Which statement best describes facilitated diffusion?

• A. The movement of small charged or water-soluble molecules from high concentration to low concentration across a semi-permeable membrane through protein channels or carrier proteins, and it is passive
• B. It requires energy to move substances up the gradient
• C. It moves large particles via vesicles
• D. It only involves water moving through aquaporins

Answer: (A)

Facilitated diffusion is a passive transport process that uses membrane proteins to help specific small charged or polar molecules cross the cell membrane down their concentration gradient. The proteins provide a passage (channels) or bind and change shape to shuttle the molecule (carriers) without the cell supplying energy. This down-the-gradient movement means no cellular energy is required, unlike active transport which moves substances against their gradient using energy. Facilitated diffusion can handle more than just water—ions and other polar molecules use channels or carriers, even though aquaporins specialize in water transport. Vesicle-based bulk transport and energy-driven pumps do not describe this process.