Enzyme Activation Energy Reduction — NEET Biology

Correct answer: A — S1 and S4 only

The core concept tested is how enzymes lower activation energy and their overall effect on a chemical reaction. Let's evaluate each statement based on NCERT: S1: "The difference in average energy content of ‘S’ from that of this transition state is called ‘activation energy’. Enzymes eventually bring down this energy barrier making the transition of ‘S’ to ‘P’ more easy." This means enzymes reduce the activation energy, which is the energy difference between the substrate and the transition state. Thus, this statement is correct. S2: Figure 9.4 clearly shows that the energy levels of the substrate (S) and product (P) remain unchanged in the presence of an enzyme. Only the transition state energy is lowered. Thus, this statement is incorrect. S3: NCERT states that "all other intermediate structural states are unstable." Enzymes lower the *activation energy* (the peak of the transition state), rather than broadly stabilizing all intermediate structural states. While lowering the transition state energy makes it easier to reach, the description in S3 is not the precise mechanism described for lowering activation energy in NCERT. Thus, this statement is incorrect. S4: Enzymes lower the activation energy and increase the reaction rate, but they do not alter the fundamental energy difference between reactants and products (ΔG), nor do they change the equilibrium constant. Figure 9.4 illustrates this by showing the substrate and product energy levels remain the same. Thus, this statement is correct. S5: The NCERT describes the enzyme-substrate complex formation as a "transient phenomenon." The increased reaction rate is primarily due to the lowering of activation energy, not prolonged binding. Thus, this statement is incorrect. Therefore, only S1 and S4 are correct. The final answer is A, as S1 and S4 accurately describe how enzymes lower activation energy and their effect on reaction energetics. Concept Recap: Enzymes accelerate reactions by lowering the activation energy (energy barrier to the transition state) but do not change the overall free energy change (ΔG) or the equilibrium constant of the reaction.