Final answer:
Before the carrier protein returns to its initial state with available binding sites, Pi is released, leading to a shape change that decreases its affinity for transported ions and prepares it to bind new ions.
Step-by-step explanation:
The question relates to the function of carrier proteins and specifically to the sodium-potassium pump, which is a type of carrier protein that actively transports sodium and potassium ions across the cell membrane. Just before the carrier protein returns to its initial state, where the cytoplasmic binding site is available to calcium (or, in the context of sodium-potassium pump, to sodium), phosphate (Pi) is released from the pump.
This event is crucial as it leads to a conformational change that decreases the affinity of the carrier protein for the ions that were just transported, thereby allowing new ions from inside the cell to bind. As a result, the carrier protein's shape changes so that it is open towards the interior of the cell, ready to bind ions and start the process again.