Deck 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes

Question 1

Which of the following transporters is NOT electrogenic?

A) Na⁺-glucose symporter
B) Ca²⁺-pump
C) Na⁺-K⁺ pump
D) Na⁺-independent Cl--HCO₃- exchanger
E) Bacteriorhodopsin

Answer 1

D
Explanation: The Na⁺-independent Cl--HCO₃- exchanger transports one negative charge in and one out in each round of transport, and is therefore not electrogenic.

Question 2

Sanshool is a natural compound found in Sichuan pepper, which is commonly used in making spicy Asian food, and creates a numbing sensation in the mouth. It is known to inhibit a subset of potassium leak channels involved in maintaining the resting membrane potential in sensory neurons. How do you think sanshool affects these sensory neurons?

A) It elevates the resting potential (to less negative values) and makes it easier to excite the neuron.
B) It elevates the resting potential and makes it harder to excite the neuron.
C) It lowers the resting potential and makes it easier to excite the neuron.
D) It lowers the resting potential and makes it harder to excite the neuron.
E) It causes hyperpolarization and decreases the spontaneous firing of the neuron.

Answer 2

A
Explanation: Inhibition of the potassium leak channels by sanshool leads to depolarization of the membrane in these sensory neurons and sensitizes them.

Question 3

The permeability of a protein-free lipid bilayer to various molecules depends on their properties. Sort the following in order of low to high permeability from left to right. Your answer would be a six-digit number composed of digits 1 to 6, e.g. 123456.
(A) O²
(B) ATP
(C) RNA oligonucleotide
(D) Na⁺
(E) Glucose
(F) Urea

Answer 3

CBDEFA

Question 4

Imagine a small synthetic vesicle made from pure phospholipids enclosing an interior lumen containing 1 mM glucose and 1 mM sodium chloride. If the vesicle is placed in pure water, which of the following happens faster?

A) Na⁺ diffuses out.
B) Cl- diffuses out.
C) H₂O diffuses in.
D) Glucose diffuses out.
E) Sodium chloride diffuses out.

Question 5

The membrane potential in a particular hyperpolarized cell is measured to be -70 mV. For each of the following channels in this cell, the calculated Nernst equilibrium potential for the corresponding ion is presented. Indicate whether each channel is driven to pass the ion into (I) or out of (O) the cell. Your answer would be a four-letter string composed of letters I and O only, e.g. IIII.
( ) K? channel; the K? equilibrium potential is -60 mV.
( ) Na? channel; the Na? equilibrium potential is +70 mV.
( ) Ca²? channel; the Ca²? equilibrium potential is +130 mV.
( ) Cl- channel; the Cl- equilibrium potential is -90 mV.

Question 6

Indicate whether each of the following descriptions matches an ABC transporter (A), a P-type pump (P), or a V-type pump (V). Your answer would be a five-letter string composed of letters A, P, and V only, e.g. PPVVV.
( ) The pumps in this family are phosphorylated at a key Asp residue in each transport cycle.
( ) This family is the largest among membrane transport proteins and includes some channels as well as pumps.
( ) The pumps in this family are responsible for the acidification of synaptic vesicles.
( ) The sodium-potassium pump is a member of this family.
( ) The multidrug resistance protein is a member of this family.

Question 7

For which of the following ions is the intracellular concentration typically higher than the extracellular concentration?

A) Sodium
B) Calcium
C) Magnesium
D) Potassium
E) Chloride

Question 8

Which of the following normally functions to lower the pH of the cytosol?

A) A Na⁺-H⁺ exchanger in the plasma membrane.
B) A Na⁺-driven Cl--HCO₃- exchanger in the plasma membrane.
C) A Na⁺-independent Cl--HCO₃-exchanger in the plasma membrane.
D) A V-type ATPase in the lysosomal membrane.
E) Both answers A and B above.

Question 9

The Nernst equilibrium potential for an ion that is 10 times more concentrated in the cytosol compared to the extracellular fluid is about -60 mV. How much would the potential be if the extracellular concentration decreases 100-fold with no change in the intracellular concentration?

A) -6000 mV
B) -6 mV
C) -600 mV
D) -20 mV
E) -180 mV

Question 10

Which of the following is correct regarding the electrical properties of cellular membranes?

