Final answer:
Negative ions are attracted to the negative terminal of the primary cell due to the electrochemical gradient created by the membrane potential difference inside and outside the cell.
Step-by-step explanation:
The negative ions are attracted to the negative terminal of the primary cell due to the electrochemical gradient. The inside of the cell is negative compared to the outside, creating a membrane potential that favors the movement of positive ions (cations) into the cell and negative ions (anions) out of the cell.
This movement is driven by the combination of the ions' concentration gradient and the effect of the membrane potential on their movement.