In a patch-clamp experiment, a neuron is initially at rest (−70 mV). A toxin that selectively blocks voltage-gated K+ channels is applied. When an action potential is triggered by a brief current injection, the repolarization phase becomes slower and the after-hyperpolarization is reduced.
Which statement best explains the changes in membrane potential observed?
MCAT Biological and Biochemical Foundations of Living Systems Quiz
Practice 2a Membrane Potential Electrochemical Gradients in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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In a patch-clamp experiment, a neuron is initially at rest (−70 mV). A toxin that selectively blocks voltage-gated K+ channels is applied. When an action potential is triggered by a brief current injection, the repolarization phase becomes slower and the after-hyperpolarization is reduced.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice D is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
A neuron has intracellular K+ 140 mM and extracellular K+ 4 mM. A researcher injects KCl into the cytosol, increasing intracellular Cl− substantially while leaving K+ nearly unchanged. Opening GABAA receptors (Cl− channels) now produces depolarizing responses instead of hyperpolarizing responses.
Which statement best explains the change in response polarity?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice A is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
A neuron is voltage-clamped at −40 mV. A brief pulse opens K+-selective channels, producing an outward current. When the holding potential is changed to −90 mV, opening the same channels produces a much smaller outward current.
Which statement best explains the change in current magnitude with membrane voltage?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice D is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
A neuron is exposed to a drug that increases permeability to K+ only during the falling phase of the action potential (e.g., by prolonging K+ channel opening). Experimentally, the action potential duration shortens.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the drug selectively increases K⁺ permeability during the falling phase of the action potential, affecting repolarization dynamics. Choice A is correct because it accurately describes how enhanced outward K⁺ current speeds up repolarization, shortening the action potential duration. Choice B is incorrect because it assumes inward K⁺ current, which contradicts the electrochemical driving force during repolarization and misapplies gradient principles. To avoid similar errors, ensure understanding of ion flow directions based on driving forces during different action potential phases. Always verify predictions align with passage data on permeability changes and their timing.
A researcher studies a cell type with a resting membrane potential near −30 mV. Ion gradients are typical (high K+ inside, high Na+ outside), but the membrane expresses a large resting Na+ conductance in addition to K+ leak.
Which statement best explains why the resting membrane potential is relatively depolarized compared with neurons?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the cell has typical ion gradients but unusually high resting Na⁺ permeability alongside K⁺ leak, which alters the resting potential compared to neurons. Choice D is correct because it accurately reflects how increased Na⁺ influx pulls the membrane potential toward the positive ENa, resulting in a less negative (depolarized) resting potential. Choice B is incorrect because it mistakenly attributes the shift to EK and confuses the concentration gradient of K⁺, a common error when overlooking the role of Na⁺ conductance. To avoid similar errors, ensure understanding of how relative ion permeabilities weight the resting potential toward specific equilibrium potentials. Always verify predictions align with passage data on ion conductances and gradients.
In an epithelial monolayer, apical application of a CFTR potentiator increased Cl− conductance across the apical membrane. Intracellular [Cl−] was measured at 30 mM and extracellular (luminal) [Cl−] at 110 mM. The apical membrane potential (cell interior relative to lumen) was approximately −40 mV. After potentiator treatment, transepithelial fluid secretion increased. Which statement best explains the principle linking increased Cl− permeability to fluid movement in this setup?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Epithelial fluid secretion depends on transepithelial ion transport creating osmotic gradients that drive water movement. With intracellular [Cl⁻] = 30 mM and luminal [Cl⁻] = 110 mM, ECl is approximately -35 mV. At an apical membrane potential of -40 mV, Cl⁻ experiences an outward electrochemical gradient and will exit the cell into the lumen. Choice D is correct because Cl⁻ efflux increases luminal osmolarity, drawing water transcellularly or paracellularly into the lumen. Choice B is incorrect because CFTR is a Cl⁻ channel, not a water pump. To understand epithelial transport, consider how ion movement creates osmotic gradients that secondarily drive water flow.