A) Setting up (or changing) the membrane potential requires changing the bulk concentrations of the ions on the two sides of the membrane.
B) The resting potential in most animal cells is between 20 mV and 120 mV (positive inside).
C) If the membrane is impermeable to an ion, the membrane potential approaches the equilibrium potential for that ion.
D) The resting potential decays immediately following the inhibition of the Na⁺-K⁺ pump by a drug.
E) None of the above.

Question 11

This family of ATPases is structurally related to the turbine-like pumps that acidify lysosomes and vesicles; however, they usually function in reverse, generating ATP from ADP and Pᵢ using proton gradients across membranes. What are they called?

A) P-type pumps
B) ABC transporters
C) V-type pumps
D) F-type pumps
E) Permeases

Question 12

In contrast to transporters, the channel proteins in cellular membranes …

A) interact strongly with the solute(s) that they transport.
B) undergo a conformational change every time they transport a solute.
C) can only mediate passive transport.
D) form pores that are always open.

Question 13

Why do cells not have membrane transport proteins for O₂?

A) Because they need to keep the oxygen concentration low inside the reducing environment of the cell.
B) Because oxygen can dissolve in water and leak in via water channels.
C) Because oxygen can dissolve in the lipid bilayer and diffuse in and out rapidly without the need for a transporter.
D) Because oxygen is transported in and out of the cell in special oxygen-carrying proteins such as hemoglobin.
E) Because oxygen transport across a membrane is energetically unfavorable.

Question 14

Most eukaryotic ABC transporters are involved in exporting small molecules from the cytosol. In this subset of the ABC transporters, does the small molecule bind better to the ATP-bound (B) or the ATP-free (F) transporter? Write down your answer as B or
F. Hint: The ATP-bound transporter "faces outward."

Question 15

The lactose permease in Escherichia coli is an H⁺-lactose symporter that mediates the inward active transport of lactose if this sugar is present in the environment instead of glucose. Which of the following is true about this transporter?

A) It has a twofold pseudosymmetrical structure.
B) Lactose and H⁺ bind to two different conformations of the transporter.
C) The transporter goes through an intermediate state in which the bound lactose is open to both sides of the membrane.
D) If lactose and H⁺ concentrations are changed sufficiently, the transporter can act as an H⁺-lactose antiporter.
E) All of the above.

Question 16

Which of the following is a pump that hydrolyzes two ATP molecules per transport cycle?

A) The cystic fibrosis transmembrane conductance regulator protein
B) The multidrug resistance protein
C) The Na⁺-K⁺ pump
D) The Ca²⁺-pump
E) The V-type ATPase

Question 17

An ion channel …

A) always mediates passive transport.
B) is ion-selective.
C) is typically several orders of magnitude faster than a transporter.
D) is usually gated.
E) All of the above.

Question 18

After release into the synaptic cleft, the neurotransmitter dopamine is actively taken up by the cells via specific dopamine transporters. The drug cocaine interferes with this process and is therefore called a reuptake inhibitor. The inhibition of the transporter at a certain cocaine concentration is qualitatively represented in the following graph. Based on this graph, which of the following describes the effect of cocaine on the kinetics of dopamine reuptake by the transporter?

A) In the presence of cocaine, the maximal rate of transport (Vmₐₓ) is unaffected, but the apparent affinity of the transporter for dopamine is reduced.
B) In the presence of cocaine, the maximal rate of transport (Vmₐₓ) is reduced, but the apparent affinity of the transporter for dopamine is unaffected.
C) In the presence of cocaine, the maximal rate of transport (Vmₐₓ) is enhanced, but the apparent affinity of the transporter for dopamine is unaffected.
D) In the presence of cocaine, both the maximal rate of transport (Vmₐₓ) and the apparent affinity of the transporter for dopamine are enhanced.
E) In the presence of cocaine, both the maximal rate of transport (Vmₐₓ) and the apparent affinity of the transporter for dopamine are unaffected, but the transporter is nevertheless inhibited.

