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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 •small uncharged, polar
•larger uncharged, polar molecules
•ionsSize – polarity - ions
The Permeability of the The Permeability of the Lipid BilayerLipid Bilayer
• 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, nucleotidesThree transport processes:a. Simple diffusion – directly thru membrane
b. 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 solution
Animal 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 too polar 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 a solute 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 membrane by binding it on one side, undergoing a conformational change and then releasing it to the other side
Example: Glucose transporter GluT1 : carrier-mediated facilitated diffusion
1. Glucose binds
2. Conformational change 3. Glucose
Released-Conformational shift
inside cell
Glucoseout (HIGH)->glucose in (low)outside cell
1.2.
3.
Glucose + ATP glucose-6-phosphate + ADP hexokinase
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 transport driving force ->concentration/electrochemical gradient OR
2. Active transport against a gradient; unfavorable
requires energy input
Note: channel proteins mediate only passive transport
•Active transportActive transport– Carrier protein moves solute Carrier protein moves solute AGAINSTAGAINST its its
concentration gradientconcentration gradient
– Requires energy, usually in the form of ATP Requires energy, usually in the form of ATP hydorlysishydorlysis
– Or a favorable gradient Or a favorable gradient establishedestablished by use of ATP by use of ATP
ATP!
3 Na+ out2 K+ in
Active transport:Na+K+ Pump(Na+K+ATPase)
PP
P
P
The sodiumThe sodium-potassium-potassium pumppump
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.
3 Phosphorylation 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.
6 Loss 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 transporttransportTransport coupled to
Exergonic 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 conc
favorable favorable UnfavorableAdd energy
Figure 7.17
ATP
Passive transport