Answer
A molecule moves down its concentration gradient in the process of simple diffusion. This means that a molecule goes from an area of high concentration to one of low concentration. Because this is the normal path the molecule takes, this process doesn't require energy.
Work Step by Step
A concentration gradient is defined as the process by which a solute moves from an area of high concentration to an area of low concentration. This movement of particles is known as simple diffusion, and does not require energy in the form of ATP. Some solutes move against the concentration gradient instead of down the concentration gradient. This means the solute moves from an area of low concentration to one of high concentration, and, therefore, does require energy.
