Active transport is the movement of materials through a membrane against a concentration gradient. It requires energy in the form of ATP and specialized carrier proteins to bind to particles and transport them across the membrane. Examples include the sodium-potassium pump in cell membranes. Large particles cross membranes through active transport processes like endocytosis, exocytosis, pinocytosis, and phagocytosis, which all require energy. Endocytosis involves engulfing extracellular material, while exocytosis expels intracellular material from the cell. Pinocytosis involves drinking in liquids, while phagocytosis engulfs solids.