Final answer:
During repolarization, sodium ions are pumped out of the cell by the sodium-potassium pump, which is responsible for expelling three Na+ ions for every two K+ ions taken in, contributing to the negative charge of the cell interior.
Step-by-step explanation:
During repolarization, sodium ions are pumped out of the cell. This process is part of the action of the sodium-potassium pump which is essential for maintaining the electrical balance of the cell. The sodium-potassium pump moves three sodium ions out of the cell and two potassium ions into the cell against their respective concentration gradients. This action uses energy derived from ATP and is crucial for the restoration of the resting potential after a cell has been depolarized during an action potential.
The sodium-potassium pump helps to make the interior of the cell negatively charged by expelling more cations (positively charged ions) than are taken in. Specifically, it expels three ions of Na+ for every two K+ ions pumped into the cells. This contributes to the cell's net negative charge compared to the outside. While most body fluids are neutral in charge, the pump ensures a high concentration of potassium ions inside cells and a low concentration of sodium ions, which is vital for numerous cellular functions.