Enzymes are biological molecules that catalyze (speed up) chemical reactions in living organisms. Enzymes have a specific three-dimensional shape with an active site, a region that fits a specific substrate molecule. This active site acts like a lock-and-key mechanism, where the substrate (key) binds to the enzyme's active site (lock). This binding brings the reactants closer together and lowers the activation energy needed for the reaction to occur, thereby speeding up the chemical process. Enzymes are highly specific to their substrates, ensuring precise control of biochemical reactions in cells.