K+ channels
Summary
K+ channels are membrane proteins that let K+ diffuse down its gradient, usually out of the cell. Leak channels hold the resting potential near EK, and voltage-gated K+ channels open after depolarization to repolarize the neuron, so blocking them slows repolarization.
Detail
Meaning: K+ channels let K+ cross the membrane by facilitated diffusion. K+ is concentrated inside the cell, so open channels usually let K+ leave, making the inside more negative. Two types matter. Leak channels are open at rest and make the membrane far more permeable to K+ than to Na+, so the resting potential (about −70 mV) sits close to EK. Voltage-gated K+ channels open in response to depolarization, slightly later than voltage-gated Na+ channels. K+ efflux then repolarizes the membrane, and slow closing can cause a brief hyperpolarization (undershoot).
Equation: EK = (RT/zF) ln([K+]out/[K+]in), where R = 8.31 J/(mol·K), T is temperature in K, z = +1 (unitless charge), F = 9.65 × 10⁴ C/mol, concentrations are in mol/L, and EK is in V.
On the MCAT: A passage shows an action potential trace and adds a K+ channel blocker. Predict slowed repolarization and a prolonged action potential. Another variant raises extracellular K+, which makes EK less negative and depolarizes the resting membrane.
Common confusion: Students think the Na+/K+ ATPase creates the resting potential. The pump builds and maintains the gradients, but K+ leak channels set the potential directly. Also, K+ leaves during repolarization; it does not enter. The resting potential is slightly less negative than EK because some Na+ also leaks in.
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.