Chapter 12 Membrane Transport
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
-52.7 mV 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 mV
Eukaryotic cell membranes are highly specialized and have characteristic sets of transporters and channels. Below, match the transporter with the appropriate membrane type. More than one transporter can be matched to a membrane type. Membrane 1.plasma - nucleotide,glucose,Na+/K+,Ca 2.lysosomal - Proton 3.endoplasmic reticulum - Ca 4.inner mitochondrial - ADP/ATP Transporter A.nucleotide B.Proton C.Glucose D.ADP/ATP E.Na+/K+ F.Ca2+
1.ANS: A, C, E, F DIF: Easy REF: 12.1 OBJ: 12.1.d State the function of membrane transport proteins and differentiate between transporters and channels. MSC: Remembering 2.ANS: B DIF: Easy REF: 12.1 OBJ: 12.1.d State the function of membrane transport proteins and differentiate between transporters and channels. MSC: Remembering 3.ANS: F DIF: Easy REF: 12.1 OBJ: 12.1.d State the function of membrane transport proteins and differentiate between transporters and channels. MSC: Remembering 4.ANS: D DIF: Easy REF: 12.1 OBJ: 12.1.d State the function of membrane transport proteins and differentiate between transporters and channels. MSC: Remembering
For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. active hydrophilic noncovalent amino acid hydrophobic passive amphipathic ion channels transporter proteins A molecule moves down its concentration gradient by __________ transport, but requires __________ transport to move up its concentration gradient. Transporter proteins and ion channels function in membrane transport by providing a __________ pathway through the membrane for specific polar solutes or inorganic ions. __________ are highly selective in the solutes they transport, binding the solute at a specific site and changing conformation so as to transport the solute across the membrane. On the other hand, __________ discriminate between solutes mainly on the basis of size and electrical charge.
A molecule moves down its concentration gradient by passive transport, but requires active transport to move up its concentration gradient. Transporter proteins and ion channels function in membrane transport by providing a hydrophilic pathway through the membrane for specific polar solutes or inorganic ions. Transporter proteins are highly selective in the solutes they transport, binding the solute at a specific site and changing conformation so as to transport the solute across the membrane. On the other hand, ion channels discriminate between solutes mainly on the basis of size and electrical charge.
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, and allow the rapid passage of water molecules and small ions in and out of cells. C.The ion selectivity of a channel depends solely on the charge of the amino acids lining the pore inside the channel. D.Most ion channels are gated, which allows them to open and close in response to a specific stimulus, rather than allowing the constant, unregulated flow of ions.
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.
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.
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.
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.
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 voltage-gated K+ channels 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.
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.
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.
Indicate whether the statements below are TRUE or FALSE. If a statement is false, explain why it is false. A.The extracellular concentration of Ca2+ is approximately 104-fold higher than the concentration of Ca2+ in the cytosol. B.The low cytosolic Ca2+ concentration sensitizes the cell to an influx of Ca2+, ensuring a rapid response to environmental stimuli. C.Cytosolic Ca2+ concentration is kept low by the use of chelators such as EDTA. D.The primary mechanism by which Ca2+ acts as a signaling molecule is by increasing the net charge in the cytosol.
A.True. B.True. C.False. Ca2+ concentrations in the cytosol are kept low by the action of ATP-driven calcium pumps in the endoplasmic reticulum membrane and the plasma membrane. D.False. Ca2+ binds tightly to many proteins in the cell, which in turn changes their activity. This interaction is the primary mechanism by which Ca2+ signaling occurs.
If ATP production is blocked in an animal cell, the cell will swell up. Explain this observation.
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.
Describe the two components of an electrochemical gradient. Use your description to suggest which of the following is influenced by a larger electrochemical gradient: (1) Na+ moving into the cell; (2) K+ moving out of the cell. Explain your reasoning.
An electrochemical gradient is composed of two forces: the energy from the concentration gradient, the force derived from the charge differential across the membrane, referred to as the membrane potential. There is a larger electrochemical gradient driving the movement of Na+ into the cell compared to the net force moving K+ out of the cell. In the case of Na+ the concentration gradient and the membrane potential both drive movement of Na+ into the cell. In the case of K+ these two forces work in opposition, reducing the net magnitude of the electrochemical gradient.
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 vesicle membranes keep cytosolic calcium levels low. b.Ca2+-pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low. c.Ca2+-pumps in the Golgi apparatus keep cytosolic calcium levels low. d.Ca2+ concentrations in the cytosol increase at a steady
Ca2+-pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low.
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.PO43− d.OH−
Cl−
Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following negatively charged ions is NOT primarily used to buffer positive charges inside the cell? a.PO43− b.OH− c.Cl− d.HCO3−
Cl−
Negatively charged ions are required to balance the net positive charge from metal ions such as K+, Na+, and Ca2+. Which of the following negatively charged ions is the most abundant outside the cell and which ion most often neutralize (written in parentheses)? a.Cl− (Ca2+) b.PO43− (K+) c.PO43− (Ca2+) d.Cl− (Na+)
Cl− (Na+)
For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. antiport coupled membrane potential ATP hydrolysis electrochemical symport concentration light-driven uniport For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its __________ gradient. On the other hand, for a charged molecule, the __________ must also be considered. The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the __________ gradient. Active transport allows the movement of solutes against this gradient. The transporter proteins called __________ transporters use the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second solute. When this transporter moves both ions in the same direction across the membrane, it is considered a/an __________; if the ions move in opposite directions, the transporter is considered a/an __________.
