Cell Bio Questions Chapter 12

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) if an ion channel is open, it will allow any ion that is under a certain size and that has the correct charge to pass through. H+ is the only ion listed that is both smaller and has the same charge of +1.

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 estrogen propanol sodium (a) alanine > propanol > sodium > estrogen (b) sodium > propanol > alanine > estrogen (c) estrogen > propanol > sodium > alanine (d) estrogen > propanol > alanine > sodium

(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.

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

(a) Aquaporin channels are found in the plasma membrane of some cells, where they facilitate 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.

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 will have a negligible degree of diffusion across the synthetic bilayer. Not only is it polar, it is also larger than other molecules that are able to diffuse across the membrane.

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) Individual channels are either completely open or completely closed. However, in a given population, there will be a mixture of open and closed channels.

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) 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.

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

(b) Ca2+ is exported using ATP-powered pumps. There are no other solutes that are being moved by these pumps.

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 rate.

(b) In addition to the Ca2+-pumps in the plasma membrane, Ca2+-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.

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

(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 mV

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) The cell has K+ leak channels. At rest, the cell is mostly permeable to K+ because of the presence of these channels.

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) 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. It is not involved in the catalytic activity of endoplasmic reticulum enzymes [choice (c)]. Because the levels of Ca2+ are very low relative to the levels of K+ and Na+, the Ca2+ gradient does not have a significant effect on the osmotic balance of the cell [choice (a)] or the membrane potential [choice (d)].

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) 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.

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 phosphorylation of the pump causes the conformational change, and it occurs after the binding of Na+.

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) Inhibitory neurons release inhibitory neurotransmitters such as GABA and glycine. They bind to and open ligand-gated Cl- channels. If Na+ channels are open, Cl- ions will rush into the cell as well, neutralizing the positive charges carried by Na+.

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) Sound waves cause vibrations of the tectorial membrane. These vibrations cause the bundles of stereocilia to tilt. This tilting physically pulls the filament that links a cilium to the ion channel in neighboring cilia, which then pulls the gate on that ion channel open.

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 keeps Na+ out directly by pumping it out and keeps Cl- out indirectly by helping to maintain the negative membrane potential. Cells do not have pumps for moving water molecules across the membrane [choice (a)], since the lipid bilayer is permeable to water. Bacteria do not have Na+-K+ pumps in their plasma membranes [choice (b)]. The Na+-K+ pump cannot directly remove water molecules from the cell; it helps maintain osmotic balance by pumping out the Na+ that leaks in, which would not help if the cell were in a solution of very low ionic strength [choice (c)].

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) 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.

Which of the following statements does not accurately describe the events involved in the propagation of an action potential? (a) An initial influx of Na+ through a small cluster of channels causes local depolarization of the membrane. (b) Local depolarization causes nearby Na+ channels to open. (c) Channels in depolarized regions of the membrane are inactivated until the resting membrane potential is reestablished. (d) The opening of transmitter-gated K+ channels helps to repolarize the membrane.

(d) The outflow of K+ through voltage-gated K+ channels helps bring the membrane back to its resting 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

(d) 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.

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-

A

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

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

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

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.

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-

B

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.

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 leaf-closing response. D. _________ ion channels respond to changes in membrane potential. E. Many receptors for neurotransmitters are _________ ion channels.

A. The acetylcholine receptor in skeletal muscle cells is a *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.

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. 2+ 4 A. The extracellular concentration of Ca is approximately 10 -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. Ca 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.

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.

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-

B

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

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.

B

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

C

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-

C

Some cells have aquaporins—channels that facilitate the flow of water molecules through the plasma membrane. For these cells, 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

C

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.

C

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

C

Which of the following statements about resting membrane potential is not 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.

C

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

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 is not true about the patch-clamp method? (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.

D

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

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 does most often neutralize (written in parentheses)? (a) Cl- (Ca2+) (b) PO43- (K+) (c) PO43- (Ca2+) (d) Cl- (Na+)

D

Which of the following statements about GABA receptors is not true? (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 .

D

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(n) __________________; if the ions move in opposite directions, the transporter is considered a(n) __________________.

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*.

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 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*.