Which process involves molecules moving down their concentration gradient without energy input?

The process that involves molecules moving down their concentration gradient without energy input is passive diffusion. This mechanism occurs naturally as molecules aim to reach equilibrium, moving from areas of higher concentration to areas of lower concentration.

In passive diffusion, substances like oxygen or carbon dioxide can cross cell membranes simply due to the inherent kinetic energy of the molecules, which propels them in the direction of lower concentration. This process does not require ATP or any other energy source because it relies solely on the concentration gradient created by the unequal distribution of molecules across the membrane.

Other processes listed, like active transport, involve energy expenditure to move molecules against their concentration gradient, while facilitated diffusion still requires the involvement of specific transport proteins but does not use energy either. Endocytosis refers to the cellular process of engulfing material, which requires energy. Thus, while passive and facilitated diffusion do not require energy, passive diffusion specifically refers to the unassisted movement of molecules directly through the membrane.