Question

which statement best describes how enzymes affect the speed of a metabolic reaction? a enzymes increase the concentration of reactants, which increases the speed of the reaction. b enzymes decrease the energy needed to start a reaction, which increases the speed of the reaction. c enzymes replace the reactants in a chemical reaction which decreases the speed of the reaction. d enzymes prevent products from being converted back into reactants, which decreases the speed of the reaction.

Explanation:

Enzymes are biological catalysts. They lower the activation - energy (the energy needed to start a reaction). By reducing the activation - energy barrier, more reactant molecules can participate in the reaction, thus increasing the reaction rate. They do not increase reactant concentration, replace reactants, or prevent products from reverting to reactants in the way described in the incorrect options.

Answer:

B. Enzymes decrease the energy needed to start a reaction, which increases the speed of the reaction.