ATPPassive transportActive transportDiffusionFacilitated diffusion
2015 Pearson Education Ltd69Figure 8.16 Review: passive and active transport
BioFlix: Membrane Transport
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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 ions across a membrane
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Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membraneA chemical force (the ions concentration gradient)An electrical force (the effect of the membrane potential on the ions movement)
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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
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Figure 8.17
EXTRACELLULARFLUIDATPCYTOPLASMH+H+H+H+H+H+Proton pump++++
2015 Pearson Education Ltd74Figure 8.17 A proton pump
Cotransport: Coupled Transport by a Membrane ProteinCotransport occurs when active transport of a solute indirectly drives transport of other substances Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
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Figure 8.18
SucroseATPH+H+H+H+H+Proton pump++++Sucrose-H+cotransporterH+H+H+H+SucroseDiffusion of H+
2015 Pearson Education Ltd76Figure 8.18 Cotransport: active transport driven by a concentration gradient
Concept 8.5: Bulk transport across theplasma 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
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ExocytosisIn exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cellMany secretory cells use exocytosis to exporttheir products
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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
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Figure 8.19
PhagocytosisPinocytosisReceptor-MediatedEndocytosisSolutesPseudopodiumFoodorotherparticleFoodvacuoleCYTOPLASMEXTRACELLULARFLUIDPlasmamembraneCoated pitCoated vesicleCoatproteinReceptor
2015 Pearson Education Ltd80Figure 8.19 Exploring endocytosis in animal cells
Animation: Exocytosis
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Animation: Exocytosis and Endocytosis Introduction
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Figure 8.19a
PhagocytosisSolutesPseudopodiumFoodorotherparticleFoodvacuoleCYTOPLASMEXTRACELLULARFLUIDPseudopodiumof amoebaBacteriumFood vacuoleAn amoeba engulfing abacterium via phago-cytosis (TEM)1 m
2015 Pearson Education Ltd83Figure 8.19a Exploring endocytosis in animal cells (part 1: phagocytosis)
Animation: Phagocytosis
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Video: Phagocytosis in Action
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Figure 8.19aa
Pseudopodiumof amoebaBacteriumFood vacuoleAn amoeba engulfing abacterium via phago-cytosis (TEM)1 m
2015 Pearson Education Ltd86Figure 8.19aa Exploring endocytosis in animal cells (part 1a: phagocytosis TEM)
Figure 8.19b
PinocytosisPlasmamembraneCoated pitCoated vesicleCoatproteinPinocytotic vesiclesforming (TEMs)0.25 m
2015 Pearson Education Ltd87Figure 8.19b Exploring endocytosis in animal cells (part 2: pinocytosis)
Animation: Pinocytosis
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Figure 8.19ba
Pinocytotic vesiclesforming (TEMs)0.25 m
2015 Pearson Education Ltd89Figure 8.19ba Exploring endocytosis in animal cells (part 2a: pinocytosis TEMs)
Figure 8.19c
Receptor-MediatedEndocytosisReceptorCoatproteinPlasmamembraneTop: A coated pit.Bottom: A coated vesicleforming during receptor-mediated endocytosis (TEMs)0.25 m
2015 Pearson Education Ltd90Figure 8.19c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis)
Animation: Receptor-Mediated Endocytosis
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Figure 8.19ca
CoatproteinPlasmamembraneTop: A coated pit.Bottom: A coated vesicleforming during receptor-mediated endocytosis (TEMs)0.25 m
2015 Pearson Education Ltd92Figure 8.19ca Exploring endocytosis in animal cells (part 3a: receptor-mediated endocytosis TEMs)
In phagocytosis a cell engulfs a particle in a vacuoleThe vacuole fuses with a lysosome to digest the particle
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In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is gulped into tiny vesicles
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In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formationA ligand is any molecule that binds specificallyto a receptor site of another molecule
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Figure 8.UN01
Glucose Uptake Over Time in Guinea Pig Red Blood Cells15-day-old guinea pig1-month-old guinea pigIncubation time (min)Concentration ofradioactive glucose (mM)1008060402000 10 20 30 40 50 60
2015 Pearson Education Ltd96Figure 8.UN01 Skills exercise: interpreting a scatter plot with two sets of data
Figure 8.UN02
ChannelproteinCarrierproteinPassive transport:Facilitated diffusion
2015 Pearson Education Ltd97Figure 8.UN02 Summary of key concepts: facilitated diffusion
Figure 8.UN03
Active transportATP
2015 Pearson Education Ltd98Figure 8.UN03 Summary of key concepts: active transport
Figure 8.UN04
0.03 M sucrose0.02 M glucoseCell0.01 M sucrose0.01 M glucose0.01 M fructoseEnvironment
2015 Pearson Education Ltd99Figure 8.UN04 Test your understanding, question 6 (diffusion)
Figure 8.UN05
2015 Pearson Education Ltd100Figure 8.UN05 Test your understanding, question 11 (water balance)