What is facilitated diffusion primarily characterized by?

Facilitated diffusion is primarily characterized by the involvement of protein channels for the transport of molecules across cell membranes. This process allows substances that cannot easily pass through the lipid bilayer to cross the membrane by utilizing specific membrane proteins, which serve as channels. These channels may be selective for particular molecules, such as glucose or ions, facilitating their movement down their concentration gradient without requiring energy input.

Facilitated diffusion works based on the natural tendency of substances to move from areas of higher concentration to areas of lower concentration, relying on the concentration gradient for transport. This distinguishes it from processes that involve energy expenditure, such as active transport, which moves substances against a concentration gradient. Therefore, the key characteristic of facilitated diffusion is the use of these protein channels, making option B the correct choice.