Question 19

Many amino acids in our diet are absorbed via the transcellular transport pathway by the intestinal epithelial cells. This process requires ATP hydrolysis by …

A) the Na⁺-amino acid symporters in the apical domain of the plasma membrane.
B) the Na⁺-amino acid antiporters in the apical domain of the plasma membrane.
C) the Na⁺-K⁺ pumps in the basal and lateral domains of the plasma membrane.
D) the amino acid carriers in the basal domain of the plasma membrane.
E) F-type ATPases in the apical domain of the plasma membrane.

Question 20

Which of the following transporters mediates primary active transport when transporting the solutes in the directions indicated? The concentration of both solutes (shown as small circles and squares) is higher outside of the cell.

Question 21

A neuron's repetitive firing rate is limited by an absolute refractory period, during which a new action potential cannot be generated. Which event is chiefly responsible for this limit?

A) Opening of K⁺ leak channels.
B) Opening of voltage-gated Na⁺ channels.
C) Opening of transmitter-gated cation channels.
D) Inactivation of voltage-gated K⁺ channels.
E) Inactivation of voltage-gated Na⁺ channels.

Question 22

Aquaporin has a pair of key asparagine residues located on the wall almost halfway through its pore. These residues simultaneously bind to the oxygen atom of a passing water molecule, rendering it unavailable for hydrogen-bonding. As a result, …

A) the channel can conduct negatively charged (but not positively charged) ions.
B) the channel can conduct glycerol as well as water.
C) the channel cannot conduct glycerol.
D) the channel cannot conduct protons.
E) the channel cannot conduct chloride ions.

Question 23

Which of the following graphs better represents the change in the relative conductance of Na⁺ and K⁺ channels during an action potential in a neuronal membrane?

Question 24

Which of the following is the reason why an aquaporin does not pass ions (such as Na⁺) through?

A) The Na⁺ ions are too small to pass through the pore rapidly enough.
B) The Na⁺ ions are too large to pass through the narrow pore.
C) The Na⁺ ions do not favorably interact with the wall of the pore and therefore cannot be dehydrated.
D) Two Asn residues in the channel prevent the passage of Na⁺ ions.
E) A Na⁺ ion must become fully hydrated to pass through.

Question 25

Astroglial cells in the brain can actively import the amino acid glutamate from the synaptic cleft, convert it to the amino acid glutamine, and release the glutamine into the cleft to be subsequently taken up by the presynaptic neurons. The neurons then convert it back to glutamate. Why is this function of glial cells important?

A) Because it competes with and replaces the direct uptake of glutamate by the neurons and thus regulates neuronal function.
B) Because neurons cannot produce the amino acids glutamine and glutamate and rely on the glial cells for their supply.
C) Because glutamate cannot be taken up directly by the neurons.
D) Because glutamate is an excitatory neurotransmitter and should be cleared from the synaptic cleft rapidly after its release.
E) Because glutamate import through glutamate-Na⁺ symporters in the glial cells is required for the maintenance of Na⁺ concentration in the synaptic cleft.

Question 26

A potassium channel conducts K⁺ ions several orders of magnitude better than Na⁺ ions, because …

A) the Na⁺ ion is too large to pass through the channel pore.
B) the hydrated Na⁺ ion occupies a larger volume compared to the hydrated K⁺ ion, and is too large to pass through the channel pore.
C) the Na⁺ ion is too small to interact with the channel in a way that facilitates the loss of water from the ion.
D) the Na⁺ ion cannot bind to the high-affinity K⁺-binding sites in the channel pore.

Question 27

In a voltage-clamp experiment on a squid giant axon, the membrane potential is clamped at +20 mV, as shown below. Within the same time frame, which of the curves A to D better show qualitatively how the total membrane current changes? A positive value of the current represents an outward flow of cations.

Question 28

Which of the following situations in a neuron results in action potentials that have extended depolarization phases?

A) Inhibiting the voltage-gated Na⁺ channels.
B) Stimulating the voltage-gated K⁺ channels.
C) Compromising the inactivation mechanism in voltage-gated Na⁺ channels.
D) Compromising the inactivation mechanism in voltage-gated K⁺ channels.
E) None of the above.

