Question
A student says: ‘During the resting potential, the Na⁺/K⁺ pump pumps 3 Na⁺ in and 2 K⁺ out.’ What’s wrong, and how should they fix their understanding?
Solution — Step by Step
The pump actually moves 3 Na⁺ OUT and 2 K⁺ IN per ATP hydrolysed. The student has flipped both directions.
Extracellular fluid is Na⁺-rich, cytoplasm is K⁺-rich. The pump maintains these gradients against diffusion, so it must push Na⁺ out against its concentration gradient.
Because 3 positives leave for every 2 that enter, the pump is electrogenic — it makes the inside slightly more negative, contributing a small part of the mV resting potential.
Remember: ‘3 OUT, 2 IN, net one positive OUT.’ If you memorise the direction of the sodium, the rest follows.
Final answer: Pump is 3 Na⁺ out, 2 K⁺ in per ATP. It is electrogenic and helps maintain the resting potential.
Why This Works
The resting potential is mostly set by K⁺ leak channels (K⁺ equilibrium near mV) plus a small contribution from the pump. The pump’s job is long-term maintenance, not the fast spike.
Alternative Method
You can verify the direction by asking: ‘Which way does each ion want to diffuse?’ Na⁺ wants in, K⁺ wants out. The pump works against both, so it pumps them the opposite way.
Most neural-system numericals come down to unit hygiene and remembering what each gate does at each phase. Draw the action potential graph before reading the question — it saves time.
Common Mistake
Confusing the pump direction with the passive channel direction. Channels follow the gradient; pumps fight it.
Do not confuse passive channels (follow the gradient) with active pumps (fight the gradient). This single distinction clears half of all neural-system doubts.