In passive transport, what primarily determines the direction of diffusion of a solute across a membrane that is permeable to that solute?

• A. The direction of bulk flow generated by cytoskeletal motor proteins
• B. The solute’s molecular weight alone (heavier molecules always diffuse toward lighter molecules)
• C. The cell’s ATP availability, which powers solute movement toward higher concentration
• D. The solute’s concentration gradient (net movement from higher concentration to lower concentration)

Answer: (D)