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Blueprint Series

Complete Solution
Chapter 11

11
TRANSMEMBRANE TRANSPORT
OF IONS AND SMALL MOLECULES

REVIEW THE CONCEPTS
1
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As it is produced by an
enzyme and accumulates in the endothelial cell cytosol, NO passively diffuses down
its concentration gradient though the endothelial cell plasma membrane out of the cell
and then passively diffuses through the plasma membrane into the cytoplasm of the
smooth muscle cell, where it acts to decrease contraction
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Of the two at neutral pH, ethanol is the much more membrane permeant
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0
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It is nonpermeant
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0,
ethanol remains uncharged and membrane permeant
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Hence, acetic acid is
now membrane permeant
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3
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The transported substrate both binds to the uniporter and elicits a conformational change in the transporter
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In contrast, channel proteins form a protein-lined passageway through
which multiple water molecules or ions move simultaneously, single file, at a rapid
rate
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CHAPTER 11: Transmembrane Transport of Ions and Small Molecules

4
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Both symporters and antiporters are capable of moving organic molecules
against an electrochemical gradient by coupling an energetically unfavorable
movement to the energetically favorable movement of a small inorganic ion
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Glucose is neutrally charged and hence its DG
for transport has only a concentration term
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Hence,
these cotransporters are better referred to as examples of secondary active transporters rather than as actual active transporters
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5
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Although
H+ is smaller than H2O, it cannot form the required hydrogen bonds
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6
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Uptake of glucose by
GLUT1 exhibits Michaelis-Menten kinetics and approaches saturation as the
concentration of glucose is increased
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a
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The Km for glucose will be lowest, indicating that
GLUT1 is glucose-specific, not galactose- or mannose-specific
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Despite its smaller size, ribose cannot bind to GLUT1 as glucose does because
it cannot form the same noncovalent bonds—and thus cannot be transported
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Using Equation 11-1, we find that at 5 mM, GLUT1-expressing cells transport
glucose at 77% maximal rate, whereas at 2
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d
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e
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f
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Insulin induces
a rapid increase in the Vmax for glucose uptake by stimulating fusion of these
vesicles with the plasma membrane, thereby increasing the number of plasma
membrane glucose transporters
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The four classes of ATP-powered pumps are: P-class, V-class, F-class, and ABC
superfamily
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All other classes pump cations or protons
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e
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Today, the natural substrates of ABC superfamily pumps are thought to be small phospholipids, cholesterol, and other
small molecules
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Direct hydrolysis of the phosphoanhydride bond would result in release of the
bond energy as heat, which would thus be “lost
...

9
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Some
Ca2+-ATPase pumps are activated by Ca2+-calmodulin, which lowers the cytosolic Ca2+ concentration by pumping Ca2+ either into the sarcoplasmic reticulum/endoplasmic reticulum or out of the cell
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In skeletal muscle cells, the result
would be to prolong the length and/or strength of muscle contraction
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These drugs irreversibly inhibit the H+/K+ ATPase in the apical membrane
of stomach parietal cells
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11
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The generation of this voltage gradient involves three fundamental elements:
a membrane to separate charge, a Na+/K+ ATPase to achieve charge separation
across the membrane, and nongated K+ channels to selectively conduct current
...
Movement of K+ outward, powered by the K+ concentration
gradient generated by Na+/K+ ATPase, leaves an excess of negative charges on
the inside and creates an excess of positive charges on the outside of the membrane
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These potassium
channels are referred to as resting K+ channels
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Their opening and closing
are nonregulated; hence, the channels are called nongated
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The ion
enters the channel as a hydrated ion, the water of hydration is exchanged for
interaction with carbonyl residues within the channel, and then as the ion exits
the channel it is rehydrated
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Because of this, the energetic situation
is highly unfavorable for Na+ versus K+
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Expression of a channel protein in a normally nonexpressing cell permits the
patch clamp assessment of channel properties
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Frog oocytes do not normally express plasma membrane channel proteins
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Frog oocytes are large and hence
technically easier to inject and to patch clamp than other cells
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CHAPTER 11: Transmembrane Transport of Ions and Small Molecules

13
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This cell
wall isrelatively stiff and rigid
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Overall, a plant cell is
consideredto have a turgor pressure because of the hyperosmotic
vacuole
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Animal cells have a slow inward leakage of ions
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The main countervailing export is the
net transport of cations by Na+/K+ ATPase (3 Na+ ions out for 2 K+ in)
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The six oxygens in the main-chain carbonyl or side-chain carboxyl
groups that bind each of the two Na+ ions in the transporter are
exquisitely positioned witha geometry similar to that of the water
molecules with which Na+ associates in solution
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When Na+ ions bind to the oxygens, they lose their water of
hydration
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K+ ions
(and watermolecules themselves) are too big to bind the six oxygens in
the proper geome-try and so do not compete with Na+
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Glucose uptake from the intestinal lumen into the epithelial cells is
driven by symport with 2 Na+ ions by a 2 Na+/glucose symporter
...
Transport by this
symporter is energetically favorable because movement of
Na+ ions into the cell is driven by both its concentration gradient and the
trans-membrane voltage gradient
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16
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Cotransporters couple the energeti-cally unfavorable movement of
glucose and amino acids into epithelial cells
to the energetically favorable movement of Na+ into these cells
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Tight junctions are essential for
the process because they seal the interstitial space between cells and
hence allow the transport proteins in the apical and basolateral

membranes of the epithelial cell to be effective
...
The coordinated
transport of glucose and Na+ ions across the intestinal epithelium
creates a transepithelial osmotic gradient
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