Biol219Lec 8Fall2016
Dr.Scott
Figure5.1aBodyFluidCompartments
MembraneDynamics
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Thebodyismostlywater
Figure5.1c BodyFluid Compartments ©2016Pears onEducation,Inc.
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Biol219Lec 8Fall2016
Dr.Scott
OsmoticEquilibrium
ChemicalDisequilibrium
ØWatermovesfreelybetweentheECFand
theICF
• Thedistributionofmajorsolutesamongthe
bodyfluidcompartmentsisuneven
ØThefluidconcentrationsareequalonthe2
sidesofthecellmembrane
ChemicalDisequilibrium
ChemicalDisequilibrium
• Solutesmightleakacrossthecellmembrane
- energyisrequiredtoreturnthem
• Na+- K+ - ATPase(pump)
• Thus,maintainingthechemicaldisequilibrium
requiresacontinualoutputofenergy
Figure 5.1dBodyFluid Compartments
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Biol219Lec 8Fall2016
Dr.Scott
TheICFandtheECFhavedifferent chemicalcomposition.
ElectricalDisequilibrium
• Thebodyiselectricallyneutral
ICF
• AfewextraanionsareintheICF,givingitanetnegative
charge
• MatchingcationsareintheECF,givingitanetpositive
charge
ElectricalDisequilibrium
• Theunevendistributionofionsbetweenthe
ICFandECFàelectricaldisequilibrium
• Changesinthisdisequilibriumcreate
electricalsignals
Ø Moreonthislater!
ECF
Na+
Cl–
ECF
• Foreverycation,thereisamatchinganion
• Ionsarenotdistributedevenlybetweenthe
ECFandtheICF
ICF
K +
A–
ICF
ECF
ICF
ECF
ICF
Ø MostsolutesintheICF
andECFareions
HomeostasisDoesNotMean
Equilibrium
ØHomeostasisattemptstomaintainthedynamic
steadystatesofthebody’scompartments
• Osmotic equilibrium
• Chemical disequilibrium
• Electrical disequilibrium
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Biol219Lec 8Fall2016
Dr.Scott
HomeostasisDoesNotMean
Equilibrium
ØTransportmechanismsandselective
permeabilityofthecellmebrane areimportant
tomaintainingthesedynamicstates
• Thedistributionofsolutesdepends on
whetherasubstancecancrosscell
membranes
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Figure 5.1bBodyFluidCompartments
OsmoticEquilibrium
Inmostcells:
• Watermovesfreelyby
• crossingwater-filled ionchannels
• Moving throughaquaporins, special protein
channels
• Themovementofwateracrossamembrane
isinresponsetoasoluteconcentration
gradientandiscalledOsmosis
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Biol219Lec 8Fall2016
Dr.Scott
Osmosis
Osmosis
ØWatermovestodilutethemore
concentratedsolution
• Osmoticpressure
• Molarityversusosmolarity
ØOncetheconcentrationsareequal,net
movementstops
• Molarity expresses concentration
• Osmolarity expresses number ofparticles
• Osmolarity ismore relevant
• Osmolarity versusosmolality
• Although different definitions,practically
interchangeable inphysiology
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OsmoticPressure
HowisOsmosismeasured?
• Theunitsareatmospheres (atm)or
millimetersofmercury(mmHg)
• mm Hg= pressure exerted ona1-cm2 area by
columnofmercury 1mm high.
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Biol219Lec 8Fall2016
Dr.Scott
Two co mp artme n ts are
se p arate d b y a me mb ran e th at
is p e rme ab le to wate r b u t n o t
glu co se . So lu tio n B is mo re
co n ce n trate d th an so lu tio n A.
Se le ctive ly
p e rme ab le
me mb ran e
Glu co se
mo le cu le s
Each compartment
startswiththe same
volume ofwater
A
The pistonmoves
as thevolume inB
increases
Vo lu me
in cre ase d
A
• Isosmotic, hyperosmotic, andhyposmotic
• Tonicity- Changeinvolumeofacellheldin
thatsolution
H 2O
Vo lu me
d e cre ase d
C o mp artme n t A is p u re wate r,
an d co mp artme n t B is a
glu co se so lu tio n . Osmotic
pressure is th e p re ssu re th at
mu st b e ap p lie d to o p po se o smo sis.
Figure 5.2Osmosis
andosmotic
pressure
• Osmolarity – Numberofparticlesinsolution
B
Wate r mo ve s b y o smo sis in to th e mo re co n ce n trate d
so lu tio n . Osmo sis sto p s wh e n
co n ce n tratio n s are e q u al.
Osmosis– otherterminology
B
Fo rce is ap p lie d to
e xactly o p p o se
o smo sis fro m A to B .
