Final answer:
When the Sodium-Potassium pump is open to the cytoplasm, only sodium ions (Na+) can bind to it. It transports three Na+ out of the cell in exchange for two K+ ions with each pump cycle, driven by energy from ATP.
Step-by-step explanation:
The Sodium-Potassium pump (Na+/K+ pump) is an essential cellular mechanism, especially important in nerve cells. It operates to maintain the correct concentrations of Na+ and K+ across the cell's plasma membrane. When the pump is open to the cytoplasm, only sodium ions (Na+) can enter the pump. During each cycle of the pump's operation, three sodium ions are transported out of the cell and two potassium ions (K+) are moved into the cell. This process is powered by ATP, which provides the energy needed for the pump to change shape and transfer the ions against their respective concentration gradients. The specificity of the pump means that other ions, such as chloride (Cl-) or calcium (Ca2+) ions, cannot enter it; they have their own respective transport mechanisms or channels.