Equilibrium potentials and what sets the resting voltage · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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The voltage each ion wants

Science · The Human Body · ages 18-19
Name ______________________   Date ____________

  1. In a typical neurone, sodium-potassium pumps use ATP to move sodium out and potassium in against their concentration gradients.

    Circle one:   True   False

  2. At 37°C, K+ concentration is 140 mM inside and 14 mM outside. Using E_K = 61.5 log10(c_out/c_in) mV, what is E_K?

    • +61.5 mV
    • -61.5 mV
    • -123.0 mV

  3. In the typical resting neurone described in the lesson, which ion has the greatest membrane permeability?

    • Sodium
    • Calcium
    • Potassium

  4. A typical neurone has high K+ permeability at rest, but V_m is not exactly E_K. Why?

    • The sodium-potassium pump stops using ATP whenever the cell rests
    • Other ion currents and pump current also contribute
    • All potassium channels close, stopping potassium flow at rest

  5. Outside K+ rises in a potassium-dominated resting neurone. Inside K+, temperature, other ion concentrations and permeabilities stay fixed. How does V_m change?

    • It becomes less negative
    • It becomes more negative
    • It remains exactly unchanged

  6. More K+ channels open, increasing potassium permeability. K+ concentrations inside and outside and temperature stay fixed. What happens to E_K?

    • E_K becomes more negative because permeability multiplies the logarithm
    • E_K becomes less negative because permeability divides the logarithm
    • E_K stays fixed; permeability does not enter its calculation

  7. At an ion's equilibrium potential, no individual ions can pass through its open channels.

    Circle one:   True   False

  8. A drug increases Na+ permeability in a resting neurone. Initially V_m is below E_Na. Concentrations, temperature and other permeabilities stay fixed. In which direction does V_m shift?

    • V_m moves toward the sodium equilibrium potential
    • V_m stays at the potassium equilibrium despite the new sodium paths
    • V_m moves away from the sodium equilibrium as more sodium paths open
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Answer key

For grown-ups. Fold this page away before handing over the rest.

The voltage each ion wants W1-mt_mRT_5vDEO1-s1

  1. True · Each cycle uses one ATP to move three Na+ out and two K+ in. This maintains high sodium outside and high potassium inside.
  2. -61.5 mV · Divide outside by inside: 14/140 = 0.1. Since log10(0.1) = -1, E_K = 61.5 × (-1) = -61.5 mV.
  3. Potassium · Potassium has the greatest resting permeability in this typical neurone. Its influence keeps V_m near E_K.
  4. Other ion currents and pump current also contribute · Potassium is the main resting influence, but other ion currents also contribute. The pump exports net positive charge and has a direct electrical effect.
  5. It becomes less negative · Higher outside K+ increases c_out/c_in and makes a negative E_K less negative. The potassium-dominated resting voltage shifts in the same direction.
  6. E_K stays fixed; permeability does not enter its calculation · Potassium concentrations, charge and temperature have not changed, so E_K stays fixed. A permeability change can alter V_m without altering E_K.
  7. False · Equilibrium means no net passive flow. Individual ions can still cross both ways, with equal average flows.
  8. V_m moves toward the sodium equilibrium potential · The new sodium pathways increase sodium's influence. With V_m initially below E_Na and concentrations fixed, V_m shifts upward toward E_Na.
Worksheet · LightMySky