The Post–Albers transport cycle (e.g., Post et al., 1972) of the Na⁺,K⁺-ATPase cartooned as an ion channel with two gates. The gates open strictly alternately to allow access to cation-binding sites from only one side at a time. The extracellular-side gate (red) is closed in all E1 states, and the cytoplasmic-side gate (blue) is closed in E2 states. Parentheses denote occluded ions, and “-P” symbolizes the aspartyl-phosphate formed by autophosphorylation of E1 pumps once three cytoplasmic Na⁺ ions have bound.