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Membrane Potential and Electrochemical Gradients (2A) - Bio & Biochem

  1. 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?
    Answer: As VmV_m approaches the K+^+ equilibrium potential, the electrochemical driving force for K+^+ decreases, reducing current 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.
  2. In an isolated skeletal muscle fiber preparation, a toxin was found to selectively increase the permeability of the sarcolemma to Na+^+ at rest (no stimulation), without changing K+^+ conductance. Baseline VmV_m was −85 mV-85\ \text{mV} and shifted to −65 mV-65\ \text{mV} after toxin exposure. Assume extracellular Na+^+ greatly exceeds intracellular Na+^+, and the intracellular side is initially negative relative to the extracellular side. Which statement best explains the changes in membrane potential observed?
    Answer: Increased Na+^+ permeability causes Na+^+ influx, reducing the magnitude of the negative membrane potential (depolarization). This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). The resting membrane potential depends on the relative permeabilities of different ions and their electrochemical gradients. In this scenario, a toxin increases Na⁺ permeability, causing the membrane potential to shift from -85 mV to -65 mV (depolarization). Choice D is correct because Na⁺ has a much higher concentration outside than inside the cell, so increased Na⁺ permeability drives Na⁺ influx down its concentration gradient, bringing positive charge into the cell and reducing the magnitude of the negative membrane potential. Choice B is incorrect because it suggests Na⁺ efflux, which would go against the concentration gradient - this is a common error when students forget that Na⁺ typically has a higher extracellular concentration. To avoid this mistake, remember that Na⁺ influx depolarizes while K⁺ efflux hyperpolarizes, and always verify ion movement follows the concentration gradient.
  3. A neuropharmacology group tested a compound that inhibits the Na+^+/K+^+-ATPase in cultured neurons. Over several minutes, intracellular Na+^+ increased and intracellular K+^+ decreased; simultaneously, the resting membrane potential drifted from −70 mV-70\ \text{mV} to −55 mV-55\ \text{mV}. No acute change in voltage-gated channel kinetics was detected. Which statement best explains the changes in membrane potential observed?
    Answer: Inhibiting the Na+^+/K+^+-ATPase collapses Na+^+ and K+^+ gradients over time, reducing the driving force that supports a negative resting potential. 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 to work against concentration gradients. In this scenario, inhibiting the pump causes gradual depolarization from -70 mV to -55 mV as ion gradients dissipate. Choice A is correct because without the pump's activity, Na⁺ gradually accumulates inside while K⁺ decreases inside, reducing both the K⁺ and Na⁺ concentration gradients that normally support the negative resting potential, causing the membrane potential to drift toward a less polarized state. Choice B is incorrect because it reverses the pump's function - the Na⁺/K⁺-ATPase exports Na⁺ and imports K⁺, not the opposite. To avoid confusion, remember that the pump maintains gradients that passive channels then use to generate membrane potential, and inhibiting the pump causes gradual gradient dissipation rather than immediate channel effects.
  4. In a ventricular myocyte, a pharmacologic agent selectively inhibits the Na+^+/K+^+-ATPase without directly affecting ion channels. Over several minutes, intracellular Na+^+ increases and intracellular K+^+ decreases, while extracellular concentrations remain approximately constant (Na+^+ out high, K+^+ out low). Resting membrane potential is recorded to become less negative. Which statement best explains the changes in membrane potential observed?
    Answer: Reduced Na+^+/K+^+ pumping decreases the K+^+ gradient, reducing K+^+ efflux at rest and shifting VmV_m toward depolarization 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.
  5. A cell expresses a ligand-gated nonselective cation channel permeable to Na+^+ and K+^+ but not to Cl−^-. At rest, Vm=−65 mVV_m=-65\ \text{mV}. When the ligand is applied, VmV_m rapidly moves toward 0 mV0\ \text{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?
    Answer: Opening the channel increases net cation influx because Na+^+ influx dominates, depolarizing the membrane toward 0 mV0\ \text{mV}. 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.
  6. A lab engineered a cell line expressing a Cl−^- channel that opens in response to a small molecule. Before addition, Vm=−40 mVV_m=-40\ \text{mV}. After addition, VmV_m shifted to −65 mV-65\ \text{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?
    Answer: Opening Cl−^- channels drives Cl−^- influx, increasing negative charge inside and hyperpolarizing the membrane. 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.
  7. 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?
    Answer: Reduced extracellular Na+^+ decreases the Na+^+ electrochemical driving force for influx at −60 mV, reducing inward current 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.
