Final answer:
The movement of an ion down its concentration gradient is called passive transport, which does not require energy from the cell.
Step-by-step explanation:
The movement of an ion down its concentration gradient is known as passive transport. This process does not require energy from the cell because it involves the ion moving from an area of higher concentration to an area of lower concentration. Passive transport includes several types such as diffusion, osmosis, and facilitated diffusion. It's different from active transport, which requires energy, often in the form of ATP, for moving substances against their concentration gradient.