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.

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.

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.

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.

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.

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.

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.

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.

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.

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.