What is facilitated diffusion?

Facilitated diffusion is a process that allows substances to cross membranes with the assistance of special proteins. This mechanism is essential for transporting molecules that cannot readily diffuse through the lipid bilayer of cell membranes due to their size, polarity, or charge. In facilitated diffusion, the movement of particles occurs along their concentration gradient, meaning that substances move from an area of higher concentration to an area of lower concentration, which is a passive process requiring no energy input from the cell.

The involvement of membrane proteins, such as channel proteins or carrier proteins, is what distinguishes facilitated diffusion from simple diffusion. These proteins provide a pathway for particular molecules to pass through the membrane more efficiently, thus facilitating their movement without the expenditure of energy.

The other options describe processes that do not accurately represent facilitated diffusion. Active transport, for example, involves the movement of molecules against their concentration gradient and requires energy, which is not characteristic of facilitated diffusion. Similarly, the movement from low to high concentration describes a scenario contrary to facilitated diffusion. Lastly, the process of engulfing large particles refers to endocytosis rather than diffusion mechanisms.