Na+
Summary
Na⁺ is the sodium ion, the main extracellular cation. The Na⁺/K⁺ ATPase keeps it high outside cells and low inside (3 Na⁺ out, 2 K⁺ in per cycle). Most testable: Na⁺ flowing in down this gradient through voltage-gated channels causes the upstroke of an action potential.
Detail
Meaning: Na⁺ is the sodium ion, the main extracellular cation. The Na⁺/K⁺ ATPase uses ATP to move 3 Na⁺ out and 2 K⁺ in, so Na⁺ is high outside cells and low inside. This gradient stores energy. Na⁺ entering through voltage-gated channels causes the upstroke of an action potential, and Na⁺ entering with glucose through a symporter is secondary active transport.
Equation: ENa = (RT/zF) ln([Na⁺]out/[Na⁺]in), where ENa is the equilibrium potential (V), R = 8.31 J/(mol·K), T is temperature (K), z = +1 (unitless), F = 9.65 × 10⁴ C/mol, and concentrations are in mol/L.
On the MCAT: Passages show an action potential trace with a channel blocker or a pump inhibitor. Blocking voltage-gated Na⁺ channels prevents depolarization. Inhibiting the pump slowly dissipates the gradient, which weakens Na⁺-coupled transport. Lowering extracellular Na⁺ makes ENa less positive and shrinks the peak.
Common confusion: Na⁺ entry during the upstroke is passive, down its gradient through channels. The pump does not cause it; it restores the gradient afterward. Also, the resting potential sits near EK, not ENa, because the resting membrane is much more permeable to K⁺ than to Na⁺.
Sources
- Campbell Biology
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.