What process allows for the movement of substances against their concentration gradient?

Active transport is the process that enables cells to move substances against their concentration gradient, which means moving molecules from an area of lower concentration to an area of higher concentration. This movement requires energy, typically in the form of ATP, because it is working against the natural tendency of molecules to move toward equilibrium, where they are more evenly distributed.

An example of active transport is the sodium-potassium pump, which maintains cellular function by moving sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. This process is vital for maintaining essential cellular functions and homeostasis.

In contrast, passive diffusion and facilitated diffusion involve the movement of substances along their concentration gradient without the expenditure of energy. Filtration typically refers to the movement of substances through a filter or membrane driven by pressure differences rather than concentration gradients. Thus, these processes do not align with the definition or requirements of moving substances against their concentration gradient.