Question 29

Indicate true (T) and false (F) statements below regarding ion channels in cellular membranes. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTT.
( ) With patch-clamp recordings, it is possible to study ion transport through a single molecule of a channel protein.
( ) Cells that are not electrically excitable often lack gated ion channels in their plasma membrane.
( ) The aggregate current crossing a patch of plasma membrane represents the degree to which each individual channel is open at that time.
( ) Patch-clamp recordings have shown that any voltage-gated ion channel opens as soon as the membrane potential is altered.

Question 30

The nicotinic acetylcholine receptor is an ionotropic receptor that can be activated by nicotine, a stimulant drug found in cigarettes. Additionally, ?-bungarotoxin, a neurotoxin found in the venom of some snakes, binds preferentially and tightly to the closed conformation of the receptors. Another drug, donepezil, is a potent acetylcholinesterase inhibitor used to treat patients with Alzheimer's disease. If applied at a neuromuscular junction, indicate whether each of the compounds (1) acetylcholine, (2) nicotine, (3) ?-bungarotoxin, and (4) donepezil would enhance (E) or suppress (S) the Ca²⁺influx in the muscle cells. Your answer would be a four-letter string composed of letters E and S only, e.g. EESS.

Question 31

MscS and MscL are mechanosensitive channels that ...

A) constitute the majority of channel proteins in the plasma membrane of auditory hair cells in the human cochlea.
B) open in response to membrane tension.
C) open at the same osmotic pressure, but differ in their ion selectivity.
D) are highly ion selective.
E) All of the above.

Question 32

Voltage-gated cation channels in the plasma membrane of a neuron open when the membrane potential passes a threshold value that is about 20 mV above the resting potential. This corresponds to a change of about 4 × 10⁶ newtons per coulomb in the magnitude of the electric field across the membrane compared to the resting state. The tetrameric channels have a voltage-sensing helix S4 in each of their four subunits. Most S4 helices contain seven residues with positively charged side chains. The charge carried by each such residue is about 1.6 × 10-¹⁹ coulombs. By reaching threshold depolarization, what are the magnitude and the direction of the collective force that the four S4 helices in the channel experience as a result of the change in the electric field? The force that particles of charge q in an electric field E experience is calculated as F = q.E.

A) 18 pN, toward the cell interior
B) 18 pN, toward the cell exterior
C) 90 pN, toward the cell interior
D) 90 pN, toward the cell exterior
E) 45 pN, toward the cell interior

Question 33

A certain neuron in a mouse brain is firing about 300 times per second, whereas almost all of its neighboring neurons are firing at much lower rates of only a few times per second. This implies that ...

A) this neuron expresses more depolarizing (Na⁺ and Ca²⁺) than hyperpolarizing (K⁺) channels compared to most of its neighbors.
B) the mouse is experiencing an epileptic seizure.
C) the refractory period of its voltage-gated Na⁺ channels is at least 300 ms.
D) it releases excitatory neurotransmitters at its axon terminals.
E) it is probably of a different type compared to most of its neighbors.

Question 34

A membrane potential is plotted over time in the following graph. The Nernst equilibrium potentials for four ions (A to D) that affect the membrane potential in this system are indicated in the graph. What happened in the time period indicated by a question mark?

A) Channels conducting ion A opened.
B) Channels conducting ion B opened.
C) Channels conducting ion C opened.
D) Channels conducting ion D opened.
E) Channels conducting ion A and C opened.

Question 35

Myelination of axons in the peripheral nervous system …

A) is carried out by Schwann cells.
B) insulates the axons to reduce current leakage.
C) increases the speed of action potential propagation through saltatory conduction.
D) conserves energy because the active excitations are restricted to the nodes of Ranvier.
E) All of the above.

Question 36

From left to right, indicate whether the transmitter-gated channels for (1) calcium, (2) chloride, (3) potassium, and (4) sodium at a chemical synapse usually open in response to excitatory (E) or inhibitory (I) neurotransmitters. Your answer would be a four-letter string composed of letters E and I only, e.g. EEIE.

Question 37

Once expressed in a neuron in the mouse brain and subjected to a flash of light, how does channelrhodopsin affect the membrane potential?

