This document defines and describes several key terms related to membrane transport in cells, including: active transport, which uses ATP to move substances against a concentration gradient; diffusion, the passive movement of particles down a concentration gradient; endocytosis and exocytosis, the processes by which cells take in and release materials via membrane vesicles; facilitated diffusion, which uses membrane proteins to assist diffusion; and osmosis, the passive movement of water across a membrane in response to differences in solute concentration. Concentration gradients, semi-permeable membranes, vesicles, and transport pumps are also discussed in the context of membrane transport.