Final answer:
Sodium ions (Na⁺⁺) bind to the inner surface of the cell on the sodium-potassium pump, which exchanges three Na⁺⁺ for two K⁺ ions using ATP. The affinity for ions changes during the pump's cycle. The experimental data supports that K⁺ binds on the cell's outer surface, while Na⁺⁺ binds inside.
Step-by-step explanation:
Na⁺⁺ Ion Binding Sites on the Sodium-Potassium Pump
Three sodium ions (Na⁺⁺) bind to cytoplasmic sites on the protein pump within a cell. The sodium-potassium pump, which requires ATP, transports these ions across the cell membrane, swapping three Na⁺⁺ from inside the cell for two potassium ions (K⁺) from the outside. Initially, the pump has a high affinity for Na⁺⁺, which bind to the carrier protein when it's oriented towards the cell's interior. After ATP is used, the pump changes conformation, reduces its affinity for potassium, releases K⁺ into the cytoplasm, and resets its affinity for Na⁺⁺. Experiments with ouabain and varying ions inside and outside artificial cells indicate that Na⁺⁺ binding occurs on the inside surface of the cell, while K⁺ binding occurs on the outside surface, given that the Na⁺⁺ transport remains unaffected by ouabain (which inhibits K⁺).
Conclusion (c) is supported by the data: The binding of K⁺ occurs on the outer surface and the binding of Na⁺⁺ on the inner surface, as they are not transported when ATP is absent.