Which process requires energy input to move substances against a concentration gradient?

Active transport is the process that requires energy input to move substances against their concentration gradient. This means that it moves molecules from an area of lower concentration to an area of higher concentration, which is contrary to the natural flow of molecules, where they typically tend to move from high to low concentration to achieve equilibrium.

The energy for active transport typically comes from the hydrolysis of ATP (adenosine triphosphate). Through various transport proteins, active transport is essential for maintaining cellular homeostasis and allowing cells to accumulate necessary substances, such as ions or nutrients, even when they are less concentrated outside the cell. For instance, cells use active transport to take up glucose even when its concentration is lower outside than inside the cell after a meal.

In contrast, processes like osmosis and diffusion do not require energy as they rely on the natural tendency of molecules to move along their concentration gradients, seeking equilibrium. Passive transport also encompasses these activities, where substances move from an area of high concentration to an area of low concentration without the use of energy. Thus, the distinction lies in energy input: active transport necessitates it, while the other processes do not.