Chapter 12
True or False: B. Action potentials are usually mediated by voltage-gated Ca 2+ channels.
False. Action potentials are usually mediated by voltage-gated Na + channels.
True or false: B. Aquaporin channels are found in the plasma membrane, allowing the rapid passage of water molecules and small ions in and out of cells.
False. Charged molecules (even protons, which are very small), are not able to pass through aquaporins.
True or false: Gap junctions are large pores that connect the cytosol to the extracellular space.
False. Gap junctions are used to connect the cytosol of adjacent cells, allowing the sharing of ions and small metabolites. Because gap junctions are large channels, if they were open while facing the extracellular environment, the ability of the plasma membrane to serve as a permeability barrier would be greatly reduced.
True or False: D. The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter.
False. Glucose is an uncharged molecule, and its import is not directly affected by the voltage difference across the membrane if glucose is being transported alone. If the example given were the Na/glucose symporter, we would have to consider the charge difference across the membrane.
True or false: C. The ion selectivity of a channel completely depends solely on the charge of the amino acids liningthe pore inside the channel.
False. Selectivity depends on three parameters: the diameter, shape and charge of the ion trying to pass through the pore of the channel.
True or false: B. The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content.
False. The primary difference between cell membranes and artificial membranes is that cell membranes have proteins responsible for creating a selective permeability, which varies with the location and function of the membrane.
True or False: 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.
False. This describes coupled transport, which is one type of active transport. Facilitated diffusion can also be called passive transport, in which a solute always moves down its concentration gradient.
True or false: C. Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them.
False. Transporters work by changing conformation after specific binding of the solute to be transported. Channels exclude molecules on the basis of size and charge, but do not depend on specific recognition of the molecules moving through.
True or False: D. Voltage-gated K + channels also open immediately in response to local depolarization, reducing the magnitude of the action potential.
False. Voltage-gated K + channels respond more slowly than the voltage-gated Na + channels. Because they do not open until the action potential reaches its peak, they do not affect its magnitude. Instead, they help to restore the local membrane potential quickly while the voltage-gated Na + channels are in the inactivated conformation.
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.
a
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
a
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
a
12-8 Transporters, in contrast to channels, work by ________________. (a) specific binding to solutes (b) a gating mechanism (c) filtering solutes by charge (d) filtering solutes by size
a
12-9 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
a
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
b
12-18 You have generated antibodies that recognize the extracellular domain of the Ca 2+ pump. Adding these antibodies to animal cells blocks the active transport of Ca 2+ from the cytosol into the extracellular environment. What do you expect to observe with respect to intracellular Ca 2+ ? (a) Ca 2+ pumps in vesicles membranes keep cystosolic calcium levels low. (b) Ca 2+ pumps in the ER membrane keep cystosolic calcium levels low. (c) Ca 2+ pumps in the Golgi apparatus keep cystosolic calcium levels low. (d) Ca 2+ concentrations in the cytosol increase at a steady rate.
b
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) HCO 3 - (b) Cl - (c) PO 4 3- (d) OH-
b
12-21 Ca 2+ pumps in the plasma membrane and endoplasmic reticulum are important for _____________. (a) maintaining osmotic balance (b) preventing Ca 2+ from altering the activity of molecules in the cytosol (c) providing enzymes in the endoplasmic reticulum with Ca 2+ ions that are necessary for their catalytic activity (d) maintaining a negative membrane potential
b
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.
b
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.
b
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) Mg 2+ (b) H + (c) K + (d) Cl -
b
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.
c
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.
c
True or False: C. Voltage-gated Na + channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon.
true
True or False: Neurotransmitters are small molecules released into the synaptic cleft after the fusion of synaptic vesicles with the presynaptic membrane.
true
True or False: B. Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied.
true
True or False: 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.
true
True or false: 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.
true
True or false: CO 2 and O 2 are water-soluble molecules that diffuse freely across cell membranes.
true
True or false: D. Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.
true
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
d
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.
d
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
d
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.
d
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 Ca 2+ channels
d
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
d
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: C 6 H 12 O 6 + 2ADP + 2P i + H + 2CO 2 + 2CH 3 CH 2 OH + 2ATP + 2H 2 O 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
d