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Membrane Potentials Quiz

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Which statement best defines the membrane potential of a cell?

A The sum of the equilibrium potentials for sodium, potassium and chloride
B The electrical potential difference produced by the sodium-potassium pump across the plasma membrane
C The electrical potential difference across the nuclear envelope of the cell
D The electrical potential difference across the plasma membrane, measured inside relative to outside ✓
E The difference in sodium and potassium concentration across the plasma membrane

The answer is D: The electrical potential difference across the plasma membrane, measured inside relative to outside

Explanation

Why D is correct

See What is a Membrane Potential?. Option D is correct because the membrane potential is the electrical potential difference across the plasma membrane, written as the potential inside the cell relative to the outside, and measured in millivolts (mV). In other words, it is a voltage: a potential difference between the two sides of the membrane. It exists because two things are true of every cell. First, ions are distributed unevenly (there are concentration gradients, for example high potassium inside and high sodium outside). Second, the membrane is selectively permeable, so some ions, mainly potassium, sodium and chloride, can cross through channels more easily than others. The movement of those ions separates charge across the membrane, and that separation is the membrane potential. All cells have one, although the resting value and the ability to change it differ between cell types, and it forms the basis of cell signalling. Memory aid: membrane potential is a voltage, always quoted as inside relative to outside.

Definition: the membrane potential is the potential difference across the plasma membrane, inside relative to outside, measured in millivolts.
Concentration gradients: ions are distributed unevenly, for example high potassium inside and high sodium outside.
Selective permeability: potassium, sodium and chloride cross through channels more easily than other ions, which separates charge across the membrane.

Why the other options are incorrect

A. The sum of the equilibrium potentials for sodium, potassium and chloride: The membrane potential is not the arithmetic sum of individual ion equilibrium potentials. It is the overall voltage across the membrane, determined by the weighted permeabilities and concentrations of the ions, not by adding their equilibrium potentials together.
B. The electrical potential difference produced by the sodium-potassium pump across the plasma membrane: The sodium-potassium pump helps maintain the ion gradients and makes a small electrogenic contribution, but the membrane potential is not defined as the pump's output. The potential is the voltage difference across the membrane that results from separated charge, not the transport process itself.
C. The electrical potential difference across the nuclear envelope of the cell: A potential difference across the nuclear envelope is not the membrane potential of a cell, which conventionally refers to the plasma membrane. The nuclear envelope can have its own electrical properties, but the term membrane potential in cell physiology means the voltage across the outer cell membrane.
E. The difference in sodium and potassium concentration across the plasma membrane: A difference in sodium and potassium concentration is a concentration gradient, not an electrical potential difference. Membrane potential is specifically the voltage across the plasma membrane, so this option describes the unequal ion distribution rather than the potential itself.

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1
Which receptor type is classically responsible for fast synaptic transmission at chemical synapses?
2
A neurone membrane is assumed to be permeable only to potassium, with potassium more concentrated inside than outside. Why does a stable negative membrane potential develop?

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