What is a defining feature of active transport?

Active transport is characterized by its ability to move substances against their concentration gradient, which is a key feature that distinguishes it from passive transport mechanisms. In passive transport, substances move from areas of higher concentration to lower concentration, utilizing the natural kinetic energy of molecules. In contrast, active transport requires energy to move substances from regions of lower concentration to higher concentration, thereby allowing cells to accumulate molecules that are vital for their functions, even when those molecules are less concentrated outside the cell.

The process of active transport often relies on ATP as an energy source, particularly in the case of primary active transport, where ATP hydrolysis directly provides the energy needed to transport substances. Therefore, the movement against the concentration gradient is not spontaneous and cannot occur without this energy input.

This mechanism is critical for various cellular processes, such as nutrient uptake, ion balance, and waste removal, making it essential for maintaining homeostasis.