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BCOR 011 Lecture 10 Sept 21, 2005
Membrane Transport
BCOR 011 Lecture 10 Sept 21, 2005
Membrane Transport
Membrane Transport
1. Permeability2. Diffusion3. Role of transport proteins - facilitated
Channel proteins Carrier proteins
4. Active vs passive transport
1. Lipid bilayers are selectively permeable
Decreasing permeability
•small,nonpolar
•smalluncharged, polar
•largeruncharged, polarmolecules
•ions
Size – polarity - ions
The Permeability of the Lipid The Permeability of the Lipid BilayerBilayer
•• Hydrophobic moleculesHydrophobic molecules– Are lipid soluble and can pass through
the membrane rapidly•• Polar moleculesPolar molecules
– Do not cross membrane rapidly•• IonsIons
– Do not cross the membrane at all
Transport processes
Solutes – dissolved ions and small organic molecules
i.e., Na+,K+, H+, Ca++, Cl,-sugars, amino acids, nucleotides
Three transport processes:a. Simple diffusion – directly thru membraneb. Facilitated diffusion (passive transport)c. Active transport – requires energy
ReqCarrierprot
SimpleDiffusion:
•Tendancy of a material to spread out•Always moves toward equilibrium
Net diffusion Net diffusion Equilibrium
Net diffusion
Net diffusion
Net diffusion
Net diffusion Equilibrium
Equilibrium
Figure 7.11 B
simple diffusion example:Oxygen crossing red cell membrane
HIGH -> low
O2
CO2
O2CO22
O2
O2 CO2O2 CO2Lungs
Tissues
Driving force: concentration gradientTrying to even out concentration
HCO3-
CO2 HCO3-
HCO3-
H2O transport: diffusion from area with low [solute] to one with high [solute]
OsmosisDiffusion of water
ImpermeableSolutes
Figure 7.12
Lowerconcentrationof solute (sugar)
Higherconcentrationof sugar
Same concentrationof sugar
Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can
More free watermolecules (higher
concentration)
Water moleculescluster around sugar molecules
Fewer free watermolecules (lowerconcentration)
Water moves from an area of higher free water concentration to an area of lower free water concentration
•Osmosis
Animal cells – pump out ionsPlants, bacteria – cell walls
Hypotonic solution Isotonic solution Hypertonic solutionAnimal cell. Ananimal cell fares bestin an isotonic environ-ment unless it hasspecial adaptations tooffset the osmoticuptake or loss ofwater.
(a)
H2O H2O H2O H2O
Lysed Normal Shriveled
Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Figure 7.13
…but most things are too large or toopolar to cross at reasonable rates using simple diffusion
Facilitated diffusion:protein–mediated movement down a gradient
Transmembrane transport proteins
Figure 7.15
Carrier proteinSolute
A carrier protein alternates between two conformations, moving asolute across the membrane as the shape of the protein changes.The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
(b)
Transmembrane transport proteinsallow selective transport of hydrophilic molecules & ions
1. carrier protein Bind solute, conformational change, releaseSelective binding
“turnstile”
Figure 7.15
EXTRACELLULARFLUID
Channel proteinSolute
CYTOPLASM
A channel protein (purple) has a channel through which water molecules or a specific solute can pass.
