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