Which of the following is an example of active transport?

Step 1: Analyze the question We need to identify which process is active transport.

Step 2: The concept Active transport is the movement of substances across a cell membrane against a concentration gradient (from low to high concentration). It requires energy (ATP) and uses carrier proteins (pumps).

Passive processes (diffusion, osmosis) move substances down a concentration gradient and need no energy.

Step 3: Evaluate each option

• A. Oxygen exchanging between red blood cells and liver cells: Gases move by diffusion down their concentration gradients. Passive, so not active transport.
• B. Carbon dioxide exchanging between muscle cells and red blood cells: This is also diffusion, so it is passive.
• C. Water moving out of the small intestine: Water moves by osmosis, which is passive.
• D. Sodium being reabsorbed in the kidney: Sodium ions are pumped out of the kidney tubule cells by the sodium-potassium pump (Na⁺/K⁺-ATPase), which uses ATP. This is active transport.

Step 4: Answer The correct option is D.

Tip: Think "ions and glucose = pumps." Gases (O₂, CO₂) always move by diffusion, and water always moves by osmosis. If you see sodium, potassium, or glucose uptake and reabsorption, active transport is usually the answer. Also remember: active = ATP needed.