Final answer:
The sodium-potassium pump expels more sodium ions than it imports potassium ions, resulting in a net negative charge inside the cell.
Step-by-step explanation:
The sodium-potassium pump contributes to the net negative charge within the interior of a cell by exporting three sodium (Na+) ions out of the cell for every two potassium (K+) ions it brings in. This activity results in a net loss of positive ions because more positive charges are being expelled than are taken into the cell. Consequently, this action makes the inside of the cell more negative relative to the exterior, which is vital for functions like secondary active transport and maintaining the membrane potential.