Which process is described as moving molecules down their concentration gradient?

Facilitated diffusion is the process that moves molecules down their concentration gradient, which means that substances naturally move from an area of higher concentration to an area of lower concentration without requiring energy. This occurs with the help of specific transport proteins that assist in the passage of larger or polar molecules across the cell membrane that would otherwise be unable to freely diffuse through the lipid bilayer.

In this context, the molecule movement aligns with the principles of passive transport, where no energy input from ATP is needed. This contrasts with other processes like active transport, where molecules are moved against their concentration gradient and require energy. Similarly, endocytosis and exocytosis are vesicular transport mechanisms used for bulk transport of materials into and out of the cell, but they do not specifically describe the movement of individual molecules down their concentration gradient.