BCOR 011
Sept 21, 2005
Lecture 10
Membrane
Transport
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
The Permeability of the Lipid
Bilayer
• Hydrophobic molecules
Decreasing
permeability
– 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
Size – polarity - ions
Transport processes
Simple
Diffusion:
Solutes – dissolved ions and small
organic molecules
i.e., Na+,K+, H+, Ca++, Cl,-
sugars, amino acids, nucleotides
•Tendancy of a material to spread out
•Always moves toward equilibrium
Three transport processes:
a. Simple diffusion – directly thru membrane
Req b. Facilitated diffusion (passive transport)
Carrierc. Active transport – requires energy
prot
Net diffusion
Net diffusion
Net diffusion
Figure 7.11 B
Net diffusion
simple diffusion example:
Oxygen crossing red cell membrane
HIGH -> low
Lungs
Tissues
O2
CO2
O2
CO2
O22
CO2
CO2
O2
HCO3-
Equilibrium
Equilibrium
Net diffusion
Equilibrium
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
HCO3-
Osmosis
Diffusion
of water
HCO3-
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)
Driving force: concentration gradient
Trying to even out concentration
Net diffusion
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
Isotonic solution
H2O
H2O
Shriveled
H2O
H2O
H2O
H2O
H2O
Normal
Lysed
(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.
Hypertonic solution
…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
Turgid (normal)
Flaccid
Plasmolyzed
Transmembrane transport proteins
allow selective transport of hydrophilic molecules & ions
1. carrier protein
Transmembrane transport proteins
allow selective transport of hydrophilic molecules & ions
aqueous channel
hydrophilic pore
Bind solute,
conformational change,
release
Selective binding
2. channel protein
very rapid
EXTRACELLULAR
selective –size/charge
FLUID
“trap door”
“turnstile”
Carrier protein
Channel protein
Solute
Solute
CYTOPLASM
(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
(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
Gets
“saturated”
Maximum
rate
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)
+ + +
+
+ + +
Does
Not
Get
“saturated”
v
-60 mVolts +
+ +
+ +
++ +
+ +
+
+++
+++
---
---
---
+++
+++
---
+
+
(solute concentration gradient) ->
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
Voltage-gated channels
“Key” - acetylcholine
+ + +
+ +
+
- - - - - -
+
-
+
-
- + - +
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
Example: Glucose transporter GluT1 :
carrier-mediated facilitated diffusion
Carrier proteins:
Transport solute across membrane
by binding it on one side,
undergoing a conformational change
and then releasing it to the other side
Glucoseout (HIGH)->glucose
(low)
outside cell
2. Conformational
change
3. Glucose
T2
ReleasedConformational
shift
2.
1. Glucose binds
T1
in
1.
inside cell
3.
T1
Glucose + ATP Æ glucose-6-phosphate + ADP
hexokinase
Carrier proteins: three types
Carrier Proteins can mediate either:
1. Passive transport
driving force ->
concentration/electrochemical gradient
OR
(a) Uniport
(b) Co-transport
Uniport – one solute transported
[
Symport – two solutes in the same direction
Antiport – two solutes in opposite directions
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
Active
transport:
Na+K+ Pump
(Na+K+ATPase)
– Requires energy, usually in the form
of ATP hydorlysis
– Or a favorable gradient established
by use of ATP
P
P
P
3 Na+ out
2 K+ in
P
ATP!
1
Cytoplasmic Na+ binds to
the sodium-potassium pump.
2 Na+ binding stimulates
phosphorylation by ATP.
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
The Na+/K+ Pump:
Na+
The sodium
-potassium
pump
[Na+]
low
[K+] high
Na+
CYTOPLASM
“bilge pump”
ATP
P
ADP
Creates an electrochemical
gradient (high external [Na+ ])
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.
Na+
Na+
Na+
Na+
Na+
+
potential energy
Na+Na
– like “storing water behind a dam”
Na+
Na+
K+
P
K+
Figure 7.16
5
Loss of the phosphate
restores the protein’s
original conformation.
K+
uses ~1/3 of cell’s ATP!!
Pi
K+
6
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
Example of indirect active transport:
Na+
gradient drives other transport
Na+ glucose symport
• An electrogenic pump
– Is a transport protein that generates the voltage
across a membrane
–
–
ATP
EXTRACELLULAR
FLUID
+
+
H+
H+
Proton pump
H+
–
Glucose
Gradient
+
H+
+
Coupled transport
–
CYTOPLASM
Figure 7.18
–
• Cotransport: active transport driven by a
concentration gradient
–
+
H+
ATP
H+
+
–
+
+
–
Indirect active
transport
Transport coupled to
Exergonic rxn, i.e. ATP
hydrolysis
*Transport driven
by cotransport of ions
+
H+
–
+
Sucrose-H+
cotransporter
H+ Diffusion
of H+
H+
–
–
+
+
Sucrose
H+
Direct active
transport
H+
Proton pump
H+
Figure 7.19
H+
*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