Facilitated diffusion is a passive transport process where molecules move across a cell membrane from an area of higher concentration to an area of lower concentration with the help of specific transmembrane proteins. It does not require energy (ATP) and relies on molecular binding between the transported substance and carrier or channel proteins. This process allows polar or large molecules that cannot diffuse freely through the lipid bilayer to cross the membrane via protein channels or carrier proteins that may undergo conformational changes during transport. The movement is always down the concentration gradient, meaning substances move passively and do not need energy input.

Key points about facilitated diffusion across cell membranes:

• It is a selective process mediated by integral membrane proteins (channels or carriers).
• It is passive, so no energy is required for the transport.
• Substances move down their concentration gradient (from high to low concentration).
• Carrier proteins may change shape to transport substances, whereas channel proteins provide a pore for passage.
• It differs from simple diffusion in that polar and charged molecules require these proteins to cross the hydrophobic membrane interior.

Thus, an accurate statement describing facilitated diffusion would emphasize passive transport through specific membrane proteins, movement down the concentration gradient, and no direct energy expenditure.