Chapter 12 Answers-Transport

Chapter 12 Membrane Transport 1: Which of the following channels would not be expected to generate a change in voltage by movement of its substrate across the membrane where it is found? a. an aquaporin b. a sodium channel c. a calcium channel d. a proton channel ANS: A Aquaporin channels are found in the plasma membrane of some cells, where they facilitate the diffusion of water across the membrane. Because water is an uncharged molecule, its movement would not be expected to alter the voltage across the membrane. DIF: Easy REF: 12.1 OBJ: 12.1.k Describe the ways in which water can move across cell membranes and articulate what governs whether water will enter or exit a cell. MSC: Understanding 2: Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest). alanine estrogenpropanolsodium a. alanine > propanol > sodium > estrogen b. sodium > propanol > alanine > estrogen c. estrogen > propanol > sodium > alanine d. estrogen > propanol > alanine > sodium ANS: D Estrogen is a steroid hormone and will diffuse the fastest across the membrane. Propanol is a small, uncharged molecule with a polar group. Alanine is an amino acid, and although it has a small, nonpolar side group, amino acids are charged molecules. Sodium is an ion and will move the slowest across the bilayer. DIF: Easy REF: 12.1 OBJ: 12.1.c Review the properties that govern the rate at which a given solute can cross a protein-free lipid bilayer. MSC: Understanding

3: Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant inside a typical mammalian cell? a. Na + b. K + c. Ca 2+ d. Cl − ANS: B DIF: Easy REF: 12.1 OBJ: 12.1.e Indicate how the concentration of ions in the cell differs from that outside the cell. MSC: Remembering 4: Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant outside a typical mammalian cell? a. Na + b. K + c. Ca 2+ d. Cl − ANS: A DIF: Easy REF: 12.1 OBJ: 12.1.e Indicate how the concentration of ions in the cell differs from that outside the cell. MSC: Remembering 5: Which of the following statements about resting membrane potential is true? a. The resting membrane potential for most animal cells is 0 mV, because the positive and negative ions are in balance. b. The resting membrane potential for most animal cells is positive, because Na + ions are so plentiful inside cells. c. The resting membrane potential for most animal cells is negative, because the inside of the cell is more negatively charged than the outside of the cell. d. At the resting membrane potential, no ions enter or exit the cell. ANS: C DIF: Easy REF: 12.1 OBJ: 12.1.f Define “resting membrane potential.” MSC: Understanding

c. facilitated diffusion. d. passive transport. ANS: A DIF: Easy REF: 12.1 OBJ: 12.1.h Distinguish between active and passive transport and indicate which type of membrane transport protein carries out each. MSC: Remembering 9: 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 vesicle membranes keep cytosolic calcium levels low. b. Ca 2+ -pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low. c. Ca 2+ -pumps in the Golgi apparatus keep cytosolic calcium levels low. d. Ca 2+ concentrations in the cytosol increase at a steady rate. ANS: B In addition to the Ca 2+ -pumps in the plasma membrane, Ca 2+ -pumps are also found in the membrane of the endoplasmic reticulum (ER). Those in the ER membrane will continue to remove calcium ions from the cytosol, keeping calcium levels low. DIF: Moderate REF: 12.2 OBJ: 12.2.e Explain why—and how—cells keep the cytosolic concentration of calcium ions low. MSC: Applying 10: 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 ANS: D The high extracellular concentration of Na + is employed by a transporter that pumps protons out of animal cells as Na + is brought into the cell. The other transporters are found only in bacterial cells.

DIF: Easy REF: 12.2 OBJ: 12.2.h Express how the sodium–proton exchanger in the plasma membrane allows animal cells to control the pH of their cytosol. MSC: Remembering 11: 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. ANS: B The major purpose of the Ca 2+ -pumps is to keep the cytosolic concentration of Ca 2+ low. When Ca 2+ does move into the cytosol, it alters the activity of many proteins; hence, Ca 2+ is a powerful signaling molecule. DIF: Easy REF: 12;2 OBJ: 12.2.e Explain why—and how—cells keep the cytosolic concentration of calcium ions low. MSC: Remembering 12: Which of the following occur WITHOUT coupling transport of the solute to the movement of a second solute? a. import of glucose into gut epithelial cells b. export of Ca 2+ from the cytosol c. export of H + from animal cells for pH regulation d. the export of Na + from cells to maintain resting membrane potential ANS: B Ca 2+ is exported using ATP-powered pumps. There are no other solutes that are being moved by these pumps. DIF: Easy REF: 12.2 OBJ: 12.2.a Outline how the glucose transporter in the plasma membrane of mammalian cells imports glucose after a meal and exports glucose to provide fuel for other tissues in the body. | 12.2.e Explain why—and how—cells keep the cytosolic concentration of calcium ions low. | 12.2.h Express how the sodium–proton exchanger in the plasma membrane allows animal cells to control the pH of their cytosol. MSC: Understanding

