Q.29 In the pump Na⁺ – K⁺ ATPase during active transport, hydrolysis of ATP provides energy for:

1. 4 mole of Na⁺ out of cell and 4 mole of K⁺ into cell
2. 1 mole of Na⁺ out of cell and 1 mole of K⁺ into cell
3. 3 mole of Na⁺ out of cell and 2 mole of K⁺ into cell
4. 2 mole of Na⁺ into the cell and 3 mole of K⁺ out of cell

   Correct Answer: 3 moles of Na+ out of cell and 2 moles of K+ into cell

   The Na+-K+ ATPase pump uses ATP hydrolysis energy to actively transport 3 Na+ ions out and 2 K+ ions into the cell per cycle, maintaining electrochemical gradients essential for membrane potential and cell volume.

   Option Analysis

   Option 1: Incorrect

   No version of Na+-K+ ATPase transports 4 Na+ out and 4 K+ in; the standard stoichiometry is asymmetric (3:2) to create a net positive charge export for resting potential.

   Option 2: Incorrect

   A 1:1 ratio would not generate the electrochemical gradient needed for action potentials or secondary transport; the pump’s electrogenic nature requires unequal ion exchange.

   Option 3: Correct

   ATP binding triggers 3 Na+ binding inside, phosphorylation flips conformation to release Na+ outside, then 2 K+ bind externally, dephosphorylation returns to original state—net 3 Na+ out/2 K+ in per ATP.

   Option 4: Incorrect

   The pump expels Na+ (not imports) and imports K+ (not exports); reversal occurs only pathologically under extreme conditions, not normally.​

   Introduction to Na+ K+ ATPase Active Transport

   The Na+ K+ ATPase pump powers Na+ K+ ATPase active transport by hydrolyzing one ATP to export 3 Na+ ions from the cell and import 2 K+ ions, creating ion gradients vital for nerve signaling and osmosis. This primary active transport consumes up to 70% of neuronal ATP.

   Pump Cycle Mechanism

   In E1 conformation (cytosol-facing), 3 Na+ bind, ATP phosphorylates Asp residue, shifting to E2 (extracellular), releasing Na+. Then 2 K+ bind, dephosphorylation resets to E1, importing K+. Net effect: +1 charge out, [Na+]out high, [K+]in high.

   Physiological Importance

   Gradients drive Na+ entry in action potentials, secondary transporters (e.g., Na+/glucose), and cell volume control; inhibition by ouabain causes swelling.

   Cycle Step ​	Ions Moved	Energy Use
   Na+ Binding (E1)	3 Na+ in	ATP → ADP + Pi
   Conformational Flip	3 Na+ out	Phosphorylation
   K+ Binding (E2)	2 K+ in	Dephosphorylation
   Reset	Net 3:2	1 ATP/cycle