(a)
Transmembrane transport proteinsallow selective transport of hydrophilic molecules & ions
aqueous channelhydrophilic porevery rapidselective –size/charge
2. channel protein
“trap door”
Kinetics of simple vs facilitatedDiffusion
v
(solute concentration gradient) ->
GetsGets“saturated”“saturated”MaximumMaximumraterate
DoesDoesNotNotGetGet
“saturated”“saturated”
For CHARGED solutes (ions): net driving force is the electrochemical gradient•has both a concentration + charge component;•Ion gradients can create an electrical voltage gradient across the membrane (membrane potential)
-60 mVolts
++++
+
+ ++++ ++ +
+ +
--- --- +++ +++
+++ +++ --- ---
++ +++
Channel Proteins:facilitate passive transport
Ion channels: move ions down an electrochemical gradient; gated
Voltage Ligand Mechanosensitive
“keys” “keys”
Ligand-gated ion channel
“Wastebasket model” – step on pedal & lid opens
Ligand-gated
example: ligand-gated ion channel“Key” - acetylcholine
Voltage-gated channels
Note: channels are passive, facilitated transport systems
+ + + + + + + +
+ + - - - - - - - - -
-
-
Example of voltage-gated ion channel
Protein ion channels: -are passive, facilitated transport systems-require a membrane protein-typically move ions very rapidly from an area
of HIGH concentration to one of lower concentration
Carrier proteins:
Transport solute across membraneby binding it on one side,undergoing a conformational changeand then releasing it to the other side
Example: Glucose transporter GluT1 : carrier-mediated facilitated diffusion
1. Glucose binds
2. Conformationalchange 3. Glucose
Released-Conformationalshift
inside cell
Glucoseout (HIGH)->glucose in (low)outside cell
1.2.
3.
Glucose + ATP glucose-6-phosphate + ADPhexokinase
T1
T2
T1
Carrier proteins: three types
Antiport – two solutes in opposite directions
Uniport – one solute transported
Symport – two solutes in the same direction[
(a) Uniport (b) Co-transport
Carrier Proteins can mediate either:
1. Passive transportdriving force ->
concentration/electrochemical gradientOR
2. Active transport against a gradient; unfavorable
requires energy inputNote: channel proteins mediate only passive transport
•• Active transportActive transport–– Carrier protein moves solute Carrier protein moves solute AGAINSTAGAINST its concentration gradientits concentration gradient
–– Requires energy, usually in the form Requires energy, usually in the form of ATP of ATP hydorlysishydorlysis
–– Or a favorable gradient Or a favorable gradient establishedestablishedby use of ATPby use of ATP
ATP!
3 Na+ out2 K+ in
Active transport:Na+K+ Pump(Na+K+ATPase)
PP
P
P
The sodiumThe sodium--potassiumpotassiumpumppump
Figure 7.16
PP i
EXTRACELLULARFLUID
Na+ binding stimulatesphosphorylation by ATP.
2
Na+
Cytoplasmic Na+ binds tothe sodium-potassium pump.
1
K+ is released and Na+
sites are receptive again; the cycle repeats.
3Phosphorylation causes the
protein to change its conformation, expelling Na+ to the outside.
4
Extracellular K+ binds to the protein, triggering release of the Phosphate group.
6Loss of the phosphaterestores the protein’s original conformation.
5
CYTOPLASM
[Na+] low[K+] high
Na+
Na+
Na+
Na+
Na+
PATP
Na+
Na+
Na+
P
ADP
K+
K+
K+
K+K+
K+
[Na+] high[K+] low
The Na+/K+ Pump:
“bilge pump”
Creates an electrochemical gradient (high external [Na+ ])
potential energy – like “storing water behind a dam”
uses ~1/3 of cell’s ATP!!
Na+
Na+
Na+
Na+
Na+ Na+
Na+
Na+
Na+
Example of indirect active transport:Na+ gradient drives other transport Na+ glucose symport
GlucoseGradient
Coupled transport
• An electrogenic pump– Is a transport protein that generates the voltage
across a membrane
Figure 7.18
EXTRACELLULARFLUID
+
H+
H+
H+
H+
H+
H+Proton pump
ATP
CYTOPLASM
+
+
+
+–
–
–
–
–
+
• Cotransport: active transport driven by a concentration gradient
Figure 7.19
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
ATP H+
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+–
–
–
–
–
–
Direct active Indirect active transport transport
Transport coupled toExergonic rxn, i.e. ATPhydrolysis
*Transport drivenby cotransport of ions
*note that the favorable ion gradient was established by direct active transport
….Each membrane has its own characteristic set of transporters
Summary:
Simple diffusion Facilitated diffusion Active transport
No protein channel carrier protein protein carrier protein
HIGH to low conc HIGH to low conc low to HIGH concfavorable favorable Unfavorable
Add energy
Figure 7.17
ATP
Passive transport