Final answer:
Active transport is the energy-dependent process by which cells move molecules against a concentration gradient using ATP and protein carriers.
Step-by-step explanation:
Active transport in biology refers to the movement of molecules against their concentration gradient. This process requires cellular energy in the form of adenosine triphosphate (ATP) and often involves protein carriers to facilitate the transport. Unlike passive transport methods, which do not require energy and move substances along a concentration gradient, active transport is essential when a cell needs to move substances from an area of lower concentration to an area of higher concentration—a process considered against a concentration gradient. Therefore, active transport is characterized by movement of molecules against their concentration gradient and the requirement of ATP.