  8. 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?
    Answer: Hyperpolarize toward the K+^+ equilibrium potential because the membrane potential becomes more dominated by K+^+ electrochemical driving forces 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.
  9. A neuron is stimulated repeatedly. During high-frequency firing, intracellular Na+^+ accumulates transiently. If intracellular Na+^+ rises while extracellular Na+^+ remains constant, the peak amplitude of subsequent action potentials decreases slightly. Which statement best explains the change in action potential amplitude?
    Answer: Rising intracellular Na+^+ reduces the Na+^+ electrochemical driving force for influx during the upstroke, limiting overshoot 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.
  10. In a neuron at rest (Vm=−70 mVV_m=-70\ \text{mV}), an experimenter transiently increased membrane permeability to Ca2+^{2+} using optogenetic activation of a Ca2+^{2+}-selective channel. Ca2+^{2+} concentrations were: extracellular 2 mM, intracellular 10−410^{-4} mM. The membrane potential rapidly became less negative during illumination. Which statement best explains the changes in membrane potential observed?
    Answer: Increased Ca2+^{2+} permeability causes Ca2+^{2+} influx down its electrochemical gradient, depolarizing the membrane. This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Calcium ions have an extremely steep concentration gradient across cell membranes and a strong electrochemical driving force for influx. In this scenario, increasing Ca²⁺ permeability causes the membrane to become less negative (depolarize). Choice D is correct because Ca²⁺ has a much higher concentration outside (2 mM) than inside (10⁻⁴ mM), creating a 20,000-fold gradient, and the negative membrane potential (-70 mV) provides additional electrical driving force for Ca²⁺ influx, so increased permeability drives Ca²⁺ influx, bringing positive charge in and depolarizing the membrane. Choice B is incorrect because it suggests Ca²⁺ efflux, which would go against both the concentration gradient and electrical gradient - a fundamental error in understanding electrochemical gradients. To avoid this mistake, remember that Ca²⁺ almost always flows into cells when Ca²⁺ channels open due to its extremely low intracellular concentration.
  11. In an epithelial monolayer, apical application of a CFTR potentiator increased Cl−\text{Cl}^- conductance across the apical membrane. Intracellular [Cl−][\text{Cl}^-] was measured at 30 mM30\ \text{mM} and extracellular (luminal) [Cl−][\text{Cl}^-] at 110 mM110\ \text{mM}. The apical membrane potential (cell interior relative to lumen) was approximately −40 mV-40\ \text{mV}. After potentiator treatment, transepithelial fluid secretion increased. Which statement best explains the principle linking increased Cl−\text{Cl}^- permeability to fluid movement in this setup?
    Answer: Enhanced Cl−\text{Cl}^- permeability increases Cl−\text{Cl}^- flux, promoting osmotic water movement that follows the net movement of solute. 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.
  12. Skeletal muscle fibers were exposed to a ryanodine receptor (RyR1) inhibitor that prevents Ca2+\text{Ca}^{2+} release from the sarcoplasmic reticulum (SR) into the cytosol. Resting cytosolic [Ca2+][\text{Ca}^{2+}] remained low, and SR [Ca2+][\text{Ca}^{2+}] remained high. Electrical stimulation still produced normal depolarization of the sarcolemma and T-tubules, but force generation was markedly reduced. Which process is most likely affected by the described changes in ion concentration?
    Answer: Reduced Ca2+\text{Ca}^{2+} binding to troponin C, decreasing actin-myosin cross-bridge formation despite normal membrane depolarization. This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). Excitation-contraction coupling in skeletal muscle requires Ca²⁺ release from the sarcoplasmic reticulum to activate the contractile machinery. While membrane depolarization triggers this process, the actual force generation depends on cytosolic Ca²⁺ binding to troponin C. When RyR1 is blocked, Ca²⁺ cannot be released from the SR despite normal electrical activity. Choice A is correct because reduced cytosolic Ca²⁺ prevents troponin C activation and subsequent cross-bridge formation. Choice B is incorrect because the question states that sarcolemmal depolarization was normal, indicating Na⁺ channels functioned properly. To understand muscle contraction, distinguish between electrical events (depolarization) and the downstream Ca²⁺-dependent mechanical events.
  13. A researcher records from a neuron and observes that opening a particular ligand-gated channel produces a membrane potential change that reverses direction near 0 mV (inward current at −60 mV, outward current at +20 mV). The channel is permeable to Na+^+ and K+^+ but not Cl−^−. Which statement best explains the changes in membrane potential observed?
