What process requires energy to move molecules against their concentration gradient?

Active transport is the process that requires energy to move molecules against their concentration gradient. In biological systems, molecules naturally move from regions of higher concentration to regions of lower concentration through passive processes like diffusion or osmosis, which do not require energy input. However, active transport mechanisms, such as the operation of ion pumps (e.g., sodium-potassium pump), allow cells to import substances that are less abundant outside the cell (like certain ions or nutrients) compared to inside, or to expel waste products.

This energy-dependent process is crucial for maintaining homeostasis within cells and allowing them to function optimally. The energy usually comes from adenosine triphosphate (ATP), which is produced during cellular respiration. Therefore, the ability of active transport to concentrate substances within the cell or remove them against the gradient plays a vital role in cellular activities.