• Isotonic,hypertonic, and hypotonic
H 2O
P u re wate r
H 2O
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A
B
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MolarityvsOsmolarity
• Molarity:#moles/L
• Osmolarity:#particlesinsolution
• Ionsdissociateinsolution
• Glucose vsNaCl
• Watermovesbyosmosisinresponseto
thetotalconcentration ofallparticlesin
thesolution
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Biol219Lec 8Fall2016
Dr.Scott
Osmolarity
• #ofosmoticallyactiveparticlesperliterof
solution
- osmoles/Lormilliosmoles/L
• Conversion:
Molarityxparticles/molecule=osmolarity
(mol/L)x(osmol/mol)=(osmol/L)
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Osmolarity vsOsmolality
Tonicity
Osmolarity =osmol/L
• Penetratingversusnonpenetrating solutes
Osmolality = osmol/kg H 2O
Usedclinically toassesshydrationstatusfor
fluidreplacement therapy
• Tonicitydependsontheconcentrationof
nonpenetrating solutes
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Biol219Lec 8Fall2016
Dr.Scott
Tonicity
• Penetratingversusnonpenetrating solutes
• Tonicitydependsontheconcentrationof
nonpenetrating solutes
• Rulesforosmolarity andtonicity
• Clinicalimportanceofosmolarity and
tonicity
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Figure5.3Therelationshipbetween
osmolarity andtonicity
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Figure5.1c BodyFluid Compartments ©2016Pears onEducation,Inc.
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Biol219Lec 8Fall2016
Dr.Scott
Clinicalimportanceofosmolarity and
tonicity
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TransportProcesses
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Figure5.5Transportacross
membranes
• Bulkflow
• Gasesandliquidsarefluids
• Pressuregradients
• Cell membranes areselectively permeable
• Permeableversusimpermeable
• Passivetransportversusactivetransport
• Concentrationgradients
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Biol219Lec 8Fall2016
Diffusion:Properties
Dr.Scott
Figure5.6Diffusionexperiment
1. Passiveprocess
2. Highconcentrationtolowconcentration
• Chemicalgradient
3. Netmovementuntilconcentrationisequal
• Equilibrium
4. Rapidovershortdistances
5. Directlyrelatedtotemperature
6. Inverselyrelatedtomolecularweightandsize
7. Inopensystemoracrossapartition
• Ionsmoveaccordingtoanelectrochemicalgradient
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RulesforDiffusionofUnchargedMolecules:
GeneralProperties
1.DiffusionuseskineticNRGofmolecularmovement
2.Moleculesdiffusefromareaof[higher]to[lower]
3.Diffusioncontinuesuntilconcentrationsareequal;
however,molecularmovementcontinuesafter
equilibriumisreached
4.Diffusionisfaster:
- alonghigher[gradient]
- overshorterdistances
- athigherT0
- forsmallermolecules
5.Diffusioncantakeplaceinanopensystemoracrossa
partitionthatseparates2systems
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RulesforDiffusionofUnchargedMolecules:
SimpleDiffusionAcrossAMembrane
6.Therate ofdiffusion throughamembraneisfasterif:
- themembrane’ssurfaceareaislarger
- themembraneisthinner
- theconcentrationgradientislarger
- themembraneismorepermeabletothemolecule
7. Membranepermeabilitytoamoleculedependson
- themolecule’slipidsolubility
- themolecule’ssize
- thelipidcompositionofthemembrane
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Biol219Lec 8Fall2016
LipophilicMoleculesMoveby
SimpleDiffusionAcrossLipids
Dr.Scott
Figure5.7Fick’slawofdiffusion
• Additional properties tosimple diffusion
• Ratedependentonsolubilityinlipids
• Proportionaltosurfaceareaofmembrane
• Fick’slawofdiffusion
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FunctionsofMembraneProteins
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Figure5.8Mapofmembraneproteins
• Structural proteins
• Enzymes
• Membrane receptor proteins
• Transporters
• Channelproteins
• Carrierproteins
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Biol219Lec 8Fall2016
Extracellular fluid
Dr.Scott
Ligand binds to
a cellmembrane
receptor protein.
Receptor-ligand
complex triggers
intracellular response.
Receptor-ligand
complex brought
into the cell
Receptor
Events in the cell
Channels
• Water channels
• Ionchannels
• Open channels
• Gated channels
• Chemicallygatedchannels
• Voltage-gatedchannels
• Mechanicallygatedchannels
Cytoplasmic
vesicle
Intracellular fluid
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Figure 5.11The structure ofchannel proteins
Figure5.10a
Membrane
Transporters
Channel through
center of
membraneprotein
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One protein
subunit
ofchannel
Channel throughcenter
ofmembraneprotein
(viewed from above)
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