What process is characterized by the movement of molecules down their concentration gradient?

The movement of molecules down their concentration gradient describes the process of passive transport. This is a fundamental mechanism in cellular biology where substances move from an area of higher concentration to an area of lower concentration without the expenditure of energy. The driving force behind passive transport is the natural tendency for molecules to disperse and achieve equilibrium.

In biological systems, this process can involve simple diffusion, where small nonpolar molecules, like oxygen and carbon dioxide, can pass directly through the lipid bilayer of a cell membrane. However, passive transport can also include facilitated diffusion, which is a specific type of passive transport that employs transport proteins to help move larger or polar molecules across the membrane without the need for energy. Both processes rely on the concentration gradient and do not require cellular energy input, distinguishing them from active transport, which involves moving molecules against their concentration gradient and requires energy often derived from ATP.

In summary, passive transport encapsulates the essential mechanism of molecules moving down their concentration gradient, which is a vital process for maintaining homeostasis in living organisms.