What type of substances does facilitated diffusion typically move across membranes?

Facilitated diffusion is a process that allows specific substances to cross cell membranes with the assistance of carrier proteins or channel proteins. This mechanism is particularly necessary for moving larger or polar molecules that cannot easily diffuse through the lipid bilayer of the cell membrane due to their size or hydrophilicity.

In the context of the choices provided, large complex substances are often too bulky or charged to pass through the membrane via simple diffusion. Instead, they require the help of these specialized proteins that create a pathway for them to move more efficiently across the membrane in response to their concentration gradient.

Simple ions typically move through ion channels, while small lipid-soluble molecules and uncharged gases can diffuse freely through the lipid bilayer without the need for facilitation. Therefore, the best representation of substances moved via facilitated diffusion is indeed large complex substances, which rely on the assistance of transport proteins to traverse the membrane effectively.