For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its concentration gradient. On the other hand, for a charged molecule, the membrane potential must also be considered. The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the electrochemical gradient. Active transport allows the movement of solutes against this gradient. The transporter proteins called coupled transporters use the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second solute. When this transporter moves both ions in the same direction across the membrane, it is considered a symport; if the ions move in opposite directions, the transporter is considered an antiport.
Describe the process by which gut epithelial cells use transporters to take up ingested glucose (against the concentration gradient) and to distribute glucose to other tissues by moving it back out of the cell (down the concentration gradient).
Gut epithelial cells use two different transporters to take glucose up from the gut and distribute it into the bloodstream and to other tissues. These transporters are located at opposite sides of the cell: the apical side of the cell (which faces the gut) contains a Na+-glucose symporter. This symporter couples the entry of Na+ down its electrochemical gradient to the active import of glucose against its concentration gradient. The Na+-glucose symporter is restricted to the apical side of the cell by tight-junction complexes in the plasma membrane, which link neighboring epithelial cells together. On the basolateral side of the cell, there is another transporter that facilitates movement of glucose down its concentration gradient, out of the cell. This transporter is a uniporter that only transports glucose in one direction: from the cytosol to the extracellular matrix. The location of this uniporter is also restricted by the presence of the tight junctions, so that the epithelial cell will not transport glucose back into the lumen of the gut.
Ion channels are classified as membrane transport proteins. They discriminate between ions based on 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−
H+
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.
It becomes more positive.
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 requires a stimulus to change from closed to open.
It opens more frequently in response to a given stimulus.
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.Ca2+ d.Cl−
K+
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
K+-coupled Because K+ is a positively charged ion and the outside of the plasma membrane is positively charged, K+ has a very small electrochemical gradient across the membrane even though its concentration gradient is large. Because there is little net movement across the membrane for K+, it would not make a good source of energy to drive the transport of other molecules against their respective gradients.
The stimulation of a motor neuron ultimately results in the release of a neurotransmitter at the synapse between the neuron and a muscle cell. How is the chemical signal converted into an electrical signal in the postsynaptic muscle cell?
Most neurotransmitter receptors function as ligand-gated ion channels. These ion channels are similar to voltage-gated channels, except that they do not open in response to a change in voltage across the membrane, but to the binding of a neurotransmitter. In the neuromuscular junction, the neurotransmitter acetylcholine binds to the acetylcholine receptor, which allows Na+ to enter the muscle cell, altering its membrane potential. In this way, a chemical signal (acetylcholine) is converted back into an electrical signal (change in membrane potential).
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.Ca2+ d.Cl−
Na+
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
Na+-H+ exchanger 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.
For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. acetylcholine receptor GABA receptorpostsynaptic Ca2+ channels K+ channels presynaptic chemical Na+ channels synapses electrical Neurons communicate with each other through specialized sites called __________. Many neurotransmitter receptors are ligand-gated ion channels that open transiently in the __________ cell membrane in response to neurotransmitters released by the __________ cell. Ligand-gated ion channels in nerve cell membranes convert __________ signals into __________ ones. Neurotransmitter release is stimulated by the opening of voltage-gated __________ in the nerve-terminal membrane.
Neurons communicate with each other through specialized sites called synapses. Many neurotransmitter receptors are ligand-gated ion channels that open transiently in the postsynaptic cell membrane in response to neurotransmitters released by the presynaptic cell. Ligand-gated ion channels in nerve cell membranes convert chemical signals into electrical ones. Neurotransmitter release is stimulated by the opening of voltage-gated Ca2+ channels in the nerve-terminal membrane.
The toxicity of mercury depends greatly upon the formulation of the metal. Which of them (elemental, metal ion, or methyl-mercury) is more likely to be absorbed through the skin? Explain your answer.
Organo-mercury compounds such as methyl-mercury are absorbed rapidly through the cellular membranes of skin cells and reach the bloodstream. Elemental mercury is a relatively large, uncharged atom, which is not absorbed to an appreciable degree. Mercury salts (the ionic form of the metal) can pass through membranes only of those cells that express transport channels with large enough pores.
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 μm 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.
Single-channel patch-clamp recordings have demonstrated that gated membrane channels will only open and close in response to specific stimuli.
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.
Some channels remain closed and some open completely.
Which of the following statements best reflects the nature of synaptic plasticity? a.New synapses are created due to the postnatal generation of neurons. b.Synaptic response changes in magnitude depending on frequency of stimulation. c.There is a change in the type of neurotransmitter used at the synapse. d.Neuronal connections are pruned during normal development.
Synaptic response changes in magnitude depending on frequency of stimulation.
