CHAPTER 12
MEMBRANE TRANSPORT
 2009 Garland Science Publishing
Principles of Membrane Transport
12-1
Indicate whether the following statements are true or false. If a statement is false, explain
why it is false.
A.
CO2 and O2 are water-soluble molecules that diffuse freely across cell
membranes.
B.
The differences in permeability between artificial lipid bilayers and cell
membranes arise from variations in phospholipid content.
C.
Transporters are similar to channels, except that they are larger, allowing folded
proteins as well as smaller organic molecules to pass through them.
D.
Cells expend energy in the form of ATP hydrolysis so as to maintain ion
concentrations that differ from those found outside the cell.
12-2
Although the extracellular environment has a high sodium ion concentration and the
intracellular environment has a high potassium ion concentration, both must be
neutralized by negatively charged molecules. In the extracellular case, what is the
principal anion?
(a)
HCO3–
(b)
Cl–
(c)
PO43–
(d)
OH–
12-3
Circle the molecule in each pair that is more likely to diffuse through the lipid bilayer.
A.
amino acids or
benzene
B.
Cl–
or
ethanol
C.
glycerol
or
RNA
D.
H2O
or
O2
E.
adenosine
or
ATP
12-4
We can test the relative permeability of a phospholipid bilayer by using a synthetic
membrane that does not contain any protein components. Some uncharged, polar
molecules are found to diffuse freely across these membranes, to varying degrees. Which
of the following has the lowest rate of diffusion across an artificial membrane? Why?
(a)
glucose
(b)
water
(c)
glycerol
(d)
ethanol
12-5
For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
should be used only once.
A molecule moves down its concentration gradient by
__________________ transport, but requires __________________
transport to move up its concentration gradient. Transporter proteins and
ion channels function in membrane transport by providing a
__________________ pathway through the membrane for specific polar
solutes or inorganic ions. __________________ are highly selective in the
solutes they transport, binding the solute at a specific site and changing its
conformation so as to transport the solute across the membrane. On the
other hand, __________________ discriminate between solutes mainly on
the basis of size and electrical charge.
active
amino acid
amphipathic
transporter proteins
hydrophilic
hydrophobic
ion channels
noncovalent
passive
Transporters and their Functions
12-6
A hungry yeast cell lands in a vat of grape juice and begins to feast on the sugars there,
producing carbon dioxide and ethanol in the process:
C6H12O6 + 2ADP + 2Pi + H+  2CO2 + 2CH3CH2OH + 2ATP + 2H2O
Unfortunately, the grape juice is contaminated with proteases that attack some of the
transport proteins in the yeast cell membrane, and the yeast cell dies. Which of the
following could account for the yeast cell’s demise?
(a)
toxic buildup of carbon dioxide inside the cell
(b)
toxic buildup of ethanol inside the cell
(c)
diffusion of ATP out of the cell
(d)
inability to import sugar into the cell
12-7
Ion channels are classified as membrane transport proteins. Channels discriminate by size
and charge. In addition to Na+, which one of the following ions would you expect to be
able to freely diffuse through a Na+ channel? Explain your answer.
(a)
Mg2+
(b)
H+
(c)
K+
(d)
Cl–
12-8
Transporters, in contrast to channels, work by ________________.
(a)
specific binding to solutes
(b)
a gating mechanism
(c)
filtering solutes by charge
(d)
12-9
filtering solutes by size
Pumps are transporters that are able to harness energy provided by other components in
the cells to drive the movement of solutes across membranes, against their concentration
gradient. This type of transport is called _____________.
(a)
active transport
(b)
free diffusion
(c)
facilitated diffusion
(d)
passive transport
12-10 Indicate whether the statements below are true or false. If a statement is false, explain
why it is false.
A.
Facilitated diffusion can be described as the favorable movement of one solute
down its concentration gradient being coupled with the unfavorable movement of
a second solute up its concentration gradient.
B.
Transporters undergo transitions between different conformations, depending on
whether the substrate-binding pocket is empty or occupied.
C.
The electrochemical gradient for K+ across the plasma membrane is small.
