Facilitated diffusion requires which of the following to occur?

Facilitated diffusion is a process that allows substances to cross membranes with the assistance of special proteins. This method is particularly useful for molecules that cannot easily pass through the lipid bilayer of the cell membrane, such as polar or large molecules.

The correct answer highlights the necessity of special carrier proteins and protein channels in facilitated diffusion. These proteins form pathways through which specific molecules can move down their concentration gradient—meaning they go from an area of higher concentration to an area of lower concentration—without requiring energy input. Carrier proteins bind to the molecules and undergo a conformational change to transport them across the membrane, while protein channels provide an open passage for ions or small molecules.

Understanding the role of these proteins is crucial, as they enable the selective transport of substances, making facilitated diffusion a vital process for nutrient uptake and waste removal in cells.