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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Biol219Lec 8Fall2016 Dr.Scott

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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