1 · Resting state · the ionic gradient
Outside the cell Na⁺ is high and K⁺ low, inside it is the reverse, and since this gradient leaks all the time, Na/K-ATPase must actively pump against it.All animal cells immerse themselves in an asymmetric salt water:· Extracellular: Na⁺ high (~ 140 mM), K⁺ low (~ 4 mM)
· Intracellular: K⁺ high (~ 140 mM), Na⁺ low (~ 12 mM)
This chemical concentration gradient + the transmembrane electrical gradient (-70 mV) together form the membrane potential — the fundamental energy reserve behind a neuron's ability to fire, a muscle's ability to contract, an enterocyte's ability to absorb nutrients, and a renal tubule's ability to reabsorb sodium.
But the gradient violates entropy: Na⁺ always wants to leak in down its gradient, K⁺ always wants to leak out. Membrane leak is passive and ongoing.
To maintain the gradient, you must actively pump against it — this is the job of Na/K-ATPase.