Essential Cell Biology 5th edition

424 CHAPTER 12 Transport Across Cell Membranes
QUESTIONS
QUESTION 12–8
The diagram in Figure 12–9 shows a transporter that
mediates the passive transfer of a solute down its
concentration gradient across the membrane. How would
you need to change the diagram to convert the transporter
into a pump that moves the solute up its concentration
gradient by hydrolyzing ATP? Explain the need for each of
the steps in your new illustration.
QUESTION 12–9
Which of the following statements are correct? Explain your
answers.
A. The plasma membrane is highly impermeable to all
charged molecules.
B. Channels have specific binding pockets for the solute
molecules they allow to pass.
C. Transporters allow solutes to cross a membrane at much
faster rates than do channels.
D. Certain H + pumps are fueled by light energy.
E. The plasma membrane of many animal cells contains
open K + channels, yet the K + concentration in the cytosol is
much higher than outside the cell.
F. A symport would function as an antiport if its orientation
in the membrane were reversed (i.e., if the portion of the
molecule normally exposed to the cytosol faced the outside
of the cell instead).
G. The membrane potential of an axon temporarily
becomes more negative when an action potential excites it.
QUESTION 12–10
List the following compounds in order of decreasing lipidbilayer
permeability: RNA, Ca 2+ , glucose, ethanol, N 2 , water.
QUESTION 12–11
Name at least one similarity and at least one difference
between the following (it may help to review the definitions
of the terms using the Glossary):
A. Symport and antiport
B. Active transport and passive transport
C. Membrane potential and electrochemical gradient
D. Pump and transporter
E. Axon and telephone wire
F. Solute and ion
QUESTION 12–12
Discuss the following statement: “The differences between
a channel and a transporter are like the differences between
a bridge and a ferry.”
QUESTION 12–13
The neurotransmitter acetylcholine is made in the cytosol
and then transported into synaptic vesicles, where its
concentration is more than 100-fold higher than in the
cytosol. When synaptic vesicles are isolated from neurons,
they can take up additional acetylcholine added to the
solution in which they are suspended, but only when ATP
is present. Na + ions are not required for the uptake, but,
curiously, raising the pH of the solution in which the synaptic
vesicles are suspended increases the rate of uptake.
Furthermore, transport is inhibited when drugs are added
that make the membrane permeable to H + ions. Suggest a
mechanism that is consistent with all of these observations.
QUESTION 12–14
The resting membrane potential of a typical animal cell is
about –70 mV, and the thickness of a lipid bilayer is about
4.5 nm. What is the strength of the electric field across the
membrane in V/cm? What do you suppose would happen
if you applied this field strength to two metal electrodes
separated by a 1-cm air gap?
QUESTION 12–15
Phospholipid bilayers form sealed, spherical vesicles
in water (discussed in Chapter 11). Assume you have
constructed lipid vesicles that contain Na + pumps as
the sole membrane protein, and assume for the sake of
simplicity that each pump transports one Na + one way and
one K + the other way in each pumping cycle. All the Na +
pumps have the portion of the molecule that normally faces
the cytosol oriented toward the outside of the vesicles. With
the help of Figures 12–11 and 12–12, determine what would
happen in each of the following cases.
A. Your vesicles were suspended in a solution containing
both Na + and K + ions and had a solution with the same ionic
composition inside them.
B. You add ATP to the suspension described in (A).
C. You add ATP, but the solution—outside as well as inside
the vesicles—contains only Na + ions and no K + ions.
D. The concentrations of Na + and K + were as in (A), but
half of the pump molecules embedded in the membrane of
each vesicle were oriented the other way around, so that
the normally cytosolic portions of these molecules faced the
inside of the vesicles. You then add ATP to the suspension.
E. You add ATP to the suspension described in (A), but in
addition to Na + pumps, the membrane of your vesicles also
contains K + leak channels.
QUESTION 12–16
Name the three ways in which an ion channel can be gated.
QUESTION 12–17
One thousand Ca 2+ channels open in the plasma membrane
of a cell that is 1000 μm 3 in size and has a cytosolic
Ca 2+ concentration of 100 nM. For how long would the
channels need to stay open in order for the cytosolic Ca 2+
concentration to rise to 5 μM? There is virtually unlimited
Ca 2+ available in the outside medium (the extracellular
Ca 2+ concentration in which most animal cells live is a few
millimolar), and each channel passes 10 6 Ca 2+ ions per
second.