What type of transport moves substances down their concentration gradient?

Passive transport is the process that moves substances down their concentration gradient, which means that molecules move from an area of higher concentration to an area of lower concentration without the need for energy input. This movement occurs naturally due to the inherent kinetic energy of molecules, seeking equilibrium.

In passive transport, substances include gases like oxygen and carbon dioxide, as well as small nonpolar molecules that can easily cross the cell membrane. The key element here is that this transport does not require cellular energy (ATP) because it utilizes the natural tendency of particles to move to areas of lower concentration until equilibrium is achieved.

Other forms of transport, like active transport, require energy to move substances against their concentration gradient, from low to high concentration. Facilitated diffusion is a type of passive transport that involves specific carrier proteins to help move substances across the membrane, particularly larger or polar molecules. Endocytosis, on the other hand, is a process where the cell membrane engulfs substances to bring them inside the cell, which also requires energy and does not simply move substances down their concentration gradient.