Define an enzyme.

• A. A carbohydrate that stores energy
• B. A lipid involved in membrane structure
• C. A biological catalyst that speeds up chemical reactions without being consumed
• D. A nucleic acid that stores genetic information

Answer: (C)

Enzymes are biological catalysts that speed up chemical reactions in living organisms, usually proteins (though some RNA molecules can also act as catalysts). They work by binding to a substrate at the active site to form an enzyme–substrate complex, which lowers the activation energy needed for the reaction to occur. Because of their specific shape, enzymes are highly selective for particular substrates or reactions, and after the reaction takes place they are free to catalyze more events. Their activity is tuned by temperature, pH, and the presence of inhibitors or activators, and many need cofactors or coenzymes to function properly. The other options describe different biomolecules with roles like energy storage (carbohydrates), membrane structure (lipids), or genetic information storage (nucleic acids), not catalytic functions.