Essential Cell Biology 5th edition

Answers A:31
1 2 3 4
OUTSIDE
Figure A12–8
INSIDE
ATP
ADP
P P P
B. False. Channels do not have binding pockets for the
solute that passes through them. Selectivity of a channel
is achieved by the size of the internal pore and by
charged regions at the entrance of the pore that attract
or repel ions of the appropriate charge.
C. False. Transporters are slower. They have enzymelike
properties; that is, they bind solutes and need to
undergo conformational changes during their functional
cycle. This limits the maximal rate of transport to about
1000 solute molecules per second, whereas channels can
pass up to 1,000,000 solute molecules per second.
D. True. The bacteriorhodopsin of some photosynthetic
bacteria pumps H + out of the cell using energy captured
from visible light.
E. True. Most animal cells contain K + leak channels in their
plasma membrane that are predominantly open. The
K + concentration inside the cell still remains higher than
outside because the membrane potential is negative and
therefore inhibits the positively charged K + from leaking
out. K + is also continually pumped into the cell by the
Na + pump.
F. False. A symport binds two different solutes on the
same side of the membrane. Turning it around would
not change it into an antiport, which must also bind
two different solutes but on opposing sides of the
membrane.
G. False. The peak of an action potential corresponds
to a transient shift of the membrane potential from a
negative to a positive value. The influx of Na + causes the
membrane potential first to move toward zero and then
to reverse, rendering the cell positively charged on its
inside. Eventually, the resting potential is restored by an
efflux of K + through voltage-gated K + channels and K +
leak channels.
ANSWER 12–10 The permeabilities are N 2 (small and
nonpolar) > ethanol (small and slightly polar) > water (small
and polar) > glucose (large and polar) > Ca 2+ (small and
charged) > RNA (very large and charged).
ANSWER 12–11
A. Both couple the movement of two different solutes
across a cell membrane. Symports transport both solutes
in the same direction, whereas antiports transport the
solutes in opposite directions.
B. Both are mediated by membrane transport proteins.
Passive transport of a solute occurs downhill, in the
direction of its concentration or electrochemical
gradient, whereas active transport occurs uphill and
therefore needs an energy source. Active transport
can be mediated by transporters but not by channels,
whereas passive transport can be mediated by either.
C. Both terms describe gradients across a membrane. The
membrane potential refers to the voltage gradient; the
electrochemical gradient is a composite of the voltage
gradient and the concentration gradient of a specific
charged solute (ion). The membrane potential is defined
independently of the solute of interest, whereas an
electrochemical gradient refers to the particular solute.
D. A pump is a specialized transporter that uses energy
to transport a solute uphill—against an electrochemical
gradient for a charged solute or a concentration
gradient for an uncharged solute.
E. Both transmit electrical signals, by means of electrons
in wires and by ion movements across the plasma
membrane in axons. Wires are made of copper, axons
are not. The signal passing down an axon does not
diminish in strength because it is self-amplifying,
whereas the signal in a wire decreases over distance (by
leakage ECB5 of EA12.09/A12.09
current across the insulating sheath).
F. Both affect the osmotic pressure in a cell. An ion is a
solute that bears a charge.
ANSWER 12–12 A bridge allows vehicles to pass over
water in a steady stream; the entrance can be designed
to exclude, for example, oversized trucks, and it can be
intermittently closed to traffic by a gate. By analogy,
gated channels allow ions to pass across a cell membrane,
imposing size and charge restrictions.
A ferry, in contrast, loads vehicles on one side of
the body of water, crosses, and unloads on the other
side—a slower process. During loading, particular vehicles
could be selected from the waiting line because they fit
particularly well on the car deck. By analogy, transporters
bind solutes on one side of the membrane and then, after a
conformational movement, release them on the other side.
Specific binding selects the molecules to be transported.
As in the case of coupled transport, sometimes you have to
wait until the ferry is full before you can go.
ANSWER 12–13 Acetylcholine is being transported into
the vesicles by an H + –acetylcholine antiport in the vesicle
membrane. The H + gradient that drives the uptake is
generated by an ATP-driven H + pump in the vesicle
membrane, which pumps H + into the vesicle (hence the
dependence of the reaction on ATP). Raising the pH of
the solution surrounding the vesicles decreases the H +
concentration of the solution, thereby increasing the
outward gradient across the vesicle membrane, explaining
the enhanced rate of acetylcholine uptake.
ANSWER 12–14 The voltage gradient across the membrane
is about 150,000 V/cm (70 × 10 –3 V/4.5 × 10 –7 cm). This
extremely powerful electric field is close to the limit at
which insulating materials—such as the lipid bilayer—
break down and cease to act as insulators. The large field
indicates what a large amount of energy can be stored in
electrical gradients across the membrane, as well as the
extreme electrical forces that proteins can experience