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Transport of Ions and Small Molecules
Across Cell Membranes
Objectives:
• Review how membrane transport is accomplished with the help of proteins – facilitated
diffusion, active transport (uniport, antiport, symport)
• Understand the characteristics, locations, and functions of the major classes of ATPpowered pump proteins
• Appreciate how non-gated channel proteins influence membrane potential, and how
membrane potential and channel functions are investigated experimentally
• Understand the process of cotransport – characteristics, examples of where it is used and
why
• Review how cells regulate water movement across the plasma membrane
• Understand how and why transepithelial transport works in certain epithelial cells. What
is necessary for this to occur?
• Appreciate how gated channels are used in neural function and how this is examined
experimentally
What do you know?
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Order the molecules on the following list according to their ability to diffuse
through a lipid bilayer, from most to least permeant. Explain your order.
Ca2+, CO2, ethanol, glucose, RNA, H2O
If a frog egg and a red blood cell are placed in pure water, the RBC will swell
and burst, but the frog egg will remain intact. Although a frog egg is about
one million times larger than a red cell, they both have nearly identical internal
concentrations of ions so that the same osmotic forces are at work in each.
Why do you suppose RBCs burst in water, while frog eggs do not?
True/False. The plasma membrane is highly impermeable to all charged
molecules. Explain your answer.
How is it possible for some molecules to be at equilibrium across a biological
membrane and yet not be at the same concentration on both sides?
True/False. A symporter would function as an antiporter if its orientation in
the membrane were reversed (that is, if the portion of the protein normally
exposed to the cytosol faced the outside of the cell instead). Explain your
answer.
Name three ways in which an ion channel can be gated.
True/False. Carrier proteins saturate at high concentration of the transported
molecule when all their binding sites are occupied; channel proteins, on the
other hand, do not bind the ions they transport and thus the flux of ions
through a channel does not saturate.
• Channels
• Carriers
• Pumps
Four Major Classes of Active Transporters
• One or more binding sites for ATP on the cytosolic face
• ATP is hydrolyzed only when a molecule is being transported
Animation Na/K pump
Therapeutic targets
Cardiac glycosides
Acid blockers
Antifungals
Acid Blockers
Proton-pump inhibitors
protonix
prevacid
Function: acidify the lumen of
their associated organelle
(vacuole, lysosome, etc.)
*Do not have a phosphorylated
intermediate
ABC = ATP Binding Cassette
ABC transporters involved in drug resistance.
Gene
Substrates
Inhibitors
ABCB1
Colchicine, doxorubicin, VP16,
Verapamil, PSC833, GG918, V-104,
Adriamycin, vinblastine, digoxin,
Pluronic L61
saquinivir, paclitaxel
ABCC1
Doxorubicin, daunorubicin, vincristine,
Cyclosporin A, V-104
VP16, colchicines, VP16, rhodamine
ABCC2
Vinblastine, sulfinpyrazone
ABCC3
Methotrexate, VP16
ABCC4
Nucleoside monophosphates
ABCC5
Nucleoside monophosphates
ABCG2
Mitoxantrone, topotecan, doxorubicin,
daunorubicin, CPT-11, rhodamine
a
VP16, etoposide.
Fumitremorgin C, GF120918
hepatomas, lung or colon carcinomas, breast cancers,
malaria, HIV
The ATP-binding cassette (ABC) superfamily, the major facilitator superfamily
(MFS), the
multidrug
and toxic-compound
extrusion
(MATE)
family, the small
Piddock
Nature Reviews
Microbiology 4, 629–636
(August 2006)
| doi:10.1038/nrmicro1464
multidrug resistance (SMR) family and the resistance nodulation division (RND)
family.
Two forces govern the
movement of ions across
selectively permeable
membranes:
1.the membrane electric
potential and
2.the ion concentration
gradient,
These forces may act in the
same or opposite directions.
If a membrane is permeable only to Na+ ions, then the measured
electric potential across the membrane equals the sodium
equilibrium potential in volts, ENa. The magnitude of ENa is given by
the Nernst equation, which is derived from basic principles of
physical chemistry:
ENa = RT ln [Nal]
ZF [Nar]
where 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), and [Nal] and [Nar] are
the Na+ concentrations on the left and right sides, respectively, at
equilibrium.
• Assume a temperature of
20oC
• RT/ZF = 0.059
• If the ratio in Na+
concentration between the
inside and outside is 0.1
and the membrane is
permeable only to Na+,
• Then you can predict the
mV difference across the
cell membrane using the
Nernst equation.
-59 mV
What will the equilibrium
membrane potential be if the
membrane is permeable only to K+?
• More open K+ channels
• Few open Na+ or Ca2+
channels
Thus resting potential is close to that of
the K+ equilibrium potential
Voltage-gated ion channels
(1) Open in response to changes in the membrane potential (voltage
gating);
(2) Over time following opening, will close and become inactive; and
(3) Have specificity for those ions that will permeate and those that will
not.
Voltage gated K+ channel
Aquaporins
Patch Clamping
(a) Na+ movement
(b) K+ movement in response to
different clamping voltages
Transporters in Disease
CFTR