Which transport mechanism requires energy to move substances against their concentration gradient?

Active transport is the process that requires energy to move substances against their concentration gradient. During active transport, molecules are transported from an area of lower concentration to an area of higher concentration, which is an energy-consuming process. This energy is typically provided by ATP (adenosine triphosphate) or other forms of chemical energy, allowing cells to maintain concentration differences of various ions and molecules essential for cellular function.

In contrast, facilitated diffusion, passive diffusion, and simple diffusion are all passive transport mechanisms that do not require energy. Facilitated diffusion still relies on concentration gradients, enabling molecules to move from higher to lower concentration through membrane proteins. Passive diffusion and simple diffusion involve the movement of molecules directly through the lipid bilayer or through pores in the membrane, respectively, but again, only from high to low concentration. Therefore, they do not require the input of energy, as they rely solely on kinetic energy and concentration gradients to drive the movement of substances.