In the context of passive transport across cell membranes, what is diffusion?

• A. The net movement of particles from a region of higher concentration to a region of lower concentration due to random thermal motion, without requiring cellular energy input
• B. The bulk movement of extracellular fluid and dissolved solutes into the cell via membrane invagination
• C. The movement of molecules across a membrane through specific carrier proteins using ATP hydrolysis
• D. The movement of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration

Answer: (A)