Transcript 10 Membrane Transport 9 21 05

BCOR 011
Sept 21, 2005
Membrane
Transport
Lecture 10
Membrane Transport
1. Permeability
2. Diffusion
3. Role of transport proteins - facilitated
Channel proteins
Carrier proteins
4. Active vs passive transport
1. Lipid bilayers are selectively permeable
•small,nonpolar
•small
uncharged, polar
•larger
uncharged, polar
molecules
•ions
Size – polarity - ions
Decreasing
permeability
The Permeability of the Lipid
Bilayer
• Hydrophobic molecules
– Are lipid soluble and can pass through
the membrane rapidly
• Polar molecules
– Do not cross membrane rapidly
• Ions
– 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 membrane
Req b. Facilitated diffusion (passive transport)
Carrierc. Active transport – requires energy
prot
Simple
Diffusion:
•Tendancy of a material to spread out
•Always moves toward equilibrium
Net diffusion
Figure 7.11 B
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Net diffusion
Equilibrium
simple diffusion example:
Oxygen crossing red cell membrane
HIGH -> low
Lungs
Tissues
O2
CO2
O2
CO2
O2
CO2
CO2
O2
HCO3-
HCO3-
HCO3-
Driving force: concentration gradient
Trying to even out concentration
H2O transport: diffusion from area with low
[solute] to one with high [solute]
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Osmosis
Diffusion
of water
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Figure 7.12
Water moves from an area of higher
free water concentration to an area
of lower free water concentration
Impermeable
Solutes
Animal cells – pump out ions
Plants, bacteria – cell walls
Hypotonic solution
(a) Animal cell. An
animal cell fares best
in an isotonic environment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
H2O
Figure 7.13
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
Isotonic solution
H2O
Turgid (normal)
H2O
H2O
Normal
Lysed
Hypertonic solution
H2O
Shriveled
H2O
H2O
Flaccid
H2O
Plasmolyzed
…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
Transmembrane transport proteins
allow selective transport of hydrophilic molecules & ions
1. carrier protein
Bind solute,
conformational change,
release
Selective binding
“turnstile”
Carrier protein
(b) 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.
Figure 7.15
Solute
Transmembrane transport proteins
allow selective transport of hydrophilic molecules & ions
aqueous channel 2. channel
hydrophilic pore
very rapid
EXTRACELLULAR
selective –size/charge
FLUID
protein
“trap door”
Channel protein
Solute
CYTOPLASM
(a) A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Figure 7.15
Kinetics of simple vs facilitated
Diffusion
v
(solute concentration gradient) ->
Gets
“saturated”
Maximum
rate
Does
Not
Get
“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
“keys”
Voltage
Ligand
Mechanosensitive
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
Glucoseout (HIGH)->glucose
(low)
outside cell
2. Conformational
3. Glucose
change
T2
ReleasedConformational
shift
2.
1. Glucose binds
T1
in
1.
inside cell
3.
Glucose + ATP  glucose-6-phosphate + ADP
hexokinase
T1
Carrier proteins: three types
(a) Uniport
(b) Co-transport
Uniport – one solute transported
[Antiport – two solutes in opposite directions
Symport – two solutes in the same direction
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 transport
– Carrier protein moves solute
AGAINST its concentration gradient
– Requires energy, usually in the form
of ATP hydorlysis
– Or a favorable gradient established
by use of ATP
Active
transport:
Na+K+ Pump
(Na+K+ATPase)
P
3 Na+ out
2 K+ in
ATP!
P
P
P
1
Cytoplasmic Na+ binds to
the sodium-potassium pump.
2 Na+ binding stimulates
[Na+] high
[K+] low
phosphorylation by ATP.
Na+
Na+
Na+
Na+
Na+
The sodium
-potassium
pump
[Na+] low
[K+] high
Na+
ATP
P
ADP
CYTOPLASM
EXTRACELLULAR
FLUID
Na+
Na+
Na+
3
K+ is released and Na+
sites are receptive again;
the cycle repeats.
4
K+
P
K+
Phosphorylation causes the
protein to change its conformation, expelling Na+ to
the outside.
K+
P
Pi
K+
Figure 7.16
5
Loss of the phosphate
restores the protein’s
original conformation.
K+
K+
6
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
The Na+/K+ Pump:
“bilge pump”
Creates an electrochemical
gradient (high external [Na+ ])
+
Na
Na+
Na+
Na+
Na+
+
+
potential energy
Na
Na
– like “storing water behind a dam”
Na+
Na+
uses ~1/3 of cell’s ATP!!
Example of indirect active transport:
Na+ gradient drives other transport
Na+ glucose symport
Glucose
Gradient
Coupled transport
• An electrogenic pump
– Is a transport protein that generates the voltage
across a membrane
–
EXTRACELLULAR
FLUID
+
–
ATP
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
Figure 7.18
–
+
+
H+
• Cotransport: active transport driven by a
concentration gradient
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
+
H+
–
+
Sucrose-H+
cotransporter
H+ Diffusion
of H+
H+
–
–
Figure 7.19
+
+
Sucrose
Direct active
transport
Indirect active
transport
Transport coupled to
Exergonic rxn, i.e. ATP
hydrolysis
*Transport driven
by 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:
Passive transport
Simple diffusion Facilitated diffusion
No protein
HIGH to low conc
favorable
Active transport
channel carrier
protein protein
carrier protein
low to HIGH conc
HIGH to low conc
favorable
Unfavorable
Add energy
ATP
Figure 7.17