population, there will be a mixture of open and closed channels. DIF: Moderate REF: 12.3 OBJ: 12.3.k Explain how voltage sensors allow voltage-gated ion channels to respond to changes in membrane potential. MSC: Understanding 16: When the net charge on either side of the plasma membrane is zero, what else is true? a. There is an equal number of K + ions on each side of the plasma membrane. b. The K + leak channels are open. c. The electrochemical potential across the membrane is zero. d. The resting membrane potential is between − 20 mV and − 200 mV. ANS: C DIF: Easy REF: 12.3 OBJ: 12.3.d Estimate the resting membrane potential in animal cells. MSC: Remembering 17: 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 C o = 15 mM and C i = 106 mM? a. 438.1 mV b. − 52.7 mV c. 52.7 mV d. − 5.3 mV ANS: B Knowing that V = 62 × log( C o / C i ), substitute the outer and inner concentration values: V = 62 × log(15/106) V = 62 × − 0.849 V = − 52.7 mV DIF: Easy REF: 12.3 OBJ: 12.3.e Review how the Nernst equation can be used to calculate the resting membrane potential across a membrane. MSC: Applying 18: 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. ANS: C As Na + ions move into the cell, the net charge becomes more positive (less negative) and the membrane potential changes to reflect the C o / C i for both Na + and K + ions. DIF: Easy REF: 12.3 OBJ: 12.3.e Review how the Nernst equation can be used to calculate the resting membrane potential across a membrane. MSC: Applying 19: In a method called patch-clamping, a glass capillary can be converted into a microelectrode that measures the electrical currents across biological membranes. Which of the following statements about the patch-clamp method is FALSE? a. The glass capillary adheres to a “patch” of membrane through the application of suction. b. The aperture in the glass capillary used to make a microelectrode is about 1 μ m in diameter. c. If the experimental conditions are held constant, fluctuations in electrical currents across the patch of membrane are still observed. d. Single-channel patch-clamp recordings have demonstrated that gated membrane channels will only open and close in response to specific stimuli. ANS: D DIF: Easy REF: 12.3 OBJ: 12.3.f Outline how patch-clamp recording can be used to study the activity of ion channels. MSC: Remembering 20: 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 ANS: D Voltage-gated Ca 2+ channels open in response to the depolarization caused as the action potential moves toward the nerve terminal. The influx of calcium from outside the cell causes the synaptic vesicles to fuse with the plasma membrane and release large amounts of neurotransmitter into the synaptic cleft.

Figure 12-32 a. effect of a depolarizing stimulus b. resting membrane potential c. threshold potential d. action potential ANS: B DIF: Easy REF: 12.4 OBJ: 12.4.c Outline how membrane depolarization triggers an action potential and how the action potential spreads along the membrane. MSC: Remembering 23: Figure 12-33 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 12-33 a. effect of a depolarizing stimulus b. resting membrane potential c. threshold potential d. action potential ANS: D DIF: Easy REF: 12.4 OBJ: 12.4.c Outline how membrane depolarization triggers an action potential and how the action potential spreads along the membrane. MSC: Remembering 24: Figure 12-34 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 12-34 a. effect of a depolarizing stimulus b. resting membrane potential c. threshold potential d. action potential ANS: A DIF: Easy REF: 12.4 OBJ: 12.4.c Outline how membrane depolarization triggers an action potential and how the action potential spreads along the membrane. MSC: Remembering

28: Which of the following statements about GABA receptors is FALSE? a. They are located on postsynaptic membranes. b. They are ligand-gated channels. c. They inhibit synaptic signaling. d. They promote neuronal uptake of Na + . ANS: D DIF: Easy REF: 12.4 OBJ: 12.4.i Summarize how transmitter-gated ion channels convert the chemical signal carried by a neurotransmitter back into an electrical signal. | 12.4.j Contrast how the receptors for excitatory and inhibitory neurotransmitters alter the activity of a postsynaptic cell. MSC: Analyzing 29: Which of the following gated ion channels are involved in inhibitory synaptic signaling? a. voltage-gated Na + channels b. voltage-gated Ca 2+ channels c. glycine-gated Cl − channels d. glutamate-gated cation channels ANS: C DIF: Easy REF: 12.4 OBJ: 12.4.j Contrast how the receptors for excitatory and inhibitory neurotransmitters alter the activity of a postsynaptic cell. MSC: Remembering