The Na⁺-K⁺-ATPase pump maintains the resting membrane potential. Which statement correctly describes the stoichiometry and energy cost of this pump?
- A Pumps 2 Na⁺ out and 2 K⁺ in per ATP hydrolysed, creating an electroneutral exchange
- B Pumps 3 Na⁺ out and 3 K⁺ in per ATP hydrolysed at rest, and changes to 2:2 during action potential repolarisation
- C Pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed, creating a net outward current that contributes to the negative resting potential ✓
- D Uses 2 ATP molecules per pump cycle to achieve the 3 Na⁺:2 K⁺ exchange across the membrane
Correct answer: C. Pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed, creating a net outward current that contributes to the negative resting potential
Explanation
The Na⁺-K⁺-ATPase (P-type ATPase) pumps 3 Na⁺ ions outward and 2 K⁺ ions inward per cycle per ATP hydrolysed. This unequal exchange creates an electrogenic (net outward) current, contributing approximately -5 to -10 mV to the resting membrane potential (which is approximately -70 mV in neurons; the bulk is due to K⁺ permeability through leak channels). The pump ratio is invariant (3:2). The Na⁺-K⁺-ATPase consumes approximately 20–30% of total body ATP, reflecting its central importance in maintaining ion gradients.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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