Therefore, any movement of K+ from the inside to the outside of the cell is driven
solely by its concentration gradient.
D.
The net negative charge on the cytosolic side of the membrane enhances the rate
of glucose import into the cell by a uniporter.
12-11 It is thought that the glucose transporter switches between two conformational states in a
completely random fashion. How is it possible for such a system to move glucose across
the membrane efficiently in a single direction?
12-12 Active transport requires the input of energy into a system so as to move solutes against
their electrochemical and concentration gradients. Which of the following is not one of
the common ways to perform active transport?
(a)
Na+-coupled
(b)
K+-coupled
(c)
ATP-driven
(d)
light-driven
12-13 The Na+-K+ ATPase is also known as the Na+-K+ pump. It is responsible for
maintaining the high extracellular sodium ion concentration and the high
intracellular potassium ion concentration. What happens immediately after the
pump hydrolyzes ATP?
(a)
Na+ is bound.
(b)
ADP is bound.
(c)
The pump is phosphorylated.
(d)
The pump changes conformation.
12-14 If ATP production is blocked in an animal cell, the cell will swell up. Explain this
observation.
12-15 Fill in Table Q12-15. In the “type of transport” column, designate whether the
transporter works by uniport, symport, or antiport mechanisms.
Table Q12-15
12-16 You have prepared lipid vesicles (spherical lipid bilayers) that contain Na+-K+ pumps as
the sole membrane protein. All of the Na+-K+ pumps are oriented in such a way that the
portion of the molecule that normally faces the cytosol is on the inside of the vesicle and
the portion of the molecule that normally faces the extracellular space is on the outside of
the vesicle. Assume that each pump transports one Na+ ion in one direction and one K+
ion in the other direction during each pumping cycle (see Figure Q12-16 for how the
Na+-K+ pump normally functions in the plasma membrane).
Figure 12-16
Predict what would happen in each of the following conditions:
A.
The solutions inside and outside the vesicles contain both Na+ and K+ ions but no
ATP.
B.
The solution outside the vesicles contains both Na+ and K+ ions; the solution
inside contains both Na+ and K+ ions and ATP.
C.
The solution outside contains Na+; the solution inside contains Na+ and ATP.
12-17 The Aeroschmidt weed contains an ATP-driven ion pump in its vacuolar membrane that
pumps potentially toxic heavy metal ions such as Zn2+ and Pb2+ into the vacuole. The
pump protein exists in a phosphorylated and an unphosphorylated form and works in a
similar way to the Na+-K+ pump of animal cells. To study its action, you incorporate the
unphosphorylated form of the protein into phospholipid vesicles containing K+ in their
interiors. (You ensure that all of the protein molecules are in the same orientation in the
lipid bilayer.) When you add Zn2+ and ATP to the solution outside such vesicles, you find
that Zn2+ is pumped into the vesicle lumen. You then expose vesicles containing the
pump protein to the solutes as shown in Table 12-17A.
Table 12-17A
You then determine the amount of phosphorylated and unphosphorylated ATP-driven ion
pump protein in each sample. Your results are summarized in Table 12-17B, where a
minus sign indicates an absence of a type of protein and a plus sign indicates its presence.
Table 12-17B
What would you expect to happen if you treat vesicles as in lane F, but before
determining the phosphorylation state of the protein you wash away the outside buffer
and replace it with a buffer containing only Zn2+?
(a)
Nothing will happen. (No Zn2+ will move into the vesicle; no K+ will move out of
the vesicle; the phosphorylation state of the protein will not change.)
(b)
No Zn2+ will move into the vesicle; no K+ will move out of the vesicle; the protein
will become unphosphorylated.
(c)
A small amount of Zn2+ will move into the vesicle; no K+ will move out of the
vesicle; the phosphorylation state of the protein will not change.
(d)
A small amount of Zn2+ will move into the vesicle; no K+ will move out of the
vesicle; the protein will become unphosphorylated.
12-18 You have generated antibodies that recognize the extracellular domain of the Ca2+ pump.
