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 interaction forms an enzyme-substrate complex, facilitating the conversion of substrates into products. Factors such as temperature and pH can affect enzyme activity, making them crucial for maintaining homeostasis in organisms.