In a ventricular myocyte, a pharmacologic agent selectively inhibits the Na+$/K^+−ATPasewithoutdirectlyaffectingionchannels.Overseveralminutes,intracellularNa^+increasesandintracellularK^+decreases,whileextracellularconcentrationsremainapproximatelyconstant(Na^+outhigh,K^+$ out low). Resting membrane potential is recorded to become less negative. Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). The Na+/K+-ATPase maintains ion gradients by pumping 3 Na+ out and 2 K+ in, consuming ATP. In this scenario, inhibiting the pump allows gradients to dissipate: intracellular K+ decreases and intracellular Na+ increases. Choice D is correct because the reduced K+ gradient (smaller concentration difference) decreases the driving force for K+ efflux, causing less negative membrane potential (depolarization). Choice B is incorrect because it suggests the K+ gradient increases, which is opposite to what occurs when the pump is inhibited. To understand pump inhibition effects, remember that ion gradients will dissipate toward equilibrium, reducing the concentration differences that normally maintain the resting potential.
In a patch-clamp experiment, a neuron was stepped from −70 mV to +20 mV. A fast inward current was observed that inactivated within milliseconds. Tetrodotoxin (TTX) eliminated this transient inward current without affecting a delayed outward current. The bath and pipette solutions maintained physiological gradients (high extracellular Na+, high intracellular K+). Which statement best explains the changes in membrane potential observed during the initial rapid phase of an action potential in this neuron?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Action potentials involve sequential changes in membrane permeability to different ions, with voltage-gated channels responding to depolarization. The fast inward current blocked by TTX is characteristic of voltage-gated Na⁺ channels, which open rapidly upon depolarization. With high extracellular Na⁺, opening these channels allows Na⁺ influx down its electrochemical gradient, driving the membrane toward ENa (+60 mV). Choice D is correct because it accurately describes the Na⁺ influx that causes the rapid depolarization phase of the action potential. Choice B is incorrect because K⁺ channels produce outward current and K⁺ efflux, not influx. To identify ion currents, consider the direction of current flow and the specific blockers used.
A myotube preparation is voltage-clamped at −80 mV while a drug is applied that selectively increases sarcolemmal permeability to Cl−. During the recording, measured intracellular and extracellular chloride concentrations are 10 mM and 110 mM, respectively, and other permeabilities are unchanged. The membrane potential shifts from −80 mV toward −60 mV after drug application. Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential is determined by the relative permeabilities and equilibrium potentials of permeant ions. In this scenario, the Cl- equilibrium potential can be calculated using the Nernst equation: ECl = -61 log(10/110) ≈ -64 mV. Choice A is correct because increasing Cl- permeability drives the membrane potential from -80 mV toward the Cl- equilibrium potential of approximately -64 mV, explaining the observed shift to -60 mV. Choice D is incorrect because it suggests Cl- efflux, but the concentration gradient (10 mM inside, 110 mM outside) favors Cl- influx, not efflux. To avoid confusion, always calculate the equilibrium potential for the ion in question and determine whether the membrane will depolarize or hyperpolarize based on the starting potential.
A researcher expresses a mutated nonselective cation channel in a cell line. In whole-cell recordings, opening the channel increases permeability equally to Na+ and K+, with negligible permeability to Cl−. Under control conditions, the membrane potential is −65 mV. Ion concentrations (mM) are Na+ in/out = 15/145 and K+ in/out = 140/5. Based on the scenario, what effect would increasing this channel's permeability have on the membrane potential?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). When a membrane becomes equally permeable to multiple ions, the membrane potential moves toward a weighted average of their equilibrium potentials. In this scenario, the Na+ equilibrium potential is approximately +60 mV and the K+ equilibrium potential is approximately -90 mV. Choice B is correct because equal permeability to Na+ and K+ drives the membrane potential toward a value between these two equilibrium potentials, resulting in depolarization from -65 mV. Choice D is incorrect because equal permeability does not mean no net current; rather, it means the membrane potential will settle where Na+ influx equals K+ efflux. To solve such problems, recognize that the membrane potential will be influenced by all permeant ions according to their relative permeabilities.