Adding these antibodies to animal cells blocks the active transport of Ca2+ from the
cytosol into the extracellular environment. What do you expect to observe with respect to
intracellular Ca2+?
(a)
Ca2+ pumps in vesicles membranes keep cystosolic calcium levels low.
(b)
Ca2+ pumps in the ER membrane keep cystosolic calcium levels low.
(c)
Ca2+ pumps in the Golgi apparatus keep cystosolic calcium levels low.
(d)
Ca2+ concentrations in the cytosol increase at a steady rate.
12-19 Cells make use of H+ electrochemical gradients in many ways. Which of the following
proton transporters is used to regulate pH in animal cells?
(a)
light-driven pump
(b)
H+ ATPase
(c)
H+ symporter
(d)
Na+-H+ exchanger
12-20 Which of the following statements is true?
(a)
Amoebae have transporter proteins that actively pump water molecules from the
cytoplasm to the cell exterior.
(b)
Bacteria and animal cells rely on the Na+-K+ pump in the plasma membrane to
prevent lysis resulting from osmotic imbalances.
(c)
The Na+-K+ pump allows animal cells to thrive under conditions of very low ionic
strength.
(d)
The Na+-K+ pump helps to keep both Na+ and Cl– ions out of the cell.
12-21 Ca2+ pumps in the plasma membrane and endoplasmic reticulum are important for
_____________.
(a)
maintaining osmotic balance
(b)
preventing Ca2+ from altering the activity of molecules in the cytosol
(c)
providing enzymes in the endoplasmic reticulum with Ca2+ ions that are necessary
for their catalytic activity
(d)
maintaining a negative membrane potential
12-22 For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
should be used only once.
For an uncharged molecule, the direction of passive transport across a
membrane is determined solely by its __________________ gradient. On
the other hand, for a charged molecule, the __________________ must
also be considered. The net driving force for a charged molecule across a
membrane therefore has two components and is referred to as the
__________________ gradient. Active transport allows the movement of
solutes against this gradient. The transporter proteins called
__________________ transporters use the movement of one solute down
its gradient to provide the energy to drive the uphill transport of a second
gradient. When this transporter moves both ions in the same direction
across the membrane, it is considered a(n) __________________; if the
ions move in opposite directions, the transporter is considered a(n)
__________________.
antiport
ATP hydrolysis
concentration
coupled
electrochemical
light-driven
membrane potential
symport
uniport
12-23 The movement of glucose into the cell, against its concentration gradient, can be powered
by the co-transport of Na+ into the cell. Explain this movement with respect to the net
entropy of the system (i.e. thermodynamics).
12-24 You are testing the rate of glucose transport into vesicles using the Na+-glucose pump.
A.
In experiment 1, you employ liposomes that have the pump in the same
orientation as that found in the plasma membrane in epithelial cells. These
liposomes contain glucose but no Na+ ions. You then transfer the liposomes to a
series of tubes with solutions containing the same glucose concentration as that
inside the vesicle and 0, 1, 2, 3, or 10 mM Na+. You measure the initial rates of
glucose transport and plot your results (Figure Q12-24). Why do the initial rates
of glucose transport into the liposome reach a plateau as the concentration of Na+
increases?
B.
In experiment 2, there is one new variable: you have included leaky Na+ channels
in the liposomal membrane. Figure Q12-24 shows your results. Explain the reason
for obtaining such different results in experiment 2 from those in experiment 1.
Figure Q12-24
Ion Channels and the Membrane Potential
12-25 Which of the following best describes the behavior of a gated channel?
(a)
It stays open continuously when stimulated.
(b)
It opens more frequently in response to a given stimulus.
(c)
It opens more widely as the stimulus becomes stronger.
(d)
It remains closed if unstimulated.
12-26 The flow of ions through a gated channel can be studied using a method called “patchclamp recording.”
A.
How is a detached patch-clamp experiment set up and what exactly does it mean
to “clamp” an ion channel?
B.
How is it possible to collect the recordings shown in figures Q12-26A and Q1226B from a single ion channel?