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.
The Na+-K+ pump helps to keep both Na+ and Cl− ions out of the cell.
For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. anions hyperpolarization neutral axon less positive cytoskeleton ligand pressure dendrites more synaptic vesicle depolarization negative voltage The action potential is a wave of __________ that spreads rapidly along the neuronal plasma membrane. This wave is triggered by a local change in the membrane potential to a value that is __________ negative than the resting membrane potential. The action potential is propagated by the opening of __________-gated channels. During an action potential, the membrane potential changes from __________ to __________. The action potential travels along the neuron's __________ to the nerve terminals. Neurons chiefly receive signals at their highly branched __________.
The action potential is a wave of depolarization that spreads rapidly along the neuronal plasma membrane. This wave is triggered by a local change in the membrane potential to a value that is less negative than the resting membrane potential. The action potential is propagated by the opening of voltage-gated channels. During an action potential, the membrane potential changes from negative to positive. The action potential travels along the neuron's axon to the nerve terminals. Neurons chiefly receive signals at their highly branched dendrites.
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.
The electrochemical potential across the membrane is zero.
Describe the two forces that drive an ion across the plasma membrane and explain how the Nernst equation takes into account both of these forces. Use the components of the equation to support your explanation and be sure to specify the assumptions being made when using the Nernst equation to calculate membrane potential.
The forces that drive the movement of an ion across the plasma membrane include a concentration gradient (that is, there is a negative change in free energy associated with an increase in entropy for ions in solution) and an electrical component (the force resulting from the attraction between molecules of opposite charges). The Nernst equation expresses the change in voltage across the membrane as it relates to a change in the ratio of ions on either side of the plasma membrane. As written below, the voltage changes by 62 millivolts with every tenfold change in the ion concentration ratio across the membrane.
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.
The plasma membrane is primarily permeable to Na+.
Because the Na+/K+ pump moves these ions at the same rate in opposing directions across the plasma membrane, it creates a net balance of charges on each side of the membrane (i.e., membrane potential = 0). Given this fact, explain how cells generate a membrane potential across the plasma membrane and suggest the consequences for cell functions if this capacity were handicapped.
The presence of K+ channels in the plasma membrane allows the free diffusion of K+ out of the cell. These K+ leak channels create a voltage difference across the membrane (membrane potential ≠ 0). Without the "leak" capability in the membrane, the transport of glucose and other small molecules that depend on the electrochemical gradient would be reduced.
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.
The pump is phosphorylated.
Which of the following statements about resting membrane potential is FALSE? 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.
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.
Identify 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
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)
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.
They keep the electrochemical gradient for K+ at zero.
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+.
They promote neuronal uptake of Na+.
What chemical principles explain the observation that a protein-free lipid bilayer is a billion times more permeable to water than to a sodium ion?
Water is a small, uncharged molecule that diffuses directly across the membrane, despite needing to pass through a hydrophobic lipid core. In cells, ions pass through lipid bilayers via ion channels or transporters. Because ions carry a charge, they have strong electrostatic interactions with water molecules. There are no compensating interactions in the hydrophobic lipid core of the bilayer, which prevents ions from entering.
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
acetylcholine
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.
active transport.
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
an aquaporin
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). alanineestrogenpropanolsodium a.alanine > propanol > sodium > estrogen b.sodium > propanol > alanine > estrogen c.estrogen > propanol > sodium > alanine d.estrogen > propanol > alanine > sodium
estrogen > propanol > alanine > sodium
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 Ca2+ 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
export of Ca2+ from the cytosol Ca2+ is exported using ATP-powered pumps. There are no other solutes that are being moved by these pumps.
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
glucose
Which of the following gated ion channels are involved in inhibitory synaptic signaling? a.voltage-gated Na+ channels b.voltage-gated Ca2+ channels c.glycine-gated Cl− channels d.glutamate-gated cation channels
glycine-gated Cl− channels
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 is the most likely cause of 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
inability to import sugar into the cell
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.
preventing Ca2+ from altering the activity of molecules in the cytosol. The major purpose of the Ca2+-pumps is to keep the cytosolic concentration of Ca2+ low. When Ca2+ does move into the cytosol, it alters the activity of many proteins; hence, Ca2+ is a powerful signaling molecule.
Some cells express aquaporin proteins—they are channel proteins that facilitate the flow of water molecules through the plasma membrane. What regulates the rate and direction of water diffusion across the membrane? a.aquaporin conformation b.resting membrane potential c.solute concentrations on either side of the membrane d.availability of ATP
solute concentrations on either side of the membrane
Transporters, in contrast to channels, work by a.specific recognition of transport substrates. b.a gating mechanism. c.filtering solutes by charge. d.filtering solutes by size.
specific recognition of transport substrates.
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
stress-gated
Approximately, how many distinct synapses are established on the dendrites and cell body of a motor neuron in the spinal cord? a.tens b.hundreds c.thousands d.millions
thousands
Figure 12-31 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
threshold potential
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
voltage-gated Ca2+ channels Voltage-gated Ca2+ 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.
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
by opening Cl− channels
