membrane dynamics - napa valley college · 2016-09-06 · simple diffusion across a membrane 6. the...
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Biol219Lec 8Fall2016 Dr.Scott
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MembraneDynamics
Figure5.1aBodyFluidCompartments
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Figure5.1cBodyFluidCompartments©2016PearsonEducation,Inc.
Thebodyismostlywater
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OsmoticEquilibrium
ØWatermovesfreelybetweentheECFandtheICF
ØThefluidconcentrationsareequalonthe2sidesofthecellmembrane
ChemicalDisequilibrium
• Thedistributionofmajorsolutesamongthebodyfluidcompartmentsisuneven
ChemicalDisequilibrium
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Figure5.1dBodyFluid Compartments
ChemicalDisequilibrium
• Solutesmightleakacrossthecellmembrane- energyisrequiredtoreturnthem• Na+- K+ - ATPase(pump)
• Thus,maintainingthechemicaldisequilibriumrequiresacontinualoutputofenergy
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ElectricalDisequilibrium
• Thebodyiselectricallyneutral• Foreverycation,thereisamatchinganion
• IonsarenotdistributedevenlybetweentheECFandtheICF• AfewextraanionsareintheICF,givingitanetnegativecharge• MatchingcationsareintheECF,givingitanetpositivecharge
ICF ECFK+ Na+
A– Cl–
TheICFandtheECFhavedifferent chemicalcomposition.
ICF
ECF ICF
ECF
ICF
ECF
ICF
Ø MostsolutesintheICFandECFareions
ElectricalDisequilibrium
• TheunevendistributionofionsbetweentheICFandECFàelectricaldisequilibrium
• Changesinthisdisequilibriumcreateelectricalsignals
Ø Moreonthislater!
HomeostasisDoesNotMeanEquilibrium
ØHomeostasisattemptstomaintainthedynamicsteadystatesofthebody’scompartments• Osmoticequilibrium
• Chemical disequilibrium
• Electrical disequilibrium
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HomeostasisDoesNotMeanEquilibrium
ØTransportmechanismsandselectivepermeabilityofthecellmebrane areimportanttomaintainingthesedynamicstates
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• Thedistributionofsolutesdependsonwhetherasubstancecancrosscellmembranes
Figure 5.1bBodyFluidCompartments
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OsmoticEquilibrium
Inmostcells:•Watermovesfreelyby• crossingwater-filled ionchannels• Moving throughaquaporins, specialproteinchannels
• ThemovementofwateracrossamembraneisinresponsetoasoluteconcentrationgradientandiscalledOsmosis
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Osmosis
ØWatermovestodilutethemoreconcentratedsolution
ØOncetheconcentrationsareequal,netmovementstops
Osmosis
•Osmoticpressure•Molarityversusosmolarity• Molarityexpressesconcentration• Osmolarity expressesnumberofparticles• Osmolarity ismore relevant
•Osmolarity versusosmolality• Althoughdifferentdefinitions,practicallyinterchangeable inphysiology
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HowisOsmosismeasured?
OsmoticPressure
• Theunitsareatmospheres (atm)ormillimetersofmercury(mmHg)• mmHg=pressureexerted ona1-cm2 areabycolumnofmercury 1mmhigh.
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Figure 5.2Osmosisandosmoticpressure
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H2O
H2O
H2O
Vo lumeincre ase d
Glucosemo le cu le s
Se le ctive lype rme ab lemembrane
A B
A B
A B
Two compartments arese parate d byamembrane thatispe rme ab le to wate rbu tno tglu cose .So lu tion B ismoreconce n trate d than so lu tion A.
Vo lumede cre ase d
Compartment A ispu re wate r,and compartmentB isaglu cose so lu tion . Osmotic pressure is the p re ssu re thatmustbe app lie d to oppose o smosis.
Fo rce isapp lie d toe xactly oppose
osmosis from A to B.
Pu re wate r
Wate rmove s by osmosis in to the more conce n trate dso lu tion . Osmosis stops whe nconce n trationsare e qual.
