Final answer:
Chemical changes in cells require enzymes (option d), which are biological catalysts composed mostly of proteins that speed up reactions by lowering activation energy, without changing the overall energy balance of the reaction.
Step-by-step explanation:
Virtually all chemical changes that take place in cells require enzymes, molecules that greatly increase the rate at which a chemical reaction occurs. Enzymes are biological catalysts that are mostly composed of proteins. They lower the activation energy needed for chemical reactions inside cells without being consumed in the reaction. Enzymes bind to the reactant molecules, helping the necessary bond-breaking and bond-forming processes to occur more readily, which enhances the reaction rate.
This allows reactions that are essential for life to proceed at a rate that supports cellular function. It's crucial to understand that enzymes do not alter the free energy of the reactants or products, meaning they don't change the overall energy balance of the reaction, but they reduce the energy barrier that must be overcome for the reaction to take place.