Deck 12: Transport Across Cell Membranes

Question 1

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⁻

Answer 1

B

Question 2

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⁻

Answer 2

B

Question 3

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

Answer 3

A
A constitutively active phospholipase C will lead to the constitutive production of IP₃ and diacylglycerol, leading to activation of PKC in a signal-independent manner; thus, Rafty activation and the lipid modification will be signal-independent.A mutation in the GPCR that bind the signal more tightly and a Ca²⁺ channel with an increased affinity for IP will increase activity of the signal transduction pathway in a signal-dependent manner.A mutation that renders Rafty such that it can no longer by phosphorylated by PKC will prevent PKC from activating Rafty and will thus prevent the lipid modifications.

Question 4

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.

Question 5

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

Question 6

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.

Question 7

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.

Question 8

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⁺)

Question 9

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⁻

Question 10

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.

Question 11

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.

Question 12

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⁻

Question 13

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

Question 14

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⁻

Question 15

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

Question 16

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

Question 17

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 + 2P_i + 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

Question 18

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.

Question 19

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

Question 20

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.

Question 21

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.

Question 22

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

Question 23

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.

Question 24

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.

Question 25

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.

Question 26

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.

Question 27

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⁺.

Question 28

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

Question 29

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

Question 30

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

Question 31

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.

Question 32

Figure 12-32 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 12-32

A)effect of a depolarizing stimulus
B)resting membrane potential
C)threshold potential
D)action potential

Question 33

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

Question 34

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

Question 35

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

Question 36

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.

Question 37

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

Question 38

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.

Question 39

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

Question 40

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? Figure 12-31

A)effect of a depolarizing stimulus
B)resting membrane potential
C)threshold potential
D)action potential

Question 41

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?

Question 43

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.

Question 45

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 __________.

Question 46

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

Question 47

The flow of ions through a gated channel can be studied using a method called "patch-clamp 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 Q12-16A and Q12-16B from a single ion channel?
Figure 12-16

Question 48

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.

Question 49

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

Question 50

Order and describe all the molecular events required for the propagation of an action potential.Is propagation along the axon bidirectional? Why or why not?

Question 52

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.

Question 53

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?

Question 54

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 .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.

Question 55

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).

Question 56

If ATP production is blocked in an animal cell, the cell will swell up.Explain this observation.

Question 58

The movement of glucose into the cell, against its concentration gradient, can be powered by the co-transport of Na⁺ into the cell.Explain the thermodynamic factors governing this transporter.Under what conditions would transport of glucose slow or come to a halt?

Question 59

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.

Question 60

Define homeostasis and illustrate your understanding of this concept by describing the mechanism of one transmembrane transport protein and how it contributes to homeostasis.

Question 61

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 12-27 for how the Na⁺-K⁺ pump normally functions in the plasma membrane).
Figure 12-27
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.

Question 62

Studies on the squid giant axon were instrumental to 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 to which the membrane is impermeable.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⁺ ions.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?

Question 63

The Aeroschmidt weed contains an ATP-driven ion pump in its vacuolar membrane that pumps potentially toxic heavy metal ions such as Zn²⁺ and Pb²⁺ into the vacuole.The pump protein cycles through a phosphorylated and an unphosphorylated state and works in a similar way to the Na⁺-K⁺ pump of animal cells.To study its action, you incorporate the unphosphorylated form of the protein into phospholipid vesicles containing K⁺ in their interiors.(You ensure that all of the protein molecules are oriented such that their cytosolic domains face the outside of your vesicles.) When you add Zn²⁺ and ATP to the solution outside such vesicles, you find that Zn²⁺ is pumped into the vesicle lumen.You then expose vesicles containing the pump protein to the solutes shown in Table 12-28.
Table 12-28
You then determine the amount of phosphorylated and unphosphorylated ATP-driven ion pump protein in each sample.Your results are summarized in Table 12-28A, where a minus sign indicates an absence of a type of protein and a plus sign indicates its presence.
Table 12-28A
You treat vesicles as in lane F, but before determining the phosphorylation state of the protein, you wash away the outside buffer and replace it with a buffer containing only Zn²⁺.What do you expect to happen in this sample, and what are the results you expect to observe with respect to phosphorylation and movement of K⁺?

Question 64

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 receptor postsynaptic
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.

Question 65

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 __________.

Question 66

The field of neurobiology is seeing rapid advances in our understanding of neural circuitry in the brain.Part of this work involves the physical mapping of all synapses (a project dubbed the connectome); another critical advance is in the area of optogenetics, which allows scientists to dissect neural circuits that determine specific behaviors in a range of organisms from fruit flies to monkeys.
A.Describe the method of optogenetics.
B.Optogenetics has great potential for deepening our understanding of behavior, learning, memory, and cognitive development.However, it is not as likely to be used directly for treatment of problems such as depression or anxiety.Why not? Consider the limitations of the method when answering this question.

Question 67

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?