CHAPTER 12 MEMBRANE TRANSPORT
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) Some channels remain closed and some open completely.
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) acetylcholine
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) active transport
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
(a) by closing Na+ channels
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? (a) effect of a depolarizing stimulus (b) resting membrane potential (c) threshold potential (d) action potential
(a) effect of a depolarizing stimulus
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) glucose
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) specific binding to solutes
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.
(b) Ca2+ pumps in the ER membrane keep cystosolic calcium levels low.
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) PO4 3- (d) OH-
(b) Cl-
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-
(b) H+
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) It opens more frequently in response to a given stimulus.
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) K+-coupled
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
(b) preventing Ca2+ from altering the activity of molecules in the cytosol
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? (a) effect of a depolarizing stimulus (b) resting membrane potential (c) threshold potential (d) action potential
(b) resting membrane potential
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) The pump is phosphorylated.
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? (a) effect of a depolarizing stimulus (b) resting membrane potential (c) threshold potential (d) action potential
(c) threshold potential
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) 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.
(d) The Na+-K+ pump helps to keep both Na+ and Cl- ions out of the cell.
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) They keep the electrochemical gradient for K+ at zero.
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? (a) effect of a depolarizing stimulus (b) resting membrane potential (c) threshold potential (d) action potential
(d) action potential
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
(d) inability to import sugar into the cell
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) stress-gated
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
(d) voltage-gated Ca2+ channels
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.
12-1 A. True. B. 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. C. 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. D. True.
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.
12-10 A. 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. B. True. C True. D. 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.
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?
12-11 Although the opening of the glucose transporter on one side of the membrane or the other is random, the binding of glucose into the binding site of the transporter is not a random event. The affinity between the glucose molecule and the transporter governs the binding event: transporter + glucose ⇔ transporter-glucose At high glucose concentrations the complex formation is favored; at low glucose concentrations dissociation of glucose from the transporter is favored. So, as long as there is a large concentration gradient, efficient transport can occur by the simple rules of binding equilibria.
12-14 If ATP production is blocked in an animal cell, the cell will swell up. Explain this observation.
12-14 ATP is required to power the Na+-K+ pump, which is necessary for maintaining osmotic balance. The pump requires ATP hydrolysis to drive its pumping cycle. So, in the absence of ATP production, the Na+ concentration inside the cell will increase. This is followed by passive diffusion of water across the membrane, causing the cell to swell.
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). 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.
12-16 A. Without any ATP to provide energy for the Na+-K+ pumps, no ions will be pumped. B. The pumps will use the energy from ATP hydrolysis to transport Na+ out of the vesicles and K+ into the vesicles. (The pumps will stop working either when the amount of ATP inside the vesicle is depleted or when the K+ outside the vesicles is depleted.) C. The pump will bind a molecule of Na+, causing the ATPase activity to hydrolyze ATP and transfer the phosphate group onto the pump. A conformational change will occur, leading to the release of Na+ from the vesicle. However, because there is no K+ outside the vesicle, the pump will get stuck at that step and subsequent steps of the cycle will not occur.
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).
12-23 The movement of Na+ ions from an area that has a high Na+ concentration to a new area of low Na+ concentration is energetically favorable because the net entropy in the system is increasing. As long as the difference in Na+ ion concentration across the membrane is large, the entropic factor will be sufficient to drive the import of glucose into the cell, which represents a decrease in entropy with respect to the population of glucose molecules inside the cell.
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 Q12- 26B from a single ion channel?
12-26 A. A detached patch-clamp experiment requires the removal of a portion of the cell membrane by sealing the microelectrode to the membrane surface. After lifting the patch of membrane stuck to the microelectrode, it is placed into a solution of controlled medium. The voltage applied to the membrane patch can be fixed (clamped) while other parameters are studied. B. By manipulating ion concentrations in the two chambers or simply reversing the direction of the current in the system, the ion flow through the channel can be reversed, resulting in the recording of negative values for current when the channel opens
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.
12-27 A. 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. B. False. Charged molecules (even protons, which are very small), are not able to pass through aquaporins. C. False. Selectivity depends on three parameters: the diameter, shape and charge of the ion trying to pass through the pore of the channel. D. True.
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. __________________ ion channels in the mimosa plant propagate the leafclosing response. D. __________________ ion channels respond to changes in membrane potential. E. Many receptors for neurotransmitters are __________________ ion channels.
12-29 A. The acetylcholine receptor in skeletal muscle cells is a(n) ligand-gated ion channel. B. Stress-gated ion channels are found in the hair cells of the mammalian cochlea. C. Voltage-gated ion channels in the mimosa plant propagate the leaf-closing response. D. Voltage-gated ion channels respond to changes in membrane potential. E. Many receptors for neurotransmitters are ligand-gated ion channels.
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
12-3 The two basic properties governing the likelihood of whether a molecule will diffuse through a lipid bilayer are the size of the molecule and the charge of the molecule. A smaller molecule will be more likely to diffuse through the lipid bilayer than a larger molecule. A nonpolar (hydrophobic) molecule will be more likely to diffuse through the lipid bilayer than a polar molecule, which is more likely to diffuse through the lipid bilayer than a charged molecule. A. benzene (small nonpolar versus larger uncharged) B. ethanol (polar versus charged) C. glycerol (small polar versus very large, highly charged) D. O2 (nonpolar versus polar) E. adenosine (polar versus highly charged)
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
12-32 (b). Knowing that: V = 62 × log(Co/Ci), substitute the outer and inner concentration values: V = 62 × log(15/106) V = 62 × -0.849 V = -52.7
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.
12-33 (b). The cell has K+ leak channels. At rest, the cell is mostly permeable to K+, because of the presence of these channels.
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.
12-34 (c). As Na+ ions move into the cell, the net charge becomes more positive (less negative) and the membrane potential changes to reflect the Co/Ci for both Na+ and K+ ions.
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.
12-36 A. True. B. False. Action potentials are usually mediated by voltage-gated Na+ channels. C. True. D. 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.
12-46 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?
12-46 A. Increasing the concentration of K+ in the cytoplasm of the squid axon will make the membrane potential more negative. Doubling the amount of K+ increases the driving force for K+ to move out of the cell, leaving the inside of the cell more negative and thus decreasing the membrane potential. (Remember, from the Nernst equation, the driving force for an ion across a membrane is proportional to the ratio of the concentration of the ion on the outside to the concentration of the ion on the inside.) B. Doubling the amount of Na+ in the extracellular fluid will increase the height of the peak of the action potential. Again, this is because now the driving force for Na+ to enter the cell is greater than it was before. Thus, when Na+ channels open, the flux of Na+ ions is now greater. (Remember that flux is the number of ions entering per second.) C. The action potential in this case will reach a height that is less than that normally achieved. (Choline is added in this case to maintain bulk electrical neutrality. Because Na+ channels are not permeable to choline, they do not contribute to the electrochemical gradient.) You have now halved the concentration of Na+ and thus decreased the driving force for Na+ to enter the cell
