Which of the following best explains diffusion in the context of passive transport across cell membranes?

• A. The movement of ions across a membrane against their electrochemical gradient using energy from ATP hydrolysis
• B. The transport of specific molecules across a membrane via carrier proteins that bind solute and require ATP to change conformation
• C. The movement of water across a selectively permeable membrane from lower solute concentration to higher solute concentration, driven by differences in water potential
• D. The net movement of molecules from a region of higher concentration to a region of lower concentration due to random thermal motion

Answer: (D)