Which of the following is a likely characteristic of facilitated diffusion?

Facilitated diffusion is a process that allows substances to cross membranes with the assistance of special proteins. The characteristic of involving protein channels is essential to this process, as these proteins aid in transporting molecules that cannot easily pass through the lipid bilayer of the cell membrane due to their size, polarity, or charge.

In facilitated diffusion, molecules move from an area of higher concentration to an area of lower concentration, which means it occurs along the concentration gradient rather than against it. This is a key aspect that distinguishes facilitated diffusion from active transport, which does require ATP and moves substances against their concentration gradient. The selective nature of the protein channels also means that they are not significantly influenced by the molecular weight of the substances being transported, although larger molecules may indeed need specific channels. This makes the involvement of protein channels the defining characteristic of facilitated diffusion.