What defines active transport in cellular processes?

Active transport is characterized by the movement of molecules against their concentration gradient, which requires energy to occur. This process typically involves the use of ATP, the primary energy currency in cells. In active transport, substances move from areas of lower concentration to areas of higher concentration, which is contrary to the natural tendency for molecules to move from high concentration to low concentration.

By utilizing energy from ATP, cells can maintain concentration differences across membranes, which is crucial for various cellular functions, such as nutrient uptake and waste removal. This energy-dependent mechanism contrasts sharply with passive transport processes, such as diffusion, where substances move along their concentration gradients without the input of energy.

Understanding this fundamental difference is key to grasping how cells regulate their internal environments and respond to changes in their external surroundings.