In biology, active transport is the process by which cells move molecules across their membrane against their concentration gradient—from an area of low concentration to an area of high concentration. Because this is an "uphill" movement that defies natural diffusion, it requires energy, typically in the form of ATP (adenosine triphosphate).

Active transport is essential for functions like nutrient uptake, waste removal, and maintaining ion balances (e.g., sodium, potassium, calcium).

There are two primary kinds of active transport: and Secondary Active Transport . 1. Primary Active Transport In primary active transport, energy is directly used to move molecules. The energy source is almost always ATP . The protein responsible for this process is a "pump" that hydrolyzes ATP (breaks it down into ADP + phosphate) to change its shape and shuttle the molecule across the membrane.