What type of movement involves substances moving down their concentration gradient without energy input?

The type of movement that involves substances moving down their concentration gradient without energy input is known as simple diffusion. Simple diffusion is a passive transport mechanism where molecules, such as gases or small nonpolar substances, move directly through the lipid bilayer of cell membranes from an area of higher concentration to an area of lower concentration. This process does not require any energy because it relies on the natural tendency of molecules to disperse evenly in a space.

In contrast, facilitated diffusion, while also a form of passive transport, occurs when molecules move down their concentration gradient through specific protein channels or carriers in the membrane. Although it doesn't require energy, it does involve specific transport proteins to assist in the movement of larger or polar molecules.

Active transport, on the other hand, requires energy input to transport substances against their concentration gradient, meaning it is not a passive process. Filtration involves the movement of water and solutes through a membrane due to a pressure difference, which also doesn't strictly refer to concentration gradient movement as seen in diffusion.

Thus, simple diffusion best describes the straightforward movement of substances down their concentration gradient without the need for energy or assistance from transport proteins.