What role do enzymes play in chemical reactions inside cells?

• A. They provide energy for reactions.
• B. They permanently change the substrate.
• C. They are consumed in the reaction.
• D. They act as biological catalysts, lowering activation energy and increasing reaction rate.

Answer: (D)

Enzymes speed up chemical reactions in cells by acting as biological catalysts. They bind to substrates at an active site, orienting them and stabilizing the transition state, which lowers the activation energy—the energy required to start the reaction. Because they provide a lower energy path rather than supplying energy themselves, the reaction proceeds faster. Enzymes are not consumed in the reaction and do not permanently change the substrates; they are ready to catalyze more reactions after the current one finishes. That’s why describing enzymes as lowering activation energy and increasing the reaction rate is the best way to capture their role.