    Answer: A reversal near 0 mV is consistent with a mixed Na+^+/K+^+ conductance whose net current changes sign when VmV_m crosses its reversal potential 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.
  14. 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?
    Answer: High resting Na+^+ permeability shifts VmV_m toward ENaE_{Na}, making the resting potential less negative despite normal gradients 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.
  15. In a smooth muscle cell, a drug increases opening of voltage-gated Ca2+^{2+} channels at a given membrane potential. Extracellular Ca2+^{2+} is ~1.2 mM and cytosolic free Ca2+^{2+} is ~0.1 μ\muM at rest. After drug application, cytosolic Ca2+^{2+} rises and contraction increases. Which statement best explains the effect in terms of electrochemical gradients?
    Answer: Opening Ca2+^{2+} channels allows Ca2+^{2+} influx down a strong electrochemical gradient, elevating cytosolic Ca2+^{2+} to promote contraction 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.
  16. A neuron expresses a ligand-gated channel that conducts both Na+\text{Na}^+ and K+\text{K}^+ (no Cl−\text{Cl}^- permeability). At rest, Vm=−70 mVV_m=-70\ \text{mV}. When the ligand is applied, the membrane potential depolarizes to approximately −10 mV-10\ \text{mV} and stabilizes there while the ligand remains present. Intracellular and extracellular ion gradients are physiological (high [Na+]out[\text{Na}^+]_\text{out}, high [K+]in[\text{K}^+]_\text{in}). Which statement best explains the changes in membrane potential observed?
    Answer: The stabilized voltage reflects the channel's reversal potential, where net Na+\text{Na}^+ influx balances net K+\text{K}^+ efflux. This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). When a channel conducts multiple ions, the membrane potential stabilizes at the reversal potential where net current is zero. For a channel permeable to both Na⁺ and K⁺, this occurs where inward Na⁺ current equals outward K⁺ current. At -10 mV, Na⁺ still flows inward (though reduced from -70 mV) while K⁺ flows outward, and these currents balance. Choice B is correct because it describes this balance at the channel's reversal potential. Choice C is incorrect because K⁺ would flow out, not in, at negative voltages given its concentration gradient. To understand mixed conductance channels, recognize that the reversal potential lies between the individual ion equilibrium potentials.
  17. A neuron at −65 mV is exposed to a ligand that opens a channel permeable only to Na+^+. The membrane depolarizes and action potentials are triggered more easily. Based on the scenario, what effect would increasing ion permeability have?
    Answer: Increasing Na+^+ permeability drives VmV_m toward the Na+^+ equilibrium potential by increasing inward Na+^+ current 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 C 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.
  18. Mitochondria isolated from liver cells were loaded with a fluorescent dye sensitive to membrane potential across the inner mitochondrial membrane (matrix relative to intermembrane space). Addition of an uncoupler increased proton permeability across the inner membrane and rapidly decreased the magnitude of the measured potential difference. No inhibitors of the electron transport chain were present. Based on the scenario, what effect would increasing ion permeability have that is most consistent with the observed change?
    Answer: Increasing H+\text{H}^+ permeability dissipates the proton electrochemical gradient, reducing the membrane potential by allowing H+\text{H}^+ to move down its driving force. This question tests understanding of membrane potential and electrochemical gradients (Foundational Concept 2: Cells and Cellular Organization). The inner mitochondrial membrane maintains a proton gradient with higher [H⁺] in the intermembrane space than the matrix, creating both a chemical gradient and electrical potential (matrix negative). Uncouplers increase H⁺ permeability, allowing protons to flow down their electrochemical gradient back into the matrix. This dissipates both the pH gradient and membrane potential without inhibiting electron transport. Choice D is correct because H⁺ movement down its gradient reduces the membrane potential magnitude. Choice B is incorrect because uncouplers allow H⁺ to escape the intermembrane space, not trap it there. To understand mitochondrial bioenergetics, recognize that the proton-motive force includes both chemical (ΔpH) and electrical (Δψ) components.
  19. A neuron is stimulated to fire an action potential. During the rising phase, voltage-gated Na+^+ channels open rapidly. During the falling phase, Na+^+ channels inactivate and voltage-gated K+^+ channels open. Which statement best explains the changes in membrane potential observed during repolarization?
    Answer: Repolarization is driven primarily by outward K+^+ current as K+^+ moves down its electrochemical gradient 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.
  20. 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?
    Answer: Blocking K+^+ channels reduces outward K+^+ current, slowing repolarization and limiting hyperpolarization after the spike 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.

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