Figure Q12-26
12-27 Indicate whether the statements below are true or false. If a statement is false,
explain why it is false.
A.
Gap junctions are large pores that connect the cytosol to the extracellular space.
B.
Aquaporin channels are found in the plasma membrane, allowing the rapid
passage of water molecules and small ions in and out of cells.
C.
The ion selectivity of a channel completely depends solely on the charge of the
amino acids liningthe pore inside the channel.
D.
Most ion channels are gated, which allow them to open and close in response to a
specific stimulus rather than allowing the constant, unregulated flow of ions.
12-28 The stimulation of auditory nerves depends on the opening and closing of channels in the
auditory hair cells. Which type of gating mechanism do these cells use?
(a)
voltage-gated
(b)
extracellular ligand-gated
(c)
intracellular ligand-gated
(d)
stress-gated
12-29 For each of the following sentences, fill in the blank with the appropriate type of gating
for the ion channel described. You can use the same type of gating mechanism more than
once.
A.
The acetylcholine receptor in skeletal muscle cells is a(n) __________________
ion channel.
B.
__________________ ion channels are found in the hair cells of the mammalian
cochlea.
C.
D.
E.
__________________ ion channels in the mimosa plant propagate the leafclosing response.
__________________ ion channels respond to changes in membrane potential.
Many receptors for neurotransmitters are __________________ ion channels.
12-30 Voltage-gated channels contain charged protein domains, which are sensitive to
changes in membrane potential. By responding to a threshold in the membrane
potential, these voltage sensors trigger the opening of the channels. Which of the
following best describes the behavior of a population of channels exposed to such
a threshold?
(a)
Some channels remain closed and some open completely.
(b)
All channels open completely.
(c)
All channels open partly, to the same degree.
(d)
All channels open partly, each to a different degree.
12-31 K+ leak channels are found in the plasma membrane. These channels open and close in an
unregulated, random fashion. What do they accomplish in a resting cell?
(a)
They set the K+ concentration gradient to zero.
(b)
They set the membrane potential to zero.
(c)
They disrupt the resting membrane potential.
(d)
They keep the electrochemical gradient for K+ at zero.
12-32 The Nernst equation can be used to calculate the membrane potential based on the ratio
of the outer and inner ion concentration. In a resting cell, membrane potential is
calculated taking only K+ ions into account. What is V when Co = 15 mM and Ci =
106 mM?
(a)
438.1 mV
(b)
–52.7 mV
(c)
52.7 mV
(d)
–5.3 mV
12-33 When using the Nernst equation to calculate membrane potential, we are making
several assumptions about conditions in the cell. Which of the following is not a
good assumption?
(a)
The temperature is 37°C.
(b)
The plasma membrane is primarily permeable to Na+.
(c)
At rest, the interior of the cell is more negatively charged than the exterior.
(d)
K+ is the principal positive ion in the cell.
12-34 If Na+ channels are opened in a cell that was previously at rest, how will the resting
membrane potential be affected?
(a)
The membrane potential is not affected by Na+.
(b)
It becomes more negative.
(c)
It becomes more positive.
(d)
It is permanently reset.
Ion Channels and Signaling in Nerve Cells
12-35 Match the numbered lines with the following structures:
A.
nerve terminal
B.
cell body
C.
axon
D.
dendrite
12-36 Indicate whether the statements below are true or false. If a statement is false, explain
why it is false.
A.
Neurotransmitters are small molecules released into the synaptic cleft after the
fusion of synaptic vesicles with the presynaptic membrane.
B.
Action potentials are usually mediated by voltage-gated Ca2+ channels.
C.
Voltage-gated Na+ channels become automatically inactivated shortly after
opening, which ensures that the action potential cannot move backward along the
axon.
D.
Voltage-gated K+ channels also open immediately in response to local
depolarization, reducing the magnitude of the action potential.
12-37 Which of the following is required for the secretion of neurotransmitters in response to an
action potential?
(a)
neurotransmitter receptors
(b)
Na+-K+ pumps
(c)
voltage-gated K+ channels
(d)
voltage-gated Ca2+ channels
12-38 Figure Q12-38 illustrates changes in membrane potential during the formation of an
action potential. What membrane characteristic or measurement used to study action
potentials is indicated by the arrow?
