Enzyme-catalyzed reactions: Identify the TRUE statement about activation energy and substrate population.
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AThe activation energy is lowered so that a larger proportion of substrate molecules can surmount it.
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BAdditional substrate molecules are energized by the enzyme so they can overcome the activation energy.
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CThe activation energy is increased, decreasing the chance that substrate molecules will react.
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DThe activation energy is lowered so that fewer substrate molecules can overcome it.
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EThe activation energy and reaction pathway are unchanged by enzymes; only product stability changes.
Answer
Correct Answer: The activation energy is lowered so that a larger proportion of substrate molecules can surmount it.
Explanation
Introduction:This question targets the central idea of enzymatic catalysis: enzymes increase reaction rates by lowering the activation energy (Ea) required to reach the transition state, without altering overall reaction thermodynamics (ΔG).
Given Data / Assumptions:
- Standard Michaelis–Menten framework.
- Constant temperature and pH; no change to ΔG or equilibrium position.
- Comparison is qualitative, focusing on populations of molecules able to react per unit time.
Concept / Approach:Lowering Ea increases the fraction of molecules with sufficient energy at a given temperature (per the Boltzmann distribution), which raises the observed rate constant and velocity v for a given substrate concentration [S].
Step-by-Step Solution:1) Enzymes stabilize the transition state, effectively lowering Ea.2) With a lower barrier, more substrate molecules possess sufficient energy to proceed to product within the same time interval.3) Reaction velocity increases (until limited by saturation), but the equilibrium ratio of products to reactants (Keq) is unchanged.
Verification / Alternative check:Arrhenius-type reasoning: a reduction in Ea increases the rate constant k, raising initial velocity v0 at the same [S]. Empirically observed in Lineweaver–Burk and progress curves.
Why Other Options Are Wrong:
- (b) Wrong: enzymes do not supply energy to “energize” substrates; they provide an alternative, lower-Ea pathway.
- (c) Wrong: increasing Ea would slow, not speed, the reaction.
- (d) Wrong: lowering Ea increases, not decreases, the fraction overcoming the barrier.
- (e) Wrong: enzymes do change the pathway (transition state stabilization) though they do not alter product stability (ΔG).
Common Pitfalls:Assuming enzymes shift equilibrium or “add energy” to substrates; confusing kinetics with thermodynamics.
Final Answer:The activation energy is lowered so that a larger proportion of substrate molecules can surmount it.