Which of the following is an example of active transport?

A. Oxygen exchanging between red blood cells and liver cells
B. Carbon dioxide exchanging between muscle cells and red blood cells
C. Water moving out of the small intestine
D. Sodium being reabsorbed in the kidney

Correct Answer:

Option D – Sodium being reabsorbed in the kidney

Explanation

Although sodium ions are small, their charge prevents them from moving across the cell membrane through simple diffusion. They must be transported using active transport.