In a cardiac myocyte, investigators increased extracellular K+ from 4 mM to 12 mM while keeping intracellular K+ constant. No drugs were applied, and membrane permeability remained dominated by K+ at rest. Following the change, the resting membrane potential became less negative.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). The resting membrane potential in cells with high K⁺ permeability is strongly influenced by the K⁺ equilibrium potential, which depends on the ratio of extracellular to intracellular K⁺ concentrations. In this scenario, raising extracellular K⁺ from 4 to 12 mM makes the resting potential less negative. Choice D is correct because increasing extracellular K⁺ reduces the concentration gradient for K⁺ efflux (the ratio of internal to external K⁺ decreases), which reduces the driving force for K⁺ to leave the cell and carry positive charge out, resulting in less negative membrane potential. Choice B is incorrect because it states the opposite - raising extracellular K⁺ actually reduces, not increases, the chemical gradient for efflux. To avoid this error, remember that the K⁺ equilibrium potential follows the Nernst equation and becomes less negative as extracellular K⁺ increases.
A cell expresses a ligand-gated nonselective cation channel permeable to Na+ and K+ but not to Cl−. At rest, Vm=−65 mV. When the ligand is applied, Vm rapidly moves toward 0 mV. Ion concentrations (mM) are: Na+ out 145, in 15; K+ out 5, in 140; Cl− out 110, in 10.
Which statement best explains the changes in membrane potential observed upon ligand application?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). When a nonselective cation channel opens, the membrane potential moves toward a weighted average of the equilibrium potentials for the permeant ions. In this scenario, opening a channel permeable to both Na⁺ and K⁺ causes depolarization from -65 mV toward 0 mV. Choice A is correct because Na⁺ has a large inward driving force (high outside, low inside) while K⁺ has an outward driving force (high inside, low outside), and at typical physiological concentrations, the Na⁺ influx dominates over K⁺ efflux through the same channel, bringing net positive charge in and depolarizing the membrane. Choice C is incorrect because it ignores Na⁺ permeability through the channel and incorrectly states that K⁺ efflux alone would depolarize (it would actually hyperpolarize). To avoid errors, consider that nonselective cation channels allow multiple ions to flow simultaneously, with the net current determined by the sum of individual ion movements.
Researchers examined how changing ion channel permeability alters membrane potential in a non-spiking epithelial cell. The cell's resting membrane potential was −60 mV. A drug was applied that selectively increased K+ channel open probability without directly affecting Na+ or Cl− channels. Ion concentrations (mM) were: K+ in 140, out 5; Na+ in 15, out 145; Cl− in 20, out 110. Immediately after drug addition, Vm shifted to −80 mV.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential is determined by the relative permeabilities of different ions and their concentration gradients across the membrane. In this scenario, increasing K⁺ permeability shifts the membrane potential from -60 mV to -80 mV (hyperpolarization). Choice A is correct because K⁺ has a much higher concentration inside (140 mM) than outside (5 mM), so increased K⁺ permeability drives K⁺ efflux down its concentration gradient, removing positive charge from the cell interior and making it more negative. Choice B is incorrect because it suggests K⁺ influx, which would go against the concentration gradient - a common error when students confuse electrical and chemical driving forces. To avoid this mistake, always consider the concentration gradient first when determining ion movement direction, then verify that the resulting charge movement matches the observed potential change.
In a patch-clamp study of cultured hippocampal neurons, the resting membrane potential was stable at −70 mV. A selective ligand-gated channel was activated while the cell was held near rest, producing a rapid shift of the membrane potential toward −90 mV without triggering an action potential. The extracellular and intracellular ion concentrations were (mM):
Na+: out 145, in 12; K+: out 4, in 140; Cl−: out 110, in 10; Ca2+: out 2, in 0.0001. The ligand did not measurably change Na+ or K+ conductance.
