Which example best demonstrates diffusion?

• A. A cell using ATP to pump sodium ions out of the cytoplasm
• B. Water moving through a semipermeable membrane from low solute concentration to high solute concentration
• C. Oxygen molecules moving from an area of high concentration in the lungs to a lower concentration in the blood
• D. Glucose being transported into a cell via a specific carrier protein against its concentration gradient

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of diffusion. Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, driven by the concentration gradient, without requiring energy input (ATP).
2. Step 2: Analyze the first option: 'A cell using ATP to pump sodium ions out of the cytoplasm.' This describes active transport, which requires energy (ATP) to move substances against their concentration gradient, not diffusion.
3. Step 3: Analyze the second option: 'Water moving through a semipermeable membrane from low solute concentration to high solute concentration.' This describes osmosis, a specific type of passive transport involving water, not diffusion of other molecules.
4. Step 4: Analyze the third option: 'Oxygen molecules moving from an area of high concentration in the lungs to a lower concentration in the blood.' This fits the definition of diffusion, as oxygen moves passively along its concentration gradient without energy input.
5. Step 5: Analyze the fourth option: 'Glucose being transported into a cell via a specific carrier protein against its concentration gradient.' This describes active transport, which requires energy and a carrier protein, not diffusion.