In kidney function, how is the electrochemical gradient used to reabsorb ions?

• A. The gradient of chloride ions is used to synthesize ATP.
• B. The gradient of calcium ions is used to facilitate passive diffusion.
• C. The gradient of potassium ions is used to reabsorb sodium ions passively.
• D. The gradient of sodium ions is used to reabsorb other ions against their gradient.

Answer: (D)