What type of transport mechanism does facilitated diffusion use?

Facilitated diffusion primarily employs a carrier system for large and complex substances. This transport mechanism allows molecules that are too large or polar to cross the lipid bilayer of the cell membrane unaided, using specialized proteins that facilitate their passage.

During facilitated diffusion, specific carrier proteins or channels in the membrane recognize and bind to the target substance, undergoing a conformational change that translocates the substance across the membrane. This process does not require energy in the form of ATP because it moves substances down their concentration gradient, meaning it relies on the natural movement of particles from an area of higher concentration to an area of lower concentration.

The distinction from other mechanisms highlights why the other options are not appropriate descriptions of facilitated diffusion. Active transport, for example, involves moving substances against their concentration gradient and requires energy, which is not the case here. Simple diffusion refers to the movement of small, nonpolar substances directly through the lipid bilayer without assistance. Lastly, endocytosis involves engulfing substances to bring them into the cell, a significantly different process from facilitated diffusion. Thus, the choice of a carrier system for large, complex substances accurately characterizes the mechanism of facilitated diffusion.