Figure Q12-38
(a)
(b)
(c)
(d)
effect of a depolarizing stimulus
resting membrane potential
threshold potential
action potential
12-39 Figure Q12-39 illustrates changes in membrane potential during the formation of an
action potential. What membrane characteristic or measurement used to study action
potentials is indicated by the arrow?
Figure Q12-39
(a)
(b)
(c)
(d)
effect of a depolarizing stimulus
resting membrane potential
threshold potential
action potential
12-40 Figure Q12-40 illustrates changes in membrane potential during the formation of an
action potential. What membrane characteristic or measurement used to study action
potentials is indicated by the arrow?
Figure Q12-40
(a)
(b)
(c)
(d)
effect of a depolarizing stimulus
resting membrane potential
threshold potential
action potential
12-41 Figure Q12-41 illustrates changes in membrane potential during the formation of an
action potential. What membrane characteristic or measurement used to study action
potentials is indicated by the arrow?
Figure Q12-41
(a)
(b)
(c)
(d)
effect of a depolarizing stimulus
resting membrane potential
threshold potential
action potential
12-42 For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
should be used only once.
The action potential is a wave of __________________ that spreads
rapidly along the neuronal plasma membrane. This wave is triggered by a
local change in the membrane potential to a value that is
__________________ negative than the resting membrane potential. The
action potential is propagated by the opening of __________________gated channels. During an action potential, the membrane potential
changes from __________________ to __________________. The action
potential travels along the neuron’s __________________ to the nerve
terminals. Neurons chiefly receive signals at their highly branched
__________________.
anions
axon
synaptic vesicle
cytoskeleton
dendrites
depolarization
hyperpolarization
less
ligand
more
negative
neutral
positive
pressure
voltage
12-43 The stimulation of a motor neuron ultimately results in the release of a neurotransmitter
at the synapse between the neuron and a muscle cell. What type of neurotransmitter is
used at these neuromuscular junctions?
(a)
acetylcholine
(b)
glutamate
(c)
GABA
(d)
glycine
12-44 Both excitatory and inhibitory neurons form junctions with muscles. By what mechanism
do inhibitory neurotransmitters prevent the postsynaptic cell from firing an action
potential?
(a)
by closing Na+ channels
(b)
by preventing the secretion of excitatory neurotransmitters
(c)
by opening K+ channels
(d)
by opening Cl– channels
12-45 For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
should be used only once.
Neurons communicate with each other through specialized sites called
__________________. Many neurotransmitter receptors are ligand-gated
ion channels that open transiently in the __________________ cell
membrane in response to neurotransmitters released by the
__________________ cell. Ligand-gated ion channels in nerve cell
membranes convert __________________ signals into
__________________ ones. Neurotransmitter release is stimulated by the
opening of voltage-gated __________________ in the nerve terminal
membrane.
acetylcholine receptor
Ca2+ channels
chemical
postsynaptic
GABA receptor
K+ channels
Na+ channels
presynaptic
synapses
electrical
How We Know: Squid Reveal Secrets of Membrane Excitability
12-46 Studies on the squid giant axon were instrumental in our current understanding of how
action potentials are generated. You decide to do some experiments on the squid giant
axon yourself.
A.
You remove the cytoplasm in an axon and replace it with an artificial cytoplasm
that contains twice the normal concentration of K+ by adding KOAc, where OAc–
is an anion that is impermeable to the membrane. In this way you double the
internal concentration of K+ while maintaining the bulk electrical balance of the
cytoplasmic solution. Will this make the resting potential of the membrane more
or less negative?
B.
You add NaCl to the extracellular fluid and effectively double the amount of
extracellular Na+ cation. How does this affect the action potential?
C.
You replace half of the NaCl in the extracellular fluid with choline chloride.
(Choline is a monovalent cation much larger than Na+. Note that the presence of
choline will not impede the flow of Na+ through its channels.) How will this
affect the action potential?