Enzymes are biological catalysts that speed up chemical reactions in living organisms. They work by lowering the activation energy needed for a reaction to occur, allowing processes like digestion and metabolism to happen more efficiently. Each enzyme is specific to a particular substrate, meaning it only works on certain molecules. The structure of an enzyme includes an active site, where the substrate binds. This binding forms an enzyme-substrate complex, facilitating the conversion of substrates into products. After the reaction, the enzyme is released unchanged and can be used again, making it an essential component of many biological functions, including those in cells and organisms.