Based on the scenario, what effect would increasing ion permeability have that is most consistent with the observed membrane potential change?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the membrane potential shifts from -70 mV to -90 mV (hyperpolarization), and we need to identify which ion movement could cause this change. Choice B is correct because Cl⁻ has a higher concentration outside (110 mM) than inside (10 mM), so increased Cl⁻ permeability would drive Cl⁻ influx down its concentration gradient, bringing negative charge into the cell and hyperpolarizing it. Choice A is incorrect because Na⁺ influx would depolarize (not hyperpolarize) the membrane, moving it toward positive values. To avoid similar errors, remember that hyperpolarization requires either anion influx or cation efflux, and verify that the ion movement follows its concentration gradient.
A lab engineered a cell line expressing a Cl− channel that opens in response to a small molecule. Before addition, Vm=−40 mV. After addition, Vm shifted to −65 mV. Measured Cl− concentrations were: extracellular 110 mM and intracellular 30 mM. Assume the channel is selective for Cl− and does not transport cations.
Based on the scenario, what effect would increasing ion permeability have that best explains the observed voltage change?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Chloride ion movement across membranes depends on both its concentration gradient and the existing membrane potential. In this scenario, opening Cl⁻ channels causes hyperpolarization from -40 mV to -65 mV. Choice B is correct because Cl⁻ has a higher concentration outside (110 mM) than inside (30 mM), and at -40 mV, the electrochemical gradient favors Cl⁻ influx down its concentration gradient, bringing negative charge into the cell and hyperpolarizing the membrane. Choice A is incorrect because it suggests Cl⁻ efflux, which would go against the concentration gradient and would depolarize rather than hyperpolarize the membrane. To avoid errors, calculate or estimate the Cl⁻ equilibrium potential using the Nernst equation - here it would be around -65 mV, explaining why the membrane potential moves toward this value.
A voltage-clamped neuron is held at −60 mV while extracellular Na+ is rapidly reduced from 145 mM to 20 mM (osmolarity maintained with an impermeant cation). Intracellular Na+ remains ~12 mM over the brief measurement. No other ions are changed. The peak inward current evoked by a brief opening of voltage-gated Na+ channels decreases markedly.
Which statement best explains the changes in membrane current observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice D is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
A bacterial cell maintains a proton motive force (PMF) across its plasma membrane. A weak acid uncoupler is added that increases H+ permeability, allowing protons to cross the membrane down their electrochemical gradient. Shortly after addition, ATP synthesis by ATP synthase decreases.
Which statement best explains the changes observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice D is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
In cultured hippocampal neurons (37°C), whole-cell current clamp recorded a stable resting membrane potential of −70 mV. The extracellular solution was then switched to one containing a selective K+ channel opener that increased K+ permeability without changing Na+ or Cl− permeability. Ion concentrations (mM) were:
Inside: K+ 140, Na+ 12, Cl− 10 Outside: K+ 4, Na+ 145, Cl− 110
After drug addition, the membrane potential shifted to −82 mV and stabilized. Based on the scenario, what effect would increasing ion permeability have on the membrane potential, and why?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice B is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice A is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data. Increasing K+ permeability drives the potential toward the K+ equilibrium potential, which is more negative than rest.
A neuron is experimentally manipulated so that extracellular K+ increases from 4 mM to 20 mM while other ions remain unchanged. The resting membrane potential depolarizes from −70 mV to −50 mV.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice A is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.
A neuron is exposed to a drug that selectively blocks fast voltage-gated Na+ channels. Upon current injection, the membrane depolarizes passively but does not generate a rapid upstroke or overshoot.
Which statement best explains the changes in membrane potential observed?
Explanation: This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Membrane potential arises from the differential distribution of ions across a cell membrane, influenced by permeability and concentration gradients. In this scenario, the described changes in ion permeability directly affect membrane potential, illustrating the principle. Choice D is correct because it accurately reflects the ion movement described, consistent with the membrane potential concept. Choice B is incorrect because it assumes ion movement that contradicts the passage details, a common error when misapplying gradient principles. To avoid similar errors, ensure understanding of how ion gradients influence membrane potential and verify predictions align with passage data.