Which of the following is NOT true of enzymes?

Step 1: Analyze the question We need the statement that is NOT true of enzymes. Four statements are true and one is false.

Step 2: Key concepts about enzymes

• Enzymes are biological catalysts, usually proteins, that speed up reactions.
• They remain chemically unchanged at the end of the reaction, so they can be reused (C is true).
• They are denatured by high temperatures, which destroys their shape and function (D is true).
• They have a specific region called the active site where the substrate binds (E is true).
• Enzymes do not decide the direction of a reaction. They only lower the activation energy, so they can catalyse a reaction in both directions, depending on the concentrations of substrate and product (A is true).

Step 3: Evaluate the options

• A. Reactions are reversible: True. For example, lipase can break down fats and, under suitable conditions, help synthesise them.
• B. Reactions are irreversible: False. Enzyme-catalysed reactions are generally reversible.
• C. Enzymes remain unchanged: True.
• D. Affected by high temperature: True.
• E. Have active sites: True.

Options A and B contradict each other, so one of them must be the false statement. Since enzyme reactions are reversible, B is the one that is NOT true.

Answer: B

Tip: When two options directly contradict each other, the answer to a "NOT true" question is very likely one of them. Remember "Enzymes: reusable, specific, heat-sensitive, reversible."