A) It causes neurotransmitter release, leading to an action potential in the postsynaptic neuron.
B) It increases the membrane permeability to cations and makes the membrane potential more negative.
C) It depolarizes the membrane, leading to the opening of voltage-gated Na⁺ channels.
D) It hyperpolarizes the membrane, leading to the inactivation of voltage-gated K⁺ channels.
E) It changes the membrane potential such that voltage-gated K⁺ channels but not voltage-gated Na⁺ channels are activated.

Question 38

In neurons, the synaptotagmin-1 protein is localized to presynaptic axon terminals. When activated, it facilitates the fusion of neurotransmitter-containing synaptic vesicles to the plasma membrane, leading to neurotransmitter release into the synaptic cleft. Which of the following would you expect to directly activate synaptotagmin-1 for this function?

A) Binding to Mg²⁺
B) Binding to Ca²⁺
C) Binding to phosphatidylcholine
D) Binding to K⁺
E) Binding to the K⁺ leak channels

Question 39

If the extracellular concentration of sodium ions is artificially decreased for a neuron, …

A) both the resting potential and the peak of the action potential rise significantly.
B) the resting potential increases but the action potential peak does not.
C) the resting potential hardly changes but the action potential peak is raised.
D) the resting potential decreases but the action potential peak is raised.
E) the resting potential hardly changes but the action potential peak is lowered.

Question 40

Neuromuscular transmission involves the sequential activation of several ion channels. Sort the following events to reflect the sequence leading to muscle contraction. Your answer would be a five-digit number composed of numbers 1 to 5 only, e.g. 12543.
(A) Opening of voltage-gated Na? channels in the muscle cell plasma membrane.
(B) Opening of Ca²⁺ release channels in the sarcoplasmic reticulum membrane.
(C) Opening of ionotropic acetylcholine receptors on the muscle cell plasma membrane.
(D) Opening of voltage-gated Ca²⁺channels in the T tubules.
(E) Opening of voltage-gated Ca²⁺channels in the motor neuron.

Question 41

Which of the following is NOT common between long-term potentiation and long-term depression?

A) They are both important for synaptic plasticity in normal individuals.
B) They both require the activation of the glutamate-gated NMDA receptors.
C) They both require a rise in the intracellular concentration of Ca²⁺.
D) They both result in an enhanced postsynaptic response.
E) They can both last for a long time.

Question 42

Sort the following events in a sequence that would lead to long-term potentiation (LTP). Your answer would be a five-digit number composed of numbers 1 to 5 only, e.g. 13245.
(A) Activation of AMPA receptors
(B) Glutamate release into the synaptic clefts
(C) Activation of calcium-dependent protein kinases in the postsynaptic cell
(D) Postsynaptic membrane depolarization
(E) Activation of NMDA receptors

Question 43

The inactivation rate of voltage-gated Na⁺ and K⁺ channels can regulate the firing frequency of neurons. Which of the following combinations results in the highest firing frequency?

A) Rapid inactivation of both Na⁺ and K⁺ channels
B) Rapid inactivation of Na⁺ channels, slow inactivation of K⁺ channels
C) Slow inactivation of Na⁺ channels, rapid inactivation of K⁺ channels
D) Slow inactivation of both Na⁺ and K⁺ channels

Question 44

Which of the following better represents the process of adaptation mediated by the voltage-gated Ca²⁺ channels and Ca²⁺-activated K⁺ channels? The vertical axis in each graph is the membrane potential in the postsynaptic neuron (in millivolts), while the horizontal axis is the time of constant stimulation (in milliseconds).

Question 45

Dopamine is a neurotransmitter involved in the reward pathways in the brain, and its decreased activity has been associated with diseases such as Parkinson's disease and attention-deficit/hyperactivity disorder (ADHD). Which of the following drugs is NOT a likely candidate to treat these diseases?

A) Methylphenidate, a dopamine reuptake inhibitor
B) Amphetamine, a dopamine-releasing agent that triggers the release of dopamine into the synaptic cleft
C) Carbidopa, which enhances the availability of the dopamine synthesis precursors in the brain
D) Forskolin, a sensitizer of dopamine receptors
E) Chlorpromazine, a dopamine antagonist that binds to and inhibits dopamine receptors

Question 46

Each single neuron acts as a computational device. Typically, the magnitude of the combined postsynaptic potential is encoded in the ... of firing of action potentials in the postsynaptic neuron.