The sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed, making option 1 correct as it describes more Na⁺ exiting than K⁺ entering.

Introduction

For each ATP molecule breakdown, the sodium-potassium pump causes movement of ions against concentration gradients to maintain cell membrane potential. This active transport mechanism is fundamental in cell biology, physiology, and biochemistry exams for life sciences students.​​

Option Analysis

Each option tests the stoichiometry and directionality of Na⁺/K⁺-ATPase transport.

• More Na⁺ out and less K⁺ in
  Correct: The pump transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell per ATP hydrolyzed, creating the characteristic 3:2 ratio.​

• K⁺ out and Na⁺ in on a one-for-one basis
  Incorrect direction and ratio; this reverses physiological ion gradients and doesn’t match the pump’s electrogenic function.​

• Na⁺ out and K⁺ in on a one-for-one basis
  Wrong stoichiometry; equal exchange (1:1) wouldn’t generate the net positive charge export needed for resting membrane potential.

• K⁺ and Na⁺ in same direction
  Invalid; the pump performs antiport (opposite directions), not symport (same direction).​

Mechanism Details

The Na⁺/K⁺-ATPase undergoes conformational changes:

1. 3 intracellular Na⁺ bind, ATP phosphorylates the pump.

2. Conformational shift (E1 to E2) releases Na⁺ extracellularly.

3. 2 extracellular K⁺ bind, dephosphorylation returns E2 to E1, releasing K⁺ intracellularly.

This 3 Na⁺:2 K⁺ cycle exports net +1 charge per ATP, contributing ~10% to resting potential (-70 mV).

Physiological Importance

Maintains low intracellular Na⁺ (~10 mM) and high K⁺ (~140 mM), essential for action potentials, cell volume, and secondary active transport (e.g., glucose-Na⁺ symport). Found in all animal cells, inhibited by cardiac glycosides like ouabain.

Exam Tip

Memorize the 3:2 ratio and directions: 3 Na⁺ OUT, 2 K⁺ IN per ATP. Questions often test this exact stoichiometry in membrane physiology sections.​​