Each compartmentstartswiththe samevolume of water
The pistonmovesas thevolume inBincreases
Osmosis– otherterminology
•Osmolarity –Numberofparticlesinsolution• Isosmotic,hyperosmotic,andhyposmotic
• Tonicity- Changeinvolumeofacellheldinthatsolution• Isotonic,hypertonic,andhypotonic
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MolarityvsOsmolarity
•Molarity:#moles/L
•Osmolarity:#particlesinsolution• Ionsdissociateinsolution• Glucose vsNaCl
•Watermovesbyosmosisinresponsetothetotalconcentrationofallparticlesinthesolution
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Osmolarity
• #ofosmoticallyactiveparticlesperliterofsolution
- osmoles/Lormilliosmoles/L
• Conversion:Molarityxparticles/molecule=osmolarity
(mol/L)x(osmol/mol)=(osmol/L)
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Osmolarity vsOsmolality
Osmolarity =osmol/L
Osmolality=osmol/kg H2OUsedclinically toassesshydrationstatusforfluidreplacement therapy
Tonicity
• Penetratingversusnonpenetrating solutes
• Tonicitydependsontheconcentrationofnonpenetrating solutes
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Tonicity
• Penetratingversusnonpenetrating solutes• Tonicitydependsontheconcentrationofnonpenetrating solutes
• Rulesforosmolarity andtonicity
• Clinicalimportanceofosmolarity andtonicity
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Figure5.3Therelationshipbetweenosmolarity andtonicity
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Clinicalimportanceofosmolarity andtonicity
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TransportProcesses
• Bulkflow• Gasesandliquidsarefluids• Pressuregradients
• Cellmembranes areselectively permeable• Permeableversusimpermeable• Passivetransportversusactivetransport• Concentrationgradients
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Figure5.5Transportacrossmembranes
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Diffusion:Properties
1. Passiveprocess2. Highconcentrationtolowconcentration
• Chemicalgradient3. Netmovementuntilconcentrationisequal
• Equilibrium
4. Rapidovershortdistances5. Directlyrelatedtotemperature6. Inverselyrelatedtomolecularweightandsize7. Inopensystemoracrossapartition• Ionsmoveaccordingtoanelectrochemicalgradient
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Figure5.6Diffusionexperiment
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RulesforDiffusionofUnchargedMolecules:GeneralProperties
1.DiffusionuseskineticNRGofmolecularmovement2.Moleculesdiffusefromareaof[higher]to[lower]3.Diffusioncontinuesuntilconcentrationsareequal;however,molecularmovementcontinuesafterequilibriumisreached4.Diffusionisfaster:
- alonghigher[gradient]- overshorterdistances- athigherT0- forsmallermolecules
5.Diffusioncantakeplaceinanopensystemoracrossapartitionthatseparates2systems
RulesforDiffusionofUnchargedMolecules:SimpleDiffusionAcrossAMembrane
6.Therate ofdiffusion throughamembraneisfasterif:- themembrane’ssurfaceareaislarger- themembraneisthinner- theconcentrationgradientislarger- themembraneismorepermeabletothemolecule
7. Membranepermeabilitytoamoleculedependson- themolecule’slipidsolubility- themolecule’ssize- thelipidcompositionofthemembrane
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LipophilicMoleculesMovebySimpleDiffusionAcrossLipids• Additionalproperties tosimplediffusion• Ratedependentonsolubilityinlipids• Proportionaltosurfaceareaofmembrane• Fick’slawofdiffusion
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Figure5.7Fick’slawofdiffusion
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FunctionsofMembraneProteins
• Structuralproteins• Enzymes• Membrane receptor proteins• Transporters• Channelproteins• Carrierproteins
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Figure5.8Mapofmembraneproteins
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Ligand bindstoa cellmembranereceptor protein.
Receptor-ligandcomplex triggersintracellular response.
Intracellular fluid
Extracellular fluid
ReceptorEvents in the cell
Cytoplasmicvesicle
Receptor-ligandcomplex broughtinto the cell
Channels
• Waterchannels• Ionchannels• Openchannels• Gatedchannels• Chemicallygatedchannels• Voltage-gatedchannels• Mechanicallygatedchannels
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Figure5.10aMembraneTransporters
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Figure5.11Thestructureofchannelproteins
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Channel throughcenter of
membraneprotein
Channel throughcenterofmembraneprotein(viewed from above)
One proteinsubunit
ofchannel