Download transport across membrane

TRANSPORT ACROSS
MEMBRANES
Associate Professor Dr. Wipa Suginta
School of Biochemistry, Institute of Science
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Types of transport across membranes
Transporting process
Transporter
Passive
- Simple diffusion
- Facilitated diffusion
None
Gap junctions
Ion channels
Carriers: glucose transporter
Active
-Primary active transport
-Secondary active transporter
Pumps: Na+/K+ pump
Antiporters/Symporters
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Types of membrane transporter
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General Diffusion
1. Downhill process.
2. No energy required.
3. Extremely slow.
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Diffusion depends on permeability of molecules (P)
across a membrane which can be expressed as
P=
KD
d
P = permeability coefficient (cm/sec)
K = partition coefficient
D = diffusion coefficient (cm2/sec)
d = width of the cell membrane (cm)
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Effects of molecular weight and solubility on diffusion coefficient
Diffusion in water
Diffusion in lipid membranes
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Membrane permeability in weak acids and bases
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When molecules diffuse, free energy is stored in
concentration gradients
The free energy can be given as:
ΔG = RT ln (c2/c1 )
= 2.303 RT log10(c2/c1 )
R = gas constant (8.315 x 10-3 kJ mol-1 or 1.987 x 10-3 kcal mol-1 )
T = temperature (kelvin, K)
c = the concentration of the transported solutes
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Transport of charge species across membrane generating the electrical
membrane potential
ΔG = RT ln (c2/c1 ) + ZFV
= 2.303 RT log10(c2/c1 ) + ZFV
R = gas constant (8.315 x 10-3 kJ mol-1 or 1.987 x 10-3 kcal mol-1 )
T = temperature (kelvin, K)
c = the concentration of the transported solutes
Z = the electrical charge of the transported species
F = the Faraday constant (96.5 kJ V -1 mol-1or 23.1 kcal V -1 mol-1 )
V = the potential in volts across the membrane
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The magnitude of electrical potential Eion (volts) is given by the Nernst equation
ENa (∆V) = RTln(Nal)/Nar)
ZF
ENa = 0.059log10(Nal)/(Nar)
R (the gas constant) = 1.987 cal/(degree · mol), or
8.28 joules/(degree · mol); T (the absolute
temperature) = 293 K at 20 °C, Z (the valency) =
+1, F (the Faraday constant) = 23,062 cal/(mol ·
V) or 96,000 coulombs/(mol · V).
The ∆G of ion transport is given by
∆G = RTlnc2/c1 + ZFV
Z is the electrical charge of the transported species
ΔV is the potential in volts across the membrane
= 2.303 RT log10c2/c1 + ZF∆V
F is the faraday [23.1 kcal V-1 mol-1 (96.5 kJ V-1 mol-1)]
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Energy changes accompanying passage of the hydrophilic solute
through the lipid bilayer
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Facilitated diffusion across membranes can be achieved by
protein carriers
• Down-concentration gradient
• No energy required
• Non-specific or specific
• Not as slow as general diffusion
• Protein mediated
•. Inhibitable
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Example 1: passive diffusion through gap
junction
Small molecules of < 1200 Da pass through gap
junctions
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Kinetics of general diffusion through membranes via
non-specific mediators
DKA
J=
× ΔC
d
D = diffusion constant of solute
K = partition coefficient
A = area of membrae
D = thickness of membrane
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Example 2: Diffusion of antimicrobial agent through a
general diffusion porin
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Diffusion through membranes using a specific mediator
Example 1: Glucose Transporters (GLUTs) or glucose permeases
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Notkins, JBC, 2002
Model of the mechanism by GLUT1 is a shuttle
between two conformational states
The glucose transporter has 12
transmembrane a helices.
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A ping-pong model of GluT1 that exists in two
conformational states
1)
State "pong”; the binding sites for solute A are exposed on the outside of the
bilayer.
2) State "ping”; the same sites are exposed on the other side of the bilayer.
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Kinetics of specific carriers
-The process resembles an enzymesubstrate reaction.
- Each type of carrier protein has one or
more specific binding sites for its solute.
- When the carrier is saturated the rate
of transport is maximal (Vmax).
- Each carrier protein has a characteristic
binding constant for its solute, KM = [s];
v = vmax/2.
Glc out + GlUT
Vmax
Km

 [Glc-GLUT] 
 Glc in + GLUT

Rate (v) =
Vmax
1+
Km
[C]
C is the concentration of Sout (initially, the concentration of Sin = 0);
Vmax is the rate of transport if all molecules of the transporter contain a bound S, which occurs at high
Solute concentrations; and
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Km is the substrate concentration at which half-maximal transport occurs across the membrane.
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Diffusion through ion channels
Ion channels mediate the passage of ions
across plasma membranes.
• Down-concentration gradient
• No energy required
• Rapid diffusion
• Selective
• Protein mediated (ion channel)
• Gating controlled by ligand/voltage
• Inhibitable
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The rate of ion transport is influenced by the ion concentrations and by the
voltage (i.e., the electric potential).
Experimental system for generating a transmembrane voltage potential across a membrane
separating a 150 mM KCl/15 mM NaCl solution (conc in the cytosol) from a 15 mM KCl/150 mM
NaCl solution (conc. in blood).
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Black lipid membrane (BLM) reconstitution is a
technique to study the properties of ion channels
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The study of ion channels has been revolutionized by the
patch-clamp technique
(Erwin Neher and Bert Sakmann,1976)
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Acetylcholine receptor is a ligand-gated channels
The arrival of a nerve impulse leads to the
synchronous export of the contents of some
300 vesicles, which raises the acetylcholine
concentration in the cleft from 10 nM to 500
mM in less than a millisecond.
The electric organ of Torpedo marmorata, an
electric fish, is a choice source of
acetylcholine receptors for study.
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Binding of acetylcholine opens the Na+/K+ channel
The release of acetylcholine causes membrane
depolarization at the postsynaptic membrane by
increasing the conductance of Na+ and K+.
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Active transporters
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Primary active transport
.
Primary active transport, also called direct active transport, directly uses energy to
transport molecules across a membrane.
Most of the enzymes that perform this type of transport are transmembrane ATPases.
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Primary active transport of ions by the Na+/K+ pump in animal
cells
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Each time the Na+/K+ pump
hydrolyzes one molecule of ATP,
three Na+ ions leave the cell and
two K+ ions enter it.
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Secondary active transport
In secondary active transport, also known as coupled transport or co-transport, energy is used to transport
molecules across a membrane with no direct coupling of ATP, but the electrochemical potential difference
created by pumping ions out of the cell is used.
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Phloem loading and unloading
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Pathways affecting calcium concentrations in muscle cells
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Transport of glucose from intestinal lumen into blood stream through
basolateral